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CSS font-size: A Definitive Font-Sizing Guide

By Craig Buckler

Font sizing in CSS sounds as though it should be easy. Until you try it. Many developers use the force; they tinker with the CSS font-size property until it looks right only to find it’s different in another browser. A little understanding can go a long way…

The font-size Property

The font-size property can be set for any HTML tag (even if it would not normally contain textual content like br). It can be assigned a variety of absolute, relative, or length size parameters.

An element will inherit the font-size of its parent unless you override it. This is especially important when you specify relative sizes.

Absolute Font Sizing Keywords

Several absolute font-sizing keywords are available. The font size is determined from a browser preset and the element will not inherit its parent’s size.

  • font-size: xx-small;
  • font-size: x-small;
  • font-size: small;
  • font-size: medium;
  • font-size: large;
  • font-size: x-large;
  • font-size: xx-large;

Although most browsers support these keywords, the exact sizes will differ. They are a fairly crude method of font sizing and are generally avoided by most developers.

Relative Font Sizing Keywords

Two relative font-sizing keywords are available. The font is sized according to its parent element:

  • font-size: smaller;
  • font-size: larger;

For example, if the parent has a font size of ‘medium’, a value of ‘larger’ will set the element to ‘large’. Other font units are normally altered by a factor of around 1.2 but, again, there is no standard and browser results will differ.

Absolute Lengths

The font-size property can be assigned an absolute length:

  • mm: millimeters, e.g. 10mm.
  • cm: centimeters, e.g. 1cm ( = 10mm).
  • in: inches, e.g. 0.39in ( ~= 10mm).
  • pt: point, where 1pt is generally assumed to be 1/72 inch e.g. 12pt.
  • pc: pica, where 1pc is 12pt
  • px: pixel, e.g. 14px.

In general, there are issues with all these measurement units. Millimeters, centimeters and inches are inaccurate for a screen-based medium. Points and picas are unreliable since systems can use different dpi settings. Pixel appears to be the most suitable, but it can lead to accessibility issues because the text cannot be resized in IE.

Relative Lengths

The font-size property can be assigned a unit that it relative to its parent’s font size:

  • em: 1em is equivalent to the current font size, so 2em is twice as large.
  • %: 100% is equivalent to the current font size, so 200% is twice as large.
  • ex: 1ex is equivalent to the height of the letter ‘x’ in the current font.

Few developers use ‘ex’, but it can be useful in some situations where you need fine-grained font sizes, e.g. 1ex rather than 0.525em.

Percentage and ’em’ sizes are equivalent, e.g. 50% = 0.5em, 100% = 1em, 120% = 1.2em, etc. Some browsers exhibit subtle differences but it’s rarely a major problem. If you want to save every byte, you could choose the shortest definition, i.e. 50% is shorter than 0.5em and 1em is shorter than 100%.

Text Sizing and Page Zooming

This is where additional complexity creeps in. Most browsers allow the user to:

  1. increase or decrease the base text size (image dimensions are not changed)
  2. zoom the page in or out so all the text and graphics change accordingly, or
  3. allow both text sizing and page zooming.

Just to complicate matters further, Internet Explorer does not allow text resizing on elements which have a font size defined in pixels (px).

If you’re a designer moving to the web from a print background, it’s disconcerting to give the user that much power. Your design could be ruined by a user zooming in 200% but reducing the text size to 50%. And — no — there is nothing you can do to prevent it. Nor should you.

CSS Font Sizing Recommendations

The general consensus is that ’em’ or ‘%’ is the best solution in most situations. Web fonts can be finely scaled relative to each other and browser text sizing is supported. I would also recommend using a percentage font-size on the body tag; it results in better text-sizing in some older browsers.

There are a couple of other recommendations I would suggest when you’re developing a site:

  1. reset the font size and page zoom to their default values in all your browsers before testing (it’s caught me out a few times!)
  2. try reasonable combinations of text sizing and page zooming in a variety of browsers to ensure the text remains readable.

Has font sizing ever caused you problems? Do you have any other tips?

Craig is a freelance UK web consultant who built his first page for IE2.0 in 1995. Since that time he's been advocating standards, accessibility, and best-practice HTML5 techniques. He's created enterprise specifications, websites and online applications for companies and organisations including the UK Parliament, the European Parliament, the Department of Energy & Climate Change, Microsoft, and more. He's written more than 1,000 articles for SitePoint and you can find him @craigbuckler.

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