CSS why doesn t percentage height work

Question

How come a percentage value for height doesn   t work but a percentage value for width does   For example    lt div id  working  gt  lt  div gt   lt div id  not-working  gt  lt  div gt     working      width 80       height 140px      background orange     not-working      width 80       height 30       background green      The width of  working ends up being 80  of the viewport  but the height of  not-working ends up being 0

User · Answer

Another option is to add style to div    lt div style  position  absolute  height somePercentage   overflow auto or other overflow value   gt     to be scrolled   lt  div gt    And it means that an element is positioned relative to the nearest positioned ancestor

User · Answer

I think you just need to give it a parent container    even if that container s height is defined in percentage  This seams to work just fine  JSFiddle  html  body        margin  0       padding  0       width  100        height  100      wrapper        width  100        height  100       container        width  100        height  50

User · Answer

A percentage value in a height property has a little complication  and the width and height properties actually behave differently to each other  Let me take you on a tour through the specs   height property   Let s have a look at what CSS Snapshot 2010 spec says about height      The percentage is calculated with respect to the height of the generated box s containing block  If the height of the containing block is not specified explicitly  i e   it depends on content height   and this element is not absolutely positioned  the value computes to  auto   A percentage height on the root element is relative to the initial containing block  Note  For absolutely positioned elements whose containing block is based on a block-level element  the percentage is calculated with respect to the height of the padding box of that element    OK  let s take that apart step by step      The percentage is calculated with respect to the height of the generated box s containing block     What s a containing block  It s a bit complicated  but for a normal element in the default static position  it s      the nearest block container ancestor box   or in English  its parent box   It s well worth knowing what it would be for fixed and absolute positions as well  but I m ignoring that to keep this answer short    So take these two examples    x000D   x000D   lt div   id  a   style  width  100px  height  200px  background-color  orange  gt  x000D     lt div id  aa  style  width  100px  height  50     background-color  blue  gt  lt  div gt  x000D   lt  div gt  x000D   x000D   x000D     x000D   x000D   lt div   id  b   style  width  100px               background-color  orange  gt  x000D     lt div id  bb  style  width  100px  height  50   background-color  blue  gt  lt  div gt  x000D   lt  div gt  x000D   x000D   x000D    In this example  the containing block of  aa is  a  and so on for  b and  bb  So far  so good   The next sentence of the spec for height is the complication I mentioned in the introduction to this answer      If the height of the containing block is not specified explicitly  i e   it depends on content height   and this element is not absolutely positioned  the value computes to  auto     Aha  Whether the height of the containing block has been specified explicitly matters    50  of height 200px is 100px in the case of  aa But 50  of height auto is auto  which is 0px in the case of  bb since there is no content for auto to expand to   As the spec says  it also matters whether the containing block has been absolutely positioned or not  but let s move on to width   width property   So does it work the same way for width  Let s take a look at the spec      The percentage is calculated with respect to the width of the generated box s containing block    Take a look at these familiar examples  tweaked from the previous to vary width instead of height    x000D   x000D   lt div   id  c   style  width  200px  height  100px  background-color  orange  gt  x000D     lt div id  cc  style  width  50     height  100px  background-color  blue  gt  lt  div gt  x000D   lt  div gt  x000D   x000D   x000D     x000D   x000D   lt div   id  d   style              height  100px  background-color  orange  gt  x000D     lt div id  dd  style  width  50   height  100px  background-color  blue  gt  lt  div gt  x000D   lt  div gt  x000D   x000D   x000D     50  of width 200px is 100px in the case of  cc 50  of width auto is 50  of whatever width auto ends up being  unlike height  there is no special rule that treats this case differently    Now  here s the tricky bit  auto means different things  depending partly on whether its been specified for width or height  For height  it just meant the height needed to fit the contents   but for width  auto is actually more complicated  You can see from the code snippet that s in this case it ended up being the width of the viewport   What does the spec say about the auto value for width      The width depends on the values of other properties  See the sections below     Wahey  that s not helpful  To save you the trouble  I ve found you the relevant section to our use-case  titled  calculating widths and margins   subtitled  block-level  non-replaced elements in normal flow       The following constraints must hold among the used values of the other properties        margin-left     border-left-width     padding-left     width     padding-right     border-right-width     margin-right    width of containing block   OK  so width plus the relevant margin  border and padding borders must all add up to the width of the containing block  not descendents the way height works   Just one more spec sentence      If  width  is set to  auto   any other  auto  values become  0  and  width  follows from the resulting equality    Aha  So in this case  50  of width auto is 50  of the viewport  Hopefully everything finally makes sense now     Footnotes    At least  as far it matters in this case  spec All right  everything only kind of makes sense now

User · Answer

The height of a block element defaults to the height of the block s content  So  given something like this    lt div id  outer  gt       lt div id  inner  gt           lt p gt Where is pancakes house  lt  p gt       lt  div gt   lt  div gt     inner will grow to be tall enough to contain the paragraph and  outer will grow to be tall enough to contain  inner   When you specify the height or width as a percentage  that s a percentage with respect to the element s parent  In the case of width  all block elements are  unless specified otherwise  as wide as their parent all the way back up to  lt html gt   so  the width of a block element is independent of its content and saying width  50  yields a well defined number of pixels   However  the height of a block element depends on its content unless you specify a specific height  So there is feedback between the parent and child where height is concerned and saying height  50  doesn t yield a well defined value unless you break the feedback loop by giving the parent element a specific height

User · Answer

You need to give it a container with a height  width uses the viewport as the default width

User · Answer

Without content  the height has no value to calculate the percentage of  The width  however  will take the percentage from the DOM  if no parent is specified   Using your example  Placing the second div inside the first div  would have rendered a result   example below      lt div id  working  gt     lt div id  not-working  gt  lt  div gt   lt  div gt    The second div would be 30  of the first div s height

[css] CSS – why doesn’t percentage height work?

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