Why em instead of px

Question

I heard you should define sizes and distances in your stylesheet with em instead of in pixels  So the question is why should I use em instead of px when defining styles in css  Is there a good example that illustrates this

User · Answer

It is wrong to say that one is a better choice than the other (or both wouldn't have been given their own purpose in the spec). It may even be worth noting that StackOverflow makes extensive use of px units. It is not the poor choice Spoike was told it was.

Definition of units

px is an absolute unit of measurement (like in, pt, or cm) that also happens to be 1/96 of an in unit (more on why later). Because it is an absolute measurement, it may be used any time you want to define something to be a particular size, rather than being proportional to something else like the size of the browser window or the font size.

Like all the other absolute units, px units don't scale according to the width of the browser window. Thus, if your entire page design uses absolute units such as px rather than %, it won't adapt to the width of the browser. This is not inherently good or bad, just a choice that the designer needs to make between adhering to an exact size and being inflexible versus stretching but in the process not adhering to an exact size. It would be typical for a site to have a mix of fixed-size and flexible-sized objects.

Fixed size elements often need to be incorporated into the page - such as advertising banners, logos or icons. This ensures you almost always need at least some px-based measurements in a design. Images, for example, will (by default) be scaled such that each pixel is 1*px* in size, so if you are designing around an image you'll need px units. It is also very useful for precise font sizing, and for border widths, where due to rounding it makes the most sense to use px units for the majority of screens.

All absolute measurements are rigidly related to each other; that is, 1in is always 96px, just as 1in is always 72pt. (Note that 1in is almost never actually a physical inch when talking about screen-based media). All absolute measurements assume a nominal screen resolution of 96ppi and a nominal viewing distance of a desktop monitor, and on such a screen one px will be equal to one physical pixel on the screen and one in will be equal to 96 physical pixels. On screens that differ significantly in either pixel density or viewing distance, or if the user has zoomed the page using the browser's zoom function, px will no longer necessarily relate to physical pixels.
em is not an absolute unit - it is a unit that is relative to the currently chosen font size. Unless you have overridden font style by setting your font size with an absolute unit (such as px or pt), this will be affected by the choice of fonts in the user's browser or OS if they have made one, so it does not make sense to use em as a general unit of length except where you specifically want it to scale as the font size scales.

Use em when you specifically want the size of something to depend on the current font size.
% is also a relative unit, in this case, relative to either the height or width of a parent element. They are a good alternative to px units for things like the total width of a design if your design does not rely on specific pixel sizes to set its size.

Using % units in your design allows your design to adapt to the width of the screen/device, whereas using an absolute unit such as px does not.

User · Answer

The top voted answer here from thomasrutter is right in his response about the em  But is very very wrong about the size of a pixel   So even though it is old  I cannot let it be undebated   A computer screen is normally NOT 96dpi   Or ppi  if you want to be pedantic     A pixel does NOT have a fixed physical size   Yes  it is fixed within one screen only  but in the next screen a pixel is most likely bigger  or smaller  and certainly NOT 1 96 of an inch      Proof Draw a line  960 pixels long   Measure it with a physical ruler   Is it 10 inches   No    Connect your laptop to your TV   Is the line 10 inches now   Still not  Show the line on your iPhone   Still same size   Why not   Who the heck invented the 96dpi computer screen myth   Some religions operate with a 72dpi myth  But equally wrong

User · Answer

Avoid em or px use rem instead becuase its easier to find the computed value   But between em and px  px is better because em is hard to debug

User · Answer

It s of use for everything that has to scale according to the font size   It s especially useful on browsers which implement zoom by scaling the font size  So if you size all your elements using em they scale accordingly

User · Answer

The main reason for using em or percentages is to allow the user to change the text size without breaking the design  If you design with fonts specified in px  they do not change size  in IE 6 and others  if the user chooses text size - larger  This is very bad for users with visual handicaps    For several examples of and articles on designs like this  there are a myriad to choose from   see the latest issue of A List Apart  Fluid Grids  the older article How to Size Text in CSS or Dan Cederholm s Bulletproof Web Design    Your images should still be displayed with px sizes  but  in general  it is not considered good form to size your text with px   As much as I personally despise IE6  it is currently the only browser approved for the bulk of the users in our Fortune 200 company

User · Answer

A very practical reason is that IE 6 doesn t let you resize the font if it s specified using px  whereas it does if you use a relative unit such as em or percentages  Not allowing the user to resize the font is very bad for accessibility  Although it s in decline  there are still a lot of IE 6 users out there

User · Answer

There is a simple solution if you want to use px to specify font size  but still want the usability that em s provide by placing this in your CSS file   body     font-size  62 5       Now specify you p  and other  tags like this   p     font-size  0 8em     This is equal to 8px      font-size  1 0em     This is equal to 10px      font-size  1 2em     This is equal to 12px      font-size  2 0em     This is equal to 20px        And so on

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I have a small laptop with a high resolution and have to run Firefox in 120  text zoom to be able to read without squinting   Many sites have problems with this  The layout becomes all garbled  text in buttons is cut in half or disappears entirely  Even stackoverflow com suffers from it     Note how the top buttons and the page tabs overlap  If they would have used em units instead of px  there would not have been a problem

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The reason I asked this question was that I forgot how to use em s as it was a while I was hacking happily in CSS  People didn t notice that I kept the question general as I wasn t talking about sizing fonts per se  I was more interested in how to define styles on any given block element on the page   As Henrik Paul and others pointed out em is proportional to the font-size used in the element   It s a common practice to define sizes on block elements in px  however  sizing up fonts in browsers usually breaks this design  Resizing fonts is commonly done with the shortcut keys Ctrl   or Ctrl -  So a good practice is to use em s instead    Using px to define the width  Here is an illustrating example  Say we have a div-tag that we want to turn into a stylish date box  we may have HTML-code that looks like this    lt div class  date-box  gt       lt p class  month  gt July lt  p gt       lt p class  day  gt 4 lt  p gt   lt  div gt    A simple implementation would defining the width of the date-box class in px       margin  0  padding  0     p month   font-size  10pt     p day   font-size  24pt  font-weight  bold     div date-box       background-color   DD2222      font-family  Arial  sans-serif      color  white      width  50px      The problem  However  if we want to size the text up in our browser the design will break  The text will also bleed outside the box which is almost the same what happens with SO s design as flodin points out  This is because the box will remain the same size in width as it is locked to 50px    Using em instead  A smarter way is to define the width in ems instead   div date-box       background-color   DD2222      font-family  Arial  sans-serif      color  white      width  2 5em         margin  0  padding  0  font-size  10pt        Initial width of date-box   10 pt x 2 5 em   25 pt    Will also work if you used px instead of pt   That way you have a fluid design on the date-box  i e  the box will size up together with the text in proportion to the font-size defined for the date-box  In this example  the font-size is defined in   as 10pt and will size up 2 5 times to that font size  So when you re sizing the fonts in the browser  the box will have 2 5 times the size of that font-size

User · Answer

The general consensus is to use percentages for font sizing  because it s more consistent across browsers platforms   It s funny though  I always used to use pt for font sizing and I assumed all sites used that  You don t normally use px sizes in other apps  eg Word   I guess it s because they re for printing - but the size is the same in a web browser as in Word

User · Answer

use px for precise placement of graphical elements  use em for measurements having to do positioning and spacing around text elements like line-height etc  px is pixel accurate  em can change dynamically with the font in use

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Pixel is an absolute unit whereas rem em are relative units  For more  https   drafts csswg org css-values-3    You should use the relative unit when you want the font-size to be adaptive according to the system s font size because the system provides the font-size value to the root element which is the HTML element   In this case  where the webpage is open in google chrome  the font-size to the HTML element is set by chrome  try changing it to see the effect on webpages with fonts of rem  em units         If you use px as the unit for fonts  the fonts will not resize whereas the fonts with rem  em unit will resize when you change the system s font size   So use px when you want the size to be fixed and use rem  em when you want the size to be adaptive  dynamic to the size of the system

User · Answer

You probably want to use em for font sizes until IE6 is gone  from your site   Px will be alright when page zooming  as opposed to text zooming  becomes the standard behaviour   Traingamer already provided the neccessary links

User · Answer

Because the em  http   en wikipedia org wiki Em  typography   is directly proportional to the font size currently in use  If the font size is  say  16 points  one em is 16 points  If your font size is 16 pixels  note  not the same as points   one em is 16 pixels   This leads to two  related  things    it s easy to keep proportions  if you choose to edit your font sizes in your CSS later on  Many browsers support custom font sizes  overriding your CSS  If you design everything in pixels  your layout might break in these cases  But  if you use ems  these overridings should mitigate these problems

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example  Code  body font-size 10px     keep at 10 all sizes below correct  change this value and the rest change to be e g  1 4 of this value 1 font-size 1 2em     12px 2 font-size 1 4em     14px 3 font-size 1 6em     16px 4 font-size 1 8em     18px 5 font-size 2em     20px     body 1 2 3 4 5 by changing the value in body the rest change automatically to be a kind of times the base value    10  2 20 10  1 6 16 etc you could have the base value as 8px    so 8  2 16 8  1 6 12 8   may be rounded by browser

[css] Why em instead of px?

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