Introduction

You’re going to see a lot numbers in CSS. Here are a few examples?

/* Integers */
1

/* Pixels */
14px

/* em */
1.5em

/* rem */
3rem

/* Percentage */
50%

/* Characters */
650ch

/* Viewport units */
100vw
80vh
50dvh

/* Container units */
100cqi
50cqb

While these all mean different things, they essentially do the same thing: define an element’s dimensions in CSS. We need units in CSS because they determine how to size elements on a page, whether it’s the height of a box, the width of an image, the font-size of a heading, the margin between two elements, how long an animation runs, etc. Without them, the browser would have no way of knowing how to apply numbers to an element.

So, what the heck is px? What’s up with this thing called rem? How are these different than other length units? The unit defines what type of number we’re dealing with, and each one does something different, giving us lots of ways to size things in CSS.

Types of numbers

You may think a number is just a number, and you’re not wrong. Numbers are numbers! But we can distinguish between a few different types of numbers, which is helpful context for discussing the different types of units we attach them to since “number” can mean, well, a number of different things.

• Integers (literally a unit-less number, e.g. 3)
• Numbers (same as an integer, only measured in decimals, e.g. 3.2)
• Dimensions (either a number or integer with a unit, e.g. 3.2rem)
• Ratios (the quotient between two divided numbers, e.g. 3/2)
• Percentages (e.g. 3%)

Got that? They’re all numbers but with nuances that make them ever-so-slightly different.

From here, we can think of numbers in CSS as falling into two specific types of units: absolute and relative. Let’s start things off our deep dive on CSS length units by breaking those down.

Absolute units

An absolute unit is like Bill Murray in the movie Groundhog Day: it’s always the same. In other words, whatever the number is, that’s exactly how it computes in the browser regardless of how other elements are sized.

The most common absolute value you’ll see is the pixel value. It’s sort of hard to define, but a pixel is the smallest building block of a graphical display, like a computer screen. And it’s based on the resolution of the screen. So, if you’re on a super high-resolution screen, a pixel will be smaller than it would be on a low-resolution screen, as the resolution can pack more pixels into a smaller amount of space for higher clarity. But look at the example below. All of the boxes are sized with pixels, so you can get a sense of how large 50px is compared to 250px.

Absolute values are nice in that they are predictable. That could, however, change in some situations, particularly when it comes to zooming. If, say, a user zooms into a page using browser settings, then anything defined with an absolute value is going to increase its absolute size accordingly. So, if the font-size of a paragraph is set to 20px, the paragraph is going to be larger as the user zooms closer into the page. And because zooming is often used to make content more readable, using absolute values that retain their sizing could be a good approach for making pages more accessible by allowing users to zoom things up to a spot that more comfortable to read.

But then again, see Josh Collinsworth’s click-baity, but fantastic, post titled “Why you should never use px to set font-size in CSS” for an exhaustive explanation of how pixels behave when used to set the font-size of an element. It’s a great read to better understand the behavior and limitations of pixel units.

And, hey: pixels are only one of many types of absolute lengths that are available in CSS. In fact, we can group them by the types of things they measure:

img {
  max-width: 500px;
}

/* Double the resolution and above */
@media (min-resolution >= 2dppx) {
  img {
    max-width: 100%;
  }
}

Relative units

A relative unit is extremely well-named because whatever value we use for a relative unit depends on the size of something else. Say we have an HTML element, a <div>, and we give it an absolute height value (not a relative one) of 200px.

<div class="box">
  I am 200 pixels tall
</div>

.box {
  height: 200px;
}

That height will never change. The .box element will be 200px tall no matter what. But let’s say we give the element a relative width (not an absolute one) of 50%.

<div class="box">
  I am 200 pixels tall and 50% wide
</div>

.box {
  height: 200px;
  width: 50%;
}

What happens to our box? It takes up 50%, or half, of the available space on the screen.

See that? Go ahead and open that demo in a new window and change the width of the screen. And notice, too, how the height never changes because it’s an absolute length unit in pixels. The width, meanwhile, is fluidly resized as “50% of the available space” changes with the width of the screen.

That’s what we mean when talking about computed values with relative numbers. A relative number acts sort of like a multiplier that calculates the value used to set a length based on what type of unit it is relative to. So, a value of 3rem is going to wind up becoming a different value when it is computed.

Percentages, like 50%, are only one kind of relative unit. We have many, many others. Once again, it’s helpful to break things out into separate groups to understand the differences just as we did earlier with absolute units.

Font relative units

The em and rem units we looked at earlier are prime examples of relative units that you will see all over the place. They’re incredibly handy, as we saw, because changing the font-size value of an element’s parent or the <html> element, respectively, causes the element’s own font-size value to update in accordance with the updated value.

In other words, we do not need to directly change an element’s font-size when updating the font-size of other elements — it’s relative and scales with the change.

Unit	Name	Relative to…
em	Element	The font-size value of the element’s parent container.
rem	Root element	The font-size value of the <html> element.
ch	Character	The width of the 0 character relative to the parent element’s font. The computed width may update when replacing one font with another, except for monospace fonts that are consistently sized.
rch	Root character	The same thing as a ch unit except it is relative to the font of the root element, i.e. <html>.
lh	Line height	The line-height value of the element’s parent container.
rlh	Root element line height	The line-height value of the <html> element.
cap	Capital letter	The height of a capital letter for a particular font relative to the parent element.
rcap	Root capital letter	The same measure as cap but relative to the root element, i.e. <html>.
ic	International character	The width of a CJK character, or foreign glyph, e.g. from a Chinese font, relative to an element’s parent container.
ric	Root international character	The same measure as ic but relative to the root element, i.e. <html>.
ex	X-height	The height of the letter x of a particular font, or an equivalent for fonts that do not contain an x character, relative to the parent element.
rex	Root x-height	The same measure as ex but relative to the root element, i.e. <html>.

Some of those terms will make more sense to typography nerds than others. The following diagram highlights the lines that correspond to relative font units.

So, at the expense of beating this concept into the ground, if width: 10ch computes to a certain number of pixels when using one font, it’s likely that the computed value will change if the font is swapped out with another one with either larger or smaller characters.

Viewport relative units

Unit	Name	Relative to…
vh / vw	Viewport Height / Viewport Width	The height and width of the viewport (i.e., visible part of the screen), respectively.
vmin / vmax	Viewport Minimum / Viewport Maximum	The lesser and greater of vh and vw, respectively.
lvh / lvw	Large Viewport Height / Large Viewport Width	The height and width of the viewport when the device’s virtual keyboard or browser UI is out of view, leaving a larger amount of available space.
lvb / lvi	Large Viewport Block / Large Viewport Inline	These are the logical equivalents of lvh and lvw, indicating the block and inline directions.
svh / svw	Small Viewport Height / Small Viewport Width	The height and width of the viewport when the device’s virtual keyboard or browser UI is in view, making the amount of available space smaller.
svb / svi	Small Viewport Block / Small Viewport Inline	These are the logical equivalents of svh and svw, indicating the block and inline directions.
dvh / dvw	Dynamic Viewport Height / Dynamic Viewport Width	The height and width of the viewport accounting for the available space changing if, say, the device’s virtual keyboard or browser UI is in view.
dvb / dvi	Dynamic Viewport Block / Dynamic Viewport Inline	These are the logical equivalents of dvh and dvw, indicating the block and inline directions.
dvmin / dvmax	Dynamic Viewport Minimum / Dynamic Viewport Maximum	The lesser and greater of dvh/dvb and dvw/dvi, respectively.

Ah, viewport units! When we say that something should be 100% wide, that means it takes up the full width of the contain it is in. That’s because a percentage unit is always relative to its nearest parent element. But a  viewport unit is always relative to the size of the viewport, or browser window. If an element has a viewport height of 100vh and a viewport width of 100vw, then it will be as tall and wide as the full browser window.

This can be a neat way to create something like a hero banner at the top of your website. For example, we can make a banner that is always one half (50vh) the height of the viewport making it prominent no matter how tall someone’s browser is. Change the CSS in the top-left corner of the following demo from height: 50vh to something else, like 75vh to see how the banner’s height responds.

There’s something else that’s very important to know when working with viewport units. You know how mobile phones, like iPhone or an Android device, have virtual keyboards where you type directly on the screen? That keyboard changes the size of the viewport. That means that whenever the keyboard opens, 100vh is no longer the full height of the screen but whatever space is leftover while the keyboard is open, and the layout could get super squished as a result.

That’s why we have the svh, lvh, and dvh units in addition to vh:

• svh is equal to the “small” viewport height, which occurs when the keyboard is open.
• lvh is equal to the “large” viewport height, which is when the keyboard is disabled and out of view.
• dvh is a happy medium between svh and lvh in that it is “dynamic” and updates its value accordingly when the keyboard is displayed or not.
• dvmin / dvmax is the greater ore lesser of dvh, respectively.

It’s a bit of a tightrope walk in some cases and a good reason why container queries and their units (which we’ll get to next) are becoming more popular. Check out Ahmed Shader’s article “New Viewport Units” for a comprehensive explanation about viewport units with detailed examples of the issues you may run into. You may also be interested in Sime Vidas’s “New CSS Viewport Units Do Not Solve The Classic Scrollbar Problem” for a better understanding of how viewport units compute values.

.parent-container {
  container-type: inline-size;
}

.child-element {
  background: rebeccapurple;
  width: 100%;

  @container parent (width > 30ch) {
    .child-element {
      background: dodgeblue;
      width: 50cqi;
    }
  }
}

What about unit-less numbers?

Oh yeah, there are times when you’re going to see numbers in CSS that don’t have any unit at all — just a single integer or number without anything appended to it.

aspect-ratio: 2 / 1; /* Ratio */
column-count: 3; /* Specifies a number of columns */
flex-grow: 1; /* Allows the element to stretch in a flex layout */
grid-column-start: 4; /* Places the element on a specific grid line */
order: 2; /* Sets the order of elemnents in a flex or grid layout */
scale: 2; /* The elementis scaled up or down by a factor */
z-index: 100; /* Element is placed in a numbered layer for stacking */
zoom: 0.2;  /* The element zooms in or out by a factor */

This isn’t a comprehensive list of all the CSS properties that accept unit-less numeric values, but it is a solid picture of when you would use them. You’ll see that in most cases a unit-less number is an explicit detail, such as a specific column to position an element, a specific layer in a stacking context, a boolean that enables or disables a feature, or the order of elements. But note that anytime we declare “zero” as a number, we can write it with or without a prepended unit, as zero always evaluates to zero no matter what unit we’re dealing with.

border: 0; /* No border */
box-shadow: 0 0 5px #333; /* No shadow offset */
margin: 0; /* No margin */
padding: 0; /* No padding */

We can create our own custom units!

In some cases, we may want to work with a numeric value, but CSS doesn’t exactly recognize it as one. In these cases, the number is recognized as a “string” value instead, regardless of whether or not it contains alphabetical characters. That’s where we can use @property to create what’s called a “custom property” that evaluates a numeric value in a certain way.

Here’s a good example. There was a time when it was virtually impossible to animate a gradient that changes colors over time because to do so would require (1) a color function that allows us to change a color value’s hue (which we have with hsl()) and (2) being able to interpolate that hue value around the color spectrum, between a range of 0deg and 360deg.

Sounds simple enough, right? Define a variable that starts at 0 and then cycles through 360 degrees at the end of an animation. But this doesn’t work:

/* 👎 */
.element {
  --hue: 0;
  
  animation: rainbow 10s linear infinite;
  background: linear-gradient(hsl(--hue 50% 50%);
}

@keyframes rainbow {
  from { --hue: 0; }
  to { --hue: 360deg; }
}

That’s because CSS reads the variable as a string instead of a number. We have to register that variable as a custom property so that CSS aptly reads it as a numeric value.

@property --hue {
  syntax: "<number>";
  initial-value: 0;
  inherits: true;
}

There we go! Now that we’ve given CSS a hint that the --hue syntax is that of a <number>, we can animate away!

/* 👍 */
@property --hue {
  syntax: "<number>";
  initial-value: 0;
  inherits: true;
}

.element {
  --hue: 0;
  
  animation: rainbow 10s linear infinite;
  background: linear-gradient(hsl(--hue 50% 50%);
}

@keyframes rainbow {
  from { --huw: 0; }
  to { --hue: 360deg; }
}

Find a deeper explanation of this same example in “Interpolating Numeric CSS Variables” by Geoff Graham.

When to use one unit over another

This is super tricky because CSS is extremely flexible and there are no definitive or hard-and-fast rules for when to use a specific type of CSS length unit over another. In some cases, you absolutely have to use a specific unit because that’s how a certain CSS feature is specced, like using angle units to set the direction of a linear gradient:

.element {
  background: linear-gradient(
    135deg, red, blue;
  )
}

The same goes for the values we use in certain color functions, like using percentages to set the saturation and lightness levels in the hsl() function:

.element {
  background: hsl(0 100% 10%);
}

Speaking of color functions, we define alpha transparency with either an integer or number:

.element {
  background: hsl(0 100% 10% / 0.5); /* or simply .5 */
}

All that being said, many cases are going to be some degree of “it depends” but there are some instances where it makes sense to use a specific unit in your work.

Generally set font sizes in rem and em units

This way, you can set things up in a way where changing the font-size value of the <html> or a parent element updates the font sizes of their descendants.

html {
  font-size: 18px; /* Inherited by all other elements */
}

.parent {
  font-size: 1rem; /* Updates when the `html` size changes */
}

.child {
  font-size: 1em; /* Updates when the parent size changes */
}

Declare “zero” without units if you’d like

It’s not a big deal or anything, but leaving off the units shortens the code a smidge, and anytime we’re able to write shorter code it’s an opportunity for faster page performance. The impact may be negligible, but we’re able to do it since 0 always computes to 0, no matter what unit we’re working with.

Use container units for responsive design, where possible

Container queries in general are so gosh-darn great for responsive layouts because they look at the size of the container and let us apply styles to its descendants when the container is a certain size.

.parent {
  container: my-container / inline-size; /* Looks at width */
}

.child {
  display: flex;
  flex-direction: column;
  max-width: 100vh; /* 100% of the viewport */
}

/* When the container is greater than 600px wide */
@container my-container (width >= 600px) {
  .child {
    flex-direction: row;
    max-width: 50%; /* 50% of the parent elenent */
  }
}

So, if we going to size the .child element — or any of its children — it’s worth specifying sizes in relation to the container’s size with container units than, say, the viewport’s size with viewport units.

.parent {
  container: my-container / inline-size; /* Looks at width */
}

.child {
  display: flex;
  flex-direction: column;
  max-width: 100cqi; /* 100% of the container */
}

/* When the container is greater than 600px wide */
@container my-container (width >= 600px) {
  .child {
    flex-direction: row;
    max-width: 50cqi; /* 50% of the container */
  }
}

Use percentages when you’re unsure of the context

Yes, use container units for responsive design, but that only does you good if you know you are in the context of a container. It’s possible, though, that you use the same component in different places, and one of those places might not be a registered container.

In that case, go with a percentage value because percentages are relative to whatever parent element you’re in, regardless of whether or not it’s a container. This way, you can declare an element’s size as 100% to take up the full space of whatever parent element contains it.

The only word of caution is to note that we’re only basing the size on the parent. Meanwhile, container units can style any descendant in the container, no matter where it is located.

Viewport units are great for laying out containers

You may be thinking that viewport units are a bad thing since we’ve been advising against them in so many cases, like font sizing, but they are still incredibly useful, particularly when it comes to establishing a full-page layout.

I say “full-page” layout because container queries are the gold standard for sizing elements according to the space they have in their container. But if we’re working with a full page of containers, this is where viewport units can be used to establish a responsive layout at a higher level.

If the elements of individual containers look at their container for sizing information, then the sizing and placement of individual containers themselves probably ought to look at the viewport since it directly influences the amount of space on the page.

Examples

20px * 1.5 = 30px

/* This is the browser's default font size */
html {
  font-size: 16px;
}

.box {
  font-size: 1.5rem; /* 16px * 1.5 = 24px */
}

html {
  font-size: 18px;
}

.box {
  font-size: 1.5rem; /* 18px * 1.5 = 27px */
}

<p>The big brown dog lazily jumped over the fence.</p>

p {
  width: 10ch;
}

.parent {
  line-height: 1.5;
}

.child {
  height: 3lh; /* 3 * 1.5 = 4.5 */
}

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