font-size

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: revert;
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 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 keywords or numerical values for pixels or ems. Choose the appropriate method based on the needs of the particular web page.

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 on 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

Using an em value creates a dynamic or computed font size (historically the em unit was derived from the width of a capital "M" in a given typeface.). The numeric value acts as a multiplier of the font-size property of the element on which it is used. Consider this example:

p {
  font-size: 2em;
}

In this case, the font size of  elements will be double the computed font-size inherited by  elements. By extension, a font-size of 1em equals the computed font-size of the element on which it is used.

If a font-size has not been set on any of the 's ancestors, then 1em will equal the default browser font-size, which is usually 16px. So, by default 1em is equivalent to 16px, and 2em is equivalent to 32px. If you were to set a font-size of 20px on the <body> element say, then 1em on the  elements would instead be equivalent to 20px, and 2em would be equivalent to 40px.

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.

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 '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 16px).

Ex

Like the em unit, an element's font-size set using the ex unit is computed or dynamic. It behaves in exactly the same way, except that when setting the font-size property using ex units, the font-size equals the x-height of the first available font used on the page. The number value multiplies the element's inherited font-size and the font-size compounds relatively.

See the W3C Editor's Draft for a more detailed description of font-relative length units such as ex.

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
CSS Fonts Module Level 4 # font-size-prop

Browser compatibility

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