Maintain the aspect ratio of a div with CSS

Question

I want to create a div that can change its width height as the window s width changes   Are there any CSS3 rules that would allow the height to change according to the width  while maintaining its aspect ratio   I know I can do this via JavaScript  but I would prefer using only CSS

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I used a new solution     squares    width  30vw   height  30vw   To main aspect ratio   aspect-ratio   width  10vw   height  10vh   However  this is relative to the entire viewport  So  if you need a div that is 30  of the viewport width  you can use 30vw instead  and since you know the width  you reuse them in height using calc and vw unit

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Well  we ve recently received the ability to use the aspect-ratio property in CSS  https   twitter com Una status 1260980901934137345 photo 1 Note  Support is not the best yet     https   caniuse com  search aspect-ratio EDIT  Aspect ratio is now available   https   web dev aspect-ratio

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I ve found a way to do this using CSS  but you have to be careful as it may change depending on the flow of your own web site   I ve done it in order to embed video with a constant aspect ratio within a fluid width portion of my web site   Say you have an embedded video like this    lt object gt        lt param       gt  lt param       gt           lt embed src           lt  embed gt   lt  object gt    You could then place this all inside a div with a  video  class   This video class will probably be the fluid element in your website such that  by itself  it has no direct height constraints  but when you resize the browser it will change in width according to the flow of the web site   This would be the element you are probably trying to get your embedded video in while maintaining a certain aspect ratio of the video   In order to do this  I put an image before the embedded object within the  video  class div         The important part is that the image has the correct aspect ratio you wish to maintain   Also  make sure the size of the image is AT LEAST as big as the smallest you expect the video  or whatever you are maintaining the A R  of  to get based on your layout   This will avoid any potential issues in the resolution of the image when it is percentage-resized   For example  if you wanted to maintain an aspect ratio of 3 2  don t just use a 3px by 2px image   It may work under some circumstances  but I haven t tested it  and it would probably be a good idea to avoid     You should probably already have a minimum width like this defined for fluid elements of your web site   If not  it is a good idea to do so in order to avoid chopping elements off or having overlap when the browser window gets too small   It is better to have a scroll bar at some point   The smallest in width a web page should get is somewhere around  600px  including any fixed width columns  because screen resolutions don t come smaller unless you are dealing with phone-friendly sites       I use a completely transparent png but I don t really think it ends up mattering if you do it right  Like this    lt div class  video  gt        lt img class  maintainaspectratio  src  maintainaspectratio png    gt        lt object gt             lt param       gt  lt param       gt                lt embed src           lt  embed gt        lt  object gt   lt  div gt    Now you should be able to add CSS similar to the following   div video        position  relative    div video img maintainaspectratio   width  100     div video object   position  absolute  top  0px  left  0px  width  100   height  100     div video embed  width  100   height  100       Make sure you also remove any explicit height or width declaration within the object and embed tags that usually come with copy pasted embed code   The way it works depends on the position properties of the video class element and the item you want have maintain a certain aspect ratio   It takes advantage of the way an image will maintain its proper aspect ratio when resized in an element   It tells whatever else is in video class element to take full-advantage of the real estate provided by the dynamic image by forcing its width height to 100  of the video class element being adjusted by the image    Pretty cool  eh   You might have to play around with it a bit to get it to work with your own design  but this actually works surprisingly well for me   The general concept is there

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2020 solution - using Grid and padding of pseudo element I have found some more fresh way to solve this case  This solution is a descendant of a any padding-bottom method  but without any position  absolute children  just using display  grid  and pseudo element  Here we have  ratio  before with good old padding-bottom  XX  and grid-area  1   1   1   1   which forces the pseudo element to keep the position in grid  Although here we have width  0  to prevent overflowing main element by this one  we cold use z-index here  but this one is shorter   And our main element  ratio  gt    first-child has the same position as  ratio  before  which is grid-area  1   1   1   1   so they both shares the same grid cell place  Now we can put any content in our div  and the pseudo element is the one who determines the width height ratio  More about grid-area   x000D   x000D   ratio     display  grid    grid-template-columns  1fr    max-width  200px     just for instance  can be 100  and depends on parent          ratio  before     content        width  0    padding-bottom  calc 100     16 9       here you can place any ratio      grid-area  1   1   1   1      ratio gt   first-child     grid-area  1   1   1   1     the same as   before      background  rgba 0  0  0  0 1      just for instance      x000D   lt div class  ratio  gt     lt div gt 16 9 lt  div gt   lt  div gt  x000D   x000D   x000D    Although you can use CSS val and place you ratio in HTML using style attribute  Works with display  inline-grid as well   x000D   x000D   ratio     display  inline-grid    grid-template-columns  1fr    width  200px     just for instance  can be 100  and depends on parent       margin-right  10px     just for instance        ratio  before     content        width  0    padding-bottom  calc 100     var --r        here you can place any ratio      grid-area  1   1   1   1      ratio gt   first-child     grid-area  1   1   1   1     the same as   before      background  rgba 0  0  0  0 1      just for instance      x000D   lt div class  ratio  style  --r  4 3   gt     lt div gt 4 3 lt  div gt   lt  div gt    lt div class  ratio  style  --r  16 9   gt     lt div gt 16 9 lt  div gt   lt  div gt  x000D   x000D   x000D

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lets say you have 2 divs the outher div is a container and the inner could be any element that you need to keep its ratio  img or an youtube iframe or whatever   html looks like this    lt div class  container  gt     lt div class  element  gt     lt  div gt  lt  -- end of element -- gt   lt div gt  lt  -- end of container -- gt   lets say you need to keep the ratio of the  quot element quot  ratio   gt  4 to 1 or 2 to 1     css looks like this  container    position  relative    height  0   padding-bottom   75     for 4 to 3 ratio    25     for 4 to 1 ratio             element    width   100     height  100     position  absolute     top   0     bottom   0     background   red     just for illustration       padding when specified in   it is calculated based on width not height     so basically you it doesn t matter what your width it height will always be calculated based of that   which will keep the ratio

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I stumbled upon what I consider a smart solution for this problem  using  lt svg gt  and display grid  A display grid element allows you to occupy the same space with two  or more  of its children  using the same grid-area  This means means they are all flow content  overlapped  and out of the box the taller one sets the ratio  One of them will be an  lt svg gt  in charge of setting the ratio  The other  actual content  If actual content is short and never fills up the entire ratio  and you just want it centered in a space with this ratio   simply center it  see first runnable snippet below    lt div class  quot ratio quot  gt     lt svg viewBox  quot 0 0 1 1 quot  gt  lt  svg gt     lt div gt      I m square    lt  div gt   lt  div gt    ratio     display  grid     ratio  gt        grid-area  1 1     Set  lt svg gt s ratio to whatever you want    lt svg viewBox  quot 0 0 4 3 quot  gt  lt  svg gt   lt svg viewBox  quot 0 0 16 9 quot  gt  lt  svg gt    x000D   x000D   ratio     display  grid     ratio  gt        grid-area  1 1        below code NOT needed for setting the ratio     I just wanted to mark it visually    and center contents      ratio div     border  1px solid red    display  flex    align-items  center    justify-content  center    x000D   lt div class  ratio  gt     lt svg viewBox  0 0 7 1  gt  lt  svg gt     lt div gt      Fixed ratio 7 1    lt  div gt   lt  div gt  x000D   x000D   x000D    If you need a solution where the content element has more content you want confined into a scrollable area with desired ratio  set position relative on the parent and position absolute  height 100   overflow-y  auto  on the content  allowing the flow content element  the  lt svg gt   to set the size  therefore the ratio   x000D   x000D   ratio     display  grid    position  relative     ratio  gt        grid-area  1 1       ratio  gt  div     height  100     overflow-y  auto    position  absolute          the rest is not needed      border  1px solid red    padding  0 1rem    x000D   lt div class  ratio  gt     lt svg viewBox  0 0 7 2  gt  lt  svg gt     lt div gt       lt h1 gt Fixed ratio 7 2 lt  h1 gt       lt p gt Lorem ipsum dolor sit amet  consectetur adipiscing elit  sed do eiusmod tempor incididunt ut labore et dolore magna aliqua  A scelerisque purus semper eget  Sem nulla pharetra diam sit amet nisl suscipit adipiscing bibendum  A cras semper auctor neque vitae tempus quam pellentesque nec  Morbi enim nunc faucibus a pellentesque sit amet porttitor  Arcu odio ut sem nulla  Sed viverra ipsum nunc aliquet bibendum enim facilisis gravida neque  Cras tincidunt lobortis feugiat vivamus at augue eget  Laoreet sit amet cursus sit amet  Amet nulla facilisi morbi tempus iaculis urna id volutpat  Leo in vitae turpis massa sed elementum tempus egestas sed  Egestas integer eget aliquet nibh  Dolor sit amet consectetur adipiscing elit    lt p gt Ut aliquam purus sit amet  Eget magna fermentum iaculis eu non diam phasellus vestibulum  Diam in arcu cursus euismod quis viverra nibh  Nullam vehicula ipsum a arcu cursus vitae congue  Vel orci porta non pulvinar neque laoreet suspendisse  At tellus at urna condimentum mattis pellentesque  Tristique senectus et netus et malesuada  Vel pretium lectus quam id leo in  Interdum velit euismod in pellentesque  Velit euismod in pellentesque massa placerat duis  Vitae suscipit tellus mauris a diam maecenas sed enim    lt p gt Mauris a diam maecenas sed enim ut sem  In hendrerit gravida rutrum quisque  Amet dictum sit amet justo donec enim diam  Diam vulputate ut pharetra sit amet aliquam id  Urna porttitor rhoncus dolor purus non enim praesent  Purus in massa tempor nec feugiat nisl pretium  Sagittis vitae et leo duis ut  Facilisi nullam vehicula ipsum a arcu cursus vitae congue mauris  Volutpat odio facilisis mauris sit amet massa vitae tortor condimentum  Aliquam purus sit amet luctus venenatis lectus magna  Sit amet purus gravida quis blandit turpis  Enim eu turpis egestas pretium aenean  Consequat mauris nunc congue nisi  Nunc sed id semper risus in hendrerit gravida rutrum  Ante metus dictum at tempor  Blandit massa enim nec dui nunc mattis enim ut     lt  div gt   lt  div gt  x000D   x000D   x000D    As  emjay noted in a comment below  the ratio svg can be placed in one of the parent s pseudo-elements  as long as it s properly encoded   x000D   x000D   three-squares     display  grid    border  1px solid red      three-squares  gt      three-squares before     grid-area  1 1      three-squares before     content  url  data image svg xml  3Csvg xmlns  http   www w3 org 2000 svg  viewBox  0 0 3 1  3E 3C svg 3E      line-height  0    x000D   lt div class  three-squares  gt       lt div gt I m 3 1 lt  div gt   lt  div gt  x000D   x000D   x000D   When used inside a pseudo-element  the  lt svg gt  becomes a replaced element which  by default  sits on a baseline of variable height  4px in Chrome  3 5px in Firefox   The height of the baseline varies according to line-height  which is why we need to set line-height  0 on the pseudo to get an accurate ratio  More details here   I personally prefer the version where the  lt svg gt  is placed in markup  as I can have a single class   ratio  dealing with containers of various ratios  as opposed to having a class for each individual ratio I might need

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I d like to share this as it has been a journey of a couple frustrating days to find a solution that worked for me  I was using these padding techniques  mentioned above about using some variation of padding and absolute positioning  to achieve 1 1 aspect ratio for a button like element that was inside of a grid flex layout  The layout was set to be 100vh high so that it would always display as a single non-scrolling page  The padding technique does work very well but it can easily break your grid layout and cause blowout nasty scroll bars  The people who say that an absolute div can t affect layout are wrong in this case because the parent grows in both height and or width  that parent can mess with your layouts even if the child doesn t directly  Normally this isn t an issue but the caveat comes when using grid  Grid is a 2D layout and it has the possibility to consider sizing and layout on both axises  I m still trying to understand the exact nature of this but so far it seems like at the very least if you use this technique within a grid area that is constrained by fr units on both axises you will almost certainly experience blowout when the aspect-ratio item grows or otherwise changes the layout  display none toggling and swapping grid areas with css were also layout changing issues that caused the blowout for me   In my case it was that I constrained the height of a column that didn t need to be  Changing it to  quot auto quot  instead of  quot 1fr quot  kept all my other layout the same and prevented blowout while still letting me keep my nice square buttons  I don t know if this is the source of frustration for everyone here but it is an easy mistake to make and even using dev-tools won t give you an accurate idea of of where the blowout is coming from in many cases since it isn t really a case of an individual element blowing it out but rather the grid layout enlarging itself to keep those fr units accurate vertically and horizontally

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vw units   You can use vw units for both the width and height of the element  This allows the element s aspect ratio to be preserved  based on the viewport width      vw   1 100th of the width of the viewport    MDN    Alternatively  you can also use vh for viewport height  or even vmin vmax to use the lesser greater of the viewport dimensions  discussion here    Example  1 1 aspect ratio   x000D   x000D  div   x000D    width  20vw  x000D    height  20vw  x000D    background  gold  x000D    x000D   lt div gt  lt  div gt  x000D   x000D   x000D    For other aspect ratios  you can use the following table to calculate the value for height according to the width of the element    aspect ratio     multiply width by -----------------------------------      1 1                1      1 3                3      4 3               0 75     16 9              0 5625   Example  4x4 grid of square divs   x000D   x000D  body   x000D    display  flex  x000D    flex-wrap  wrap  x000D    justify-content  space-between  x000D    x000D  div   x000D    width  23vw  x000D    height  23vw  x000D    margin  0 5vw auto  x000D    background  gold  x000D    x000D   lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  lt div gt  lt  div gt  x000D   x000D   x000D    Here is a Fiddle with this demo and here is a solution to make a responsive grid of squares with verticaly and horizontaly centered content     Browser support for vh vw units is IE9  see canIuse for more info

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Just create a wrapper  lt div gt  with a percentage value for padding-bottom  like this   x000D   x000D   demoWrapper     padding  10px    background  white    box-sizing  border-box    resize  horizontal    border  1px dashed    overflow  auto    max-width  100     height  calc 100vh - 16px      div     width  100     padding-bottom  75     background  gold       lt -- For the demo       x000D   lt div class  demoWrapper  gt     lt div gt  lt  div gt   lt  div gt  x000D   x000D   x000D   It will result in a  lt div gt  with height equal to 75  of the width of its container  a 4 3 aspect ratio   This relies on the fact that for padding    The percentage is calculated with respect to the width of the generated box s containing block         source  w3 org  emphasis mine   Padding-bottom values for other aspect ratios and 100  width   aspect ratio    padding-bottom value -------------- ----------------------     16 9              56 25      4 3               75      3 2               66 66      8 5               62 5    Placing content in the div   In order to keep the aspect ratio of the div and prevent its content from stretching it  you need to add an absolutely positioned child and stretch it to the edges of the wrapper with  div stretchy-wrapper     position  relative     div stretchy-wrapper  gt  div     position  absolute    top  0  bottom  0  left  0  right  0     Here s a demo and another more in depth demo

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Basing on your solutions I ve made some trick   When you use it  your HTML will be only   lt div data-keep-ratio  75   gt       lt div gt Main content lt  div gt   lt  div gt    To use it this way make  CSS     data-keep-ratio        display  block      width  100       position  relative      data-keep-ratio   gt          position  absolute      left  0      right  0      top  0      bottom  0      and js  jQuery        data-keep-ratio    each function         var ratio     this  data  keep-ratio          this  css  padding-bottom   ratio         And having this you just set attr data-keep-ratio to height width and that s it

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I d like to share my solution  where I have an img-tag filling a certain aspect ratio  I couldn t use background because of lack of support of the CMS and I d not prefer to use a style tag like so   lt img style  background url         gt   Also  the width is 100   so it doesn t need to be set at a fixed size as in some of the solutions  It will scale responsively    x000D   x000D   wrapper   x000D    width  50   x000D    x000D   x000D   image-container   x000D    position  relative  x000D    width  100   x000D    x000D   x000D   image-container  before   x000D    content      x000D    display  block  x000D    x000D   x000D   image-container img   x000D    position  absolute  x000D    top  0  x000D    left  0  x000D    width  100   x000D    height  100   x000D    object-fit  cover  x000D    x000D   x000D   ratio-4-3  before   x000D    padding-top  75   x000D    x000D   x000D   ratio-3-1  before   x000D    padding-top  calc 100    3   x000D    x000D   x000D   ratio-2-1  before   x000D    padding-top  50   x000D    x000D   lt div class  wrapper  gt   lt  -- Just to make things a bit smaller -- gt  x000D     lt p gt  x000D    Example of an 4 3 aspect ratio  filled by an image with an 1 1 ratio  x000D     lt  p gt  x000D     lt div class  image-container ratio-4-3  gt   lt  -- Lets go for a 4 3 aspect ratio -- gt  x000D       lt img src  https   placekitten com 1000 1000   alt  Kittens     gt  x000D     lt  div gt  x000D     lt p gt  x000D    Just place other block elements around it  it will work just fine  x000D     lt  p gt  x000D   lt  div gt  x000D   x000D   x000D

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While most answers are very cool  most of them require to have an image already sized correctly    Other solutions only work for a width and do not care of the height available  but sometimes you want to fit the content in a certain height too   I ve tried to couple them together to bring a fully portable and re-sizable solution    The trick is to use to auto scaling of an image but use an inline svg element instead of using a pre-rendered image or any form of second HTTP request      x000D   x000D  div holder  x000D    background-color red  x000D    display inline-block  x000D    height 100px  x000D    width 400px  x000D    x000D  svg  img  x000D    background-color blue  x000D    display block  x000D    height auto  x000D    width auto  x000D    max-width 100   x000D    max-height 100   x000D    x000D   content sizer  x000D    position relative  x000D    display inline-block  x000D    height 100   x000D    x000D   content  x000D    position absolute  x000D    top 0  x000D    bottom 0  x000D    left 0  x000D    right 0  x000D    background-color rgba 155 255 0 0 5   x000D    x000D   lt div class  holder  gt  x000D     lt div class  content sizer  gt  x000D       lt svg width 10000 height 5000   gt  x000D       lt div class  content  gt  x000D       lt  div gt  x000D     lt  div gt  x000D   lt  div gt  x000D   x000D   x000D    Notice that I have used big values in the width and height attributes of the SVG  as it needs to be bigger than the maximum expected size as it can only shrink  The example makes the div s ratio 10 5

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As  web-tiki already show a way to use vh vw  I also need a way to center in the screen  here is a snippet code for 9 16 portrait    container     width  100vw    height  calc 100vw   16   9     transform  translateY calc  100vw   16   9 - 100vh    -2        translateY will keep this center in the screen  calc 100vw   16   9  is expected height for 9 16  100vw   16   9 - 100vh  is overflow height  so  pull up overflow height 2 will keep it center on screen   For landscape  and keep 16 9  you show use   container     width  100vw    height  calc 100vw   9   16     transform  translateY calc  100vw   9   16 - 100vh    -2        The ratio 9 16 is ease to change  no need to predefined 100 56 25 or 100 75 If you want to ensure height first  you should switch width and height  e g  height 100vh width  calc 100vh   9   16  for 9 16 portrait   If you want to adapted for different screen size  you may also interest   background-size cover contain   Above style is similar to contain  depends on width height ratio   object-fit   cover contain for img video tag   media  orientation  portrait   media  orientation  landscape    Media query for portrait landscape to change the ratio

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Here is my solution for maintaining an 16 9 aspect ratio in portrait or landscape on a div with optional fixed margins  It s a combination of width height and max-width max-height properties with vw units  In this sample  50px top and bottom margins are added on hover   x000D   x000D  html     height  100      body     margin  0    height  100       container     display  flex    justify-content  center    align-items  center    height  100       fixedRatio     max-width  100vw    max-height  calc 9   16   100vw     width  calc 16   9   100vh     height  100vh          DEBUG      display  flex    justify-content  center    align-items  center    background-color  blue    font-size  2rem    font-family   Arial     color  white    transition  width 0 5s ease-in-out  height 0 5s ease-in-out       fixedRatio hover     width  calc 16   9    100vh - 100px      height  calc 100vh - 100px     x000D   lt div class  container  gt     lt div class  fixedRatio  gt      16 9    lt  div gt   lt  div gt  x000D   x000D   x000D   Demo on JSFiddle

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I just created a 2 1 div that resized to occupy the full width  but then shrinks the width if it would cause the top or bottom to exceed  But note  this will only work with the size of the window  not the size of the parent   scene       position  relative      top  50vh      left  50vw      width  100vw      height  50vw      max-height  100vh      max-width  calc 100vh   2       transform  translate -50   -50       I m sure you can work out the correct   s to use for 4 3 instead of 2 1

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You can use an svg  Make the container wrapper position relative  put the svg first as staticly positioned and then put absolutely positioned content  top  0  left 0  right 0  bottom 0    Example with 16 9 proportions   image svg   can be inlined in src    lt svg xmlns  http   www w3 org 2000 svg  viewBox  0 0 16 9  width  16  height  9   gt    CSS    container     position  relative     content     position  absolute    top 0  left 0  right 0  bottom 0      HTML    lt div class  container  gt     lt img style  width  100   src  image svg    gt     lt div class  content  gt  lt  div gt   lt  div gt    Note that inline svg doesn t seem to work  but you can urlencode the svg and embed it in img src attribute like this    lt img src  data image svg xml  3Csvg 20xmlns 3D 22http 3A 2F 2Fwww w3 org 2F2000 2Fsvg 22 20viewBox 3D 220 200 2016 209 22 20width 3D 2216 22 20height 3D 229 22 2F 3E  style  width  100      gt

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Just an idea or a hack    x000D   x000D  div   x000D    background-color  blue  x000D    width  10   x000D    transition  background-color 0 5s  width 0 5s  x000D    font-size  0  x000D    x000D   x000D  div hover   x000D    width  20   x000D    background-color  red  x000D    x000D     x000D  img   x000D    width  100   x000D    height  auto  x000D    visibility  hidden  x000D    x000D   lt div gt  x000D     lt  -- use an image with target aspect ratio  sample is a square -- gt  x000D     lt img src  http   i imgur com 9OPnZNk png    gt  x000D   lt  div gt  x000D   x000D   x000D

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New in Chrome 88 and soon to follow in other browsers is the new CSS aspect-ratio property   The aspect-ratio  CSS property sets a preferred aspect ratio for the box  which will be used in the calculation of auto sizes and some other layout functions   CSS Tricks Article More Information  x000D   x000D  div     background  rebeccapurple    height 100px    margin 1em auto      square     aspect-ratio  1   1      x000D   lt div class  square  gt      lt  div gt  x000D   x000D   x000D

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As stated in here on w3schools com and somewhat reiterated in this accepted answer  padding values as percentages  emphasis mine       Specifies the padding in percent of the width of the containing element   Ergo  a correct example of a responsive DIV that keeps a 16 9 aspect ratio is as follows   CSS   parent       position  relative      width  100      child       position  relative      padding-bottom  calc 100    9   16      child  gt  div       position  absolute      top  0      bottom  0      left  0      right  0      HTML   lt div class  parent  gt       lt div class  child  gt           lt div gt Aspect is kept when resizing lt  div gt       lt  div gt   lt  div gt    Demo on JSFiddle

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If you want to fit a square inside the viewport on either portrait or landscape view  as big as possible  but nothing sticking outside   switch between using vw vh on orientation portrait landscape    media  orientation portrait        square       width  100vw      height 100vw          media  orientation landscape       square       width  100vh      height 100vh

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A simple way of maintaining the aspect ratio  using the canvas element  Try resizing the div below to see it in action  For me  this approach worked best  so I am sharing it with others so they can benefit from it as well   x000D   x000D   cont     border  5px solid blue    position  relative    width  300px    padding  0    margin  5px    resize  horizontal    overflow  hidden      ratio     width  100     margin  0    display  block      content     background-color  rgba 255  0  0  0 5     position  absolute    top  0    left  0    width  100     height  100     margin  0    x000D   lt div class  cont  gt     lt canvas class  ratio  width  16  height  9  gt  lt  canvas gt     lt div class  content  gt I am 16 9 lt  div gt   lt  div gt  x000D   x000D   x000D   Also works with dynamic height   x000D   x000D   cont     border  5px solid blue    position  relative    height  170px    padding  0    margin  5px    resize  vertical    overflow  hidden    display  inline-block     so the div doesn t automatically expand to max width        ratio     height  100     margin  0    display  block      content     background-color  rgba 255  0  0  0 5     position  absolute    top  0    left  0    width  100     height  100     margin  0    x000D   lt div class  cont  gt     lt canvas class  ratio  width  16  height  9  gt  lt  canvas gt     lt div class  content  gt I am 16 9 lt  div gt   lt  div gt  x000D   x000D   x000D

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Here s how I do it    data-aspect-ratio        display  block      max-width  100       position  relative      data-aspect-ratio  before       content          display  block      data-aspect-ratio   gt          display  block      height  100       left  0      position  absolute      top  0      width  100      data-aspect-ratio  quot 3 1 quot   before       padding-top  33 33      data-aspect-ratio  quot 2 1 quot   before       padding-top  50      data-aspect-ratio  quot 16 9 quot   before       padding-top  56 25      data-aspect-ratio  quot 3 2 quot   before       padding-top  66 66      data-aspect-ratio  quot 4 3 quot   before       padding-top  75      data-aspect-ratio  quot 1 1 quot   before       padding-top  100      data-aspect-ratio  quot 3 4 quot   before       padding-top  133 33      data-aspect-ratio  quot 2 3 quot   before       padding-top  150      data-aspect-ratio  quot 9 16 quot   before       padding-top  177 77      data-aspect-ratio  quot 1 2 quot   before       padding-top  200      data-aspect-ratio  quot 1 3 quot   before       padding-top  300      For example    lt div data-aspect-ratio  quot 16 9 quot  gt  lt iframe     gt  lt  iframe gt  lt  div gt   source

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To add to Web Designer s answer  the  lt div gt  will have a height  entirely made up of bottom padding  of 75  of the width of it s containing element  Here s a good summary  http   mattsnider com css-using-percent-for-margin-and-padding   I m not sure why this should be so  but that s how it is   If you want your div to be a width other than 100   you need another wrapping div on which to set the width   div ar-outer      width  60      container  whatever width you want        margin  0 auto     centered if you like      div ar       width 100      100  of width of container        padding-bottom  75      75  of width of container        position relative    div ar-inner       position  absolute      top  0  bottom  0  left  0  right  0      I used something similar to Elliot s image trick recently to allow me to use CSS media queries to serve a different logo file depending on device resolution  but still scale proportionally as an  lt img gt  would naturally do  I set the logo as background image to a transparent  png with the correct aspect ratio   But Web Designer s solution would save me an http request

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Elliot inspired me to this solution - thanks   aspectratio png is a completely transparent PNG-file with the size of your preferred aspect-ratio  in my case 30x10 pixels   HTML   lt div class  eyecatcher  gt     lt img src   img aspectratio png   gt   lt  div gt    CSS3   eyecatcher img     width  100     background-repeat  no-repeat    background-size  100  100     background-image  url    img autoresized-picture jpg       Please note  background-size is a css3-feature which might not work with your target-browsers  You may check interoperability  f e  on caniuse com

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I have run into this issue quite some times  so I made a JS solution for it  This basically adjust the height of the domElement according the width of the element by the ratio you specify  You could use it as followed    lt div ratio  4x3  gt  lt  div gt   Please be aware that since it is setting the height of the element  the element should be either a display block or display inline-block   https   github com JeffreyArts html-ratio-component

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Say that you like to maintain Width  100px and the Height  50px  i e   2 1   Just do this math    pb-2to1     padding-bottom  calc 50   100   100       i e   2 1

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This is an improvement on the accepted answer    Uses pseudo elements instead of wrapper divs The aspect ratio is based on the width of the box instead of its parent The box will stretch vertically when the content becomes taller    x000D   x000D   box   x000D    margin-top  1em  x000D    margin-bottom  1em  x000D    background-color   CCC  x000D    x000D   x000D   fixed-ar  before   x000D    content      x000D    float  left  x000D    width  1px  x000D    margin-left  -1px  x000D    x000D   fixed-ar  after   x000D    content      x000D    display  table  x000D    clear  both  x000D    x000D   x000D   fixed-ar-16-9  before   x000D    padding-top  56 25   x000D    x000D   fixed-ar-3-2  before   x000D    padding-top  66 66   x000D    x000D   fixed-ar-4-3  before   x000D    padding-top  75   x000D    x000D   fixed-ar-1-1  before   x000D    padding-top  100   x000D    x000D   x000D   width-50   x000D    display  inline-block  x000D    width  50   x000D    x000D   width-20   x000D    display  inline-block  x000D    width  20   x000D    x000D   lt div class  box fixed-ar fixed-ar-16-9  gt 16 9 full width lt  div gt  x000D   lt hr gt  x000D   lt div class  box fixed-ar fixed-ar-16-9 width-50  gt 16 9 lt  div gt  x000D   lt hr gt  x000D   lt div class  box fixed-ar fixed-ar-16-9 width-20  gt 16 9 lt  div gt  x000D   lt div class  box fixed-ar fixed-ar-3-2 width-20  gt 3 2 lt  div gt  x000D   lt div class  box fixed-ar fixed-ar-4-3 width-20  gt 4 3 lt  div gt  x000D   lt div class  box fixed-ar fixed-ar-1-1 width-20  gt 1 1 lt  div gt  x000D   lt hr gt  x000D   lt div class  box fixed-ar fixed-ar-16-9 width-20  gt 16 9 lt  div gt  x000D   lt div class  box fixed-ar fixed-ar-16-9 width-50  gt 16 9 lt  div gt  x000D   x000D   x000D

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2021 solution - use the aspect-ratio CSS property  currently supported in Chromium  Safari Technology Preview  and Firefox Nightly   lt div class  demo  gt  lt  div gt    demo     background  black    width  500px    aspect-ratio  4 3

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You can achieve that by using SVG   It depends on a case  but in some it is really usefull  As an example - you can set background-image without setting fixed height or use it to embed youtube  lt iframe gt  with ratio 16 9 and position absolute etc   For 3 2 ratio set viewBox  0 0 3 2  and so on   Example    x000D   x000D  div  x000D      background-color red x000D    x000D  svg  x000D      width 100   x000D      display block  x000D      visibility hidden x000D    x000D   x000D   demo-1 width 35   x000D   demo-2 width 20   x000D   lt div class  demo-1  gt  x000D     lt svg viewBox  0 0 3 2  gt  lt  svg gt  x000D   lt  div gt  x000D   x000D   lt hr gt  x000D   x000D   lt div class  demo-2  gt  x000D     lt svg viewBox  0 0 3 2  gt  lt  svg gt  x000D   lt  div gt  x000D   x000D   x000D

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I guess just using rem or em should solve the problem of fixed ratio but wont be hard bound to the screen as vw or vh or as painfull to use with flexbox as   is  Well  none of the answers worked properly for me  and in my case this attended me   lt div class  quot container quot  gt      lt div gt      lt  div gt   lt  div gt    container      height  100vh     width  100vw     container div      height  4em     width  3em     Or using rem  But anyway  any of them should make it  rem uses the default font-size value as em use the closest font-size

[html] Maintain the aspect ratio of a div with CSS

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