font-size

Summary

The font-size CSS property specifies the size of the font. Setting the font size may, in turn, change the size of other items, since it is used to compute the value of the em and ex 
{{ cssxref }} units.

Syntax

/* <absolute-size> values */
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;

/* <relative-size> values */
font-size: larger;
font-size: smaller;

/* <length> values */
font-size: 12px;
font-size: 0.8em;

/* <percentage> values */
font-size: 80%;

/* Global values */
font-size: inherit;
font-size: initial;
font-size: unset;

Values

xx-small, x-small, small, medium, large, x-large, xx-large

A set of absolute size keywords based on the user's default font size (which is medium). Similar to presentational HTML's <font size="1"> through <font size="7"> where the user's default font size is <font size="3">.

larger, smaller

Larger or smaller than the parent element's font size, by roughly the ratio used to separate the absolute size keywords above.

<length>

A positive {{ cssxref }}. When the units are specified in em or ex, the size is defined relative to the size of the font on the parent element of the element in question. For example, 0.5em is half the font size of the parent of the current element. When the units are specified in rem, the size is defined relative to the size of the font used by the html (root) element.

<percentage>

A positive <percentage> of the parent element's font size.

It is best to use values that are relative to the user's default font size, and avoid absolute values such as lengths with units other than em or ex. However, if such absolute values must be used, px are preferred over other units because their meaning does not vary depending on what the operating system thinks (generally incorrectly) the resolution of the monitor is.

Formal syntax

<absolute-size> | <relative-size> | <length> | <percentage>
where 
<absolute-size> = xx-small | s-small | small | medium | large | x-large | xx-large
<relative-size> = larger | smaller

Possible approaches

There are several ways to specify the font size, with keywords or numerical values for pixels or ems. Choose the appropriate method based on the needs for the particular web page.

Keywords

Keywords are a good way to set the size of fonts on the web. By setting a keyword font size on the body element, you can set relative font-sizing everywhere else on the page, giving you the ability to easily scale the font up or down on the entire page accordingly.

Pixels

Setting the font size in pixel values (px) is a good choice when you need pixel accuracy. A px value is static. This is an OS-independent and cross-browser way of literally telling the browsers to render the letters at exactly the number of pixels in height that you specified. The results may vary slightly across browsers, as they may use different algorithms to achieve a similar effect.

Font sizing settings can also be used in combination. For example, if a parent element is set to 16px and its child element is set to larger, the child element displays larger than the parent element in the page.

Ems

Another way of setting the font size is with em values. The size of an em value is dynamic. When defining the font-size property, an em is equal to the size of the font that applies to the parent of the element in question. If you haven't set the font size anywhere on the page, then it is the browser default, which is probably 16px. So, by default 1em = 16px, and 2em = 32px. If you set a font-size of 20px on the body element, then 1em = 20px and 2em = 40px. Note that the value 2 is essentially a multiplier of the current em size.

In order to calculate the em equivalent for any pixel value required, you can use this formula:

em = desired element pixel value / parent element font-size in pixels

For example, suppose the font-size of the body of the page is set to 1em, with the browser standard of 1em = 16px; if the font-size you want is 12px, then you should specify 0.75em (because 12/16 = 0.75). Similarly, if you want a font size of 10px, then specify 0.625em (10/16 = 0.625); for 22px, specify 1.375em (22/16).

A popular technique to use throughout the document is to set the the font-size of the body to 62.5% (that is 62.5% of the default of 16px), which equates to 10px, or 0.625em. Now you can set the font-size for any elements using em units, with an easy-to-remember conversion, by dividing the px value by 10. This way 6px = 0.6em, 8px = 0.8em, 12px = 1.2em, 14px = 1.4em, 16px = 1.6em. For example:

body {
  font-size: 62.5%; /* font-size 1em = 10px on default browser settings */
}
span {
  font-size: 1.6em; /* 1.6em = 16px */
}

The em is a very useful unit in CSS, since it automatically adapts its length relative to the font that the reader chooses to use.

One important fact to keep in mind:  em values compound. Take the following HTML and apply it with the previous CSS above:

<div>
<span>Outer <span>inner</span> outer</span>
</div>

The result is:

Assuming that the browser's default font-size is 16px, the words "outer" would be rendered at 16px, but the word "inner" would be rendered at 25.6px. This is because the inner span's font-size is 1.6 em which is relative to its parent's font-size, which is in turn relative to its parent's font-size. This is often called compounding.

Rems

rem values were invented in order to sidestep the compounding problem. rem values are relative to the root html element, not the parent element. In other words, it lets you specify a font size in a relative fashion without being affected by the size of the parent, thereby eliminating compounding.

The CSS below is nearly identical to the previous example. The only exception is that the unit has been changed to rem.

html {
  font-size: 62.5%; /* font-size 1em = 10px on default browser settings */
}
span {
  font-size: 1.6rem;
}

Then we apply this CSS to the same HTML, which looks like this:

<span>Outer <span>inner</span> outer</span>

In this example, the words "outer inner outer" are all displayed at 16px (assuming that the browser's font-size has been left at the default value of 16px).

Examples

Example 1

/* Set paragraph text to be very large. */
p { font-size: xx-large }

/* Set h1 (level 1 heading) text to be 2.5 times the size
 * of the text around it. */
h1 { font-size: 250% }

/* Sets text enclosed within span tag to be 16px */
span { font-size: 16px; }

Example 2

.small {
	font-size: xx-small;
}
.larger {
	font-size: larger;
}
.point {
	font-size: 24pt;
}
.percent {
	font-size: 200%;
}

<h1 class="small">Small H1</h1>
<h1 class="larger">Larger H1</h1>
<h1 class="point">24 point H1</h1>
<h1 class="percent">200% H1</h1>

Live Sample

Notes

em and ex units on the font-size property are relative to the parent element's font size (unlike all other properties, where they're relative to the font size on the element). This means em units and percentages do the same thing for font-size.

Specifications

Browser compatibility

[1] Internet Explorer 9 and 10 only had partial support for this feature.