When you want to draw attention to a piece of text on a page, few things are more effective than increasing the font size. And to do that, you’ll need to understand the CSS font-size property.
The size of your fonts draws the reader’s eye and establish a visual hierarchy. You’ll notice in this blog post that the title (<h1>) has the largest font size. Next, the heading elements (<h2>, <h3>, and <h4>) decrease in size until we reach the <p> elements beneath them.
Say you want to shrink or enlarge the default heading sizes, or you want to change the font size of other elements on the page. In that case, you can change the font size in CSS.
In this post, we’ll show you how to use the CSS font-size property, including:
- using an absolute-size unit
- using a relative-size unit
- using length value (like pixels, rems, or rems)
- using a percentage value
- using vw units to make font size responsive
How to Change Font Size in CSS
font-size is the CSS property that controls the size of text on a webpage. There are several values you can use to define the font-size property. The example below includes different values and units you can use in CSS. The one you choose will depend on the needs and goals of your site.
Let’s take a closer look at these values and discuss the pros and cons of each.
Absolute-size keywords are based on the default font size. Most commonly, the default font size is medium (which translates to 16 pixels or 1 em) though it can differ by browser and font family. Note that in HTML the default font size is 16 pixels.
The absolute-size keywords are:
Here’s how each one looks in a browser:
Absolute-size keywords make it easy to set text to a specified size and create a font hierarchy for your page.
However, they do not allow a user to change the text size in all browsers, which makes it a poor choice for accessibility. To be more inclusive of all users, try relative-size keywords instead.
Relative-size keywords set the font larger or smaller relative to the parent element's font size. Relative sizes are roughly based on the ratio of the absolute-size keywords described above.
So, if a parent element has a font-size of large, a child element with a font-size of smaller will look as if it’s font size were medium. Let’s look at the code of this hypothetical.
Relative-size keywords make it easy to set text size relative to surrounding elements. Their advantage over absolute-size keywords is that they allow users to change the text size in all browsers, which makes it a good choice for accessibility.
There are a few length values you can assign to the font-size property. Here, we’ll focus on the three most common: pixels, em units, and rem units.
Using pixels (px) as your length lets you set fontsize with precision, regardless of the browser a visitor is using. It specifies exactly the number of pixels in height that you want a browser to render your text.
However, the fixed nature of pixels is also a drawback. They’re not optimized for all devices — CSS-Tricks found that websites on the iPad mini render the same as websites on the iPad, for example — and they’re not an accessible length value.
Because users cannot change the font size in some browsers, there are more inclusive and responsive options you can use, like em and rem. We’ll discuss those next.
The em unit sets the font size relative to the font size of the parent element. So, giving text a font-size of 2em will make this text twice the size of its surrounding text.
Setting font size in em units is ideal for an inclusive design. Since ems are a relative unit, users can adjust the text size in all browsers.
The only drawback is that ems compound. So, say a <span> element with a font size of 2em contains another <span> element. The nested <span> element would be twice the size, or 4em. See the code below.
Fortunately, the rem unit solves this compounding problem.
Rems are a relative unit like ems, but they don’t compound. That’s because ems are font-relative units, meaning the font size is relative to the parent element's font size, while rems are root-based. This means that font size is relative to the size of the font used by the <html> element, also called the root element.
Say I set the font size of the root element to 12px so that any text in the document that isn’t modified by CSS will be 12 pixels. But, I also want to change the font size of a <div> element that’s similar to the one mentioned in the example above. Let’s look at the code for that below.
Notice how the nested <span> element is the same font size as the other <span> element.
For more guidance on when to use the px unit versus the rem unit, check out this helpful explainer video:
A percentage value sets the font size of an element relative to the parent element's font size.
Say a <div> element that’s set to 36px contains two <p> elements. The font-size of the first paragraph is set to 50% and the second paragraph is set to 200%. The first <p> element will be 18 pixels and the second will be 27 pixels. Here’s how that code looks in action:
The property values described above all have one thing in common: They are not responsive. If you’d like to set your font size to be responsive for all devices and displays, you can use the viewport width unit, shortened to vw.
The vw unit is another relative unit. However, it’s not relative to a parent element or root element, but to the width of the viewport. To be exact, 1 vw is equal to 1% of the viewport. Meaning, if the viewport is 100 pixels wide, 1vw equals 1 pixel.
Say I want the font size of my paragraph to be 10% of the width of the browser window. Here’s what the code looks like:
And here’s what the text looks like when we resize the viewport:
Max Font Size in CSS
When setting your font size in vm units, be careful that your text doesn’t get too big on large screens. Unfortunately, CSS doesn’t have “max font size” property, but you can prevent your font from getting too big by using media queries.
You just have to use the media query at a certain screen size breakpoint and force the font size back into a set pixel value. Say I want to force my font size back to 30px when the viewport exceeds 1000px. Here’s what the code looks like:
And here’s what the text looks like when we resize the viewport:
Controlling Font Size
Changing font size in CSS is complex when compared to the ease of changing font size in Google Docs or Microsoft Word — but it can be mastered with some practice in HTML and CSS.
Editor's note: This post was originally published in May 2020 and has been updated for comprehensiveness.