font-size - CSS: Cascading Style Sheets

The font-size CSS property sets the size of the font. Changing the font size also updates the sizes of the font size-relative <length> units, such as em, ex, and so forth.

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;
font-size: xxx-large;

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

/* <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;

The font-size property is specified in one of the following ways:

As one of the absolute-size or relative-size keywords
As a <length> or a <percentage>, relative to the parent element's font size

Values

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

Absolute-size keywords, based on the user's default font size (which is medium).

larger, smaller

Relative-size keywords. The font will be larger or smaller relative to the parent element's font size, roughly by the ratio used to separate the absolute-size keywords above.

<length>

A positive <length> value. For most font-relative units (such as em and ex), the font size is relative to the parent element's font size.

For font-relative units that are root-based (such as rem), the font size is relative to the size of the font used by the <html> (root) element.

<percentage>

A positive <percentage> value, relative to the parent element's font size.

Note: To maximize accessibility, it is generally best to use values that are relative to the user's default font size.

Description

There are several ways to specify the font size, including 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.

Note: Defining font sizes in px is not accessible, because the user cannot change the font size in some browsers. For example, users with limited vision may wish to set the font size much larger than the size chosen by a web designer. Avoid using them for font sizes if you wish to create an inclusive design.

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 font size of the element on which the em is used. If you haven't set the font size anywhere on the page, then it is the browser default, which is often 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 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).

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.

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.

One important fact to keep in mind: em values compound. Take the following HTML and CSS:

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

<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 1.6em 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 10px).

Formal definition

Initial value	`medium`
Applies to	all elements. It also applies to `::first-letter` and `::first-line`.
Inherited	yes
Percentages	refer to the parent element's font size
Computed value	as specified, but with relative lengths converted into absolute lengths
Animation type	a length

Formal syntax

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

Examples

Setting font sizes

CSS

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

HTML

<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>

Result

Specifications

Specification	Status	Comment
CSS Fonts Module Level 4 The definition of 'font-size' in that specification.	Working Draft	Adds `xxx-large` keyword.
CSS Fonts Module Level 3 The definition of 'font-size' in that specification.	Candidate Recommendation	No change.
CSS Level 2 (Revision 1) The definition of 'font-size' in that specification.	Recommendation	No change.
CSS Level 1 The definition of 'font-size' in that specification.	Recommendation	Initial definition.

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`font-size`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 5.5	Opera Full support 7	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0
`rem` values	Chrome Full support 31	Edge Full support 12	Firefox Full support 31 Notes Full support 31 Notes Notes Before Firefox 57, animations using em units are not affected by changes to the `font-size` of the animated element's parent (bug 1254424). Notes Before Firefox 57, some language settings' inherited `font-size` is smaller than expected (bug 1391341).	IE Full support 11 Full support 11 No support 9 — 10	Opera Full support 28	Safari Full support 7	WebView Android Full support 4.1	Chrome Android Full support 42	Firefox Android Full support 31	Opera Android Full support 28	Safari iOS Full support 7	Samsung Internet Android Full support 4.0
`xxx-large` keyword	Chrome Full support 79	Edge Full support 79	Firefox Full support 70	IE No support No	Opera No support No	Safari No support No	WebView Android Full support 79	Chrome Android Full support 79	Firefox Android No support No	Opera Android No support No	Safari iOS No support No	Samsung Internet Android No support No

Legend

Full support: Full support
No support: No support
See implementation notes.: See implementation notes.