A Complete Guide to calc() in CSS

CSS has a special calc() function for doing basic math. In this guide, let’s cover just about everything there is to know about this very useful function.

Here’s an example:

.main-content {
  /* Subtract 80px from 100vh */
  height: calc(100vh - 80px);
}

In this guide, let’s cover just about everything there is to know about this very useful function.

calc() is for values

The only place you can use the calc() function is in values. See these examples where we’re setting the value for a number of different properties.

.el {
  font-size: calc(3vw + 2px);
  width:     calc(100% - 20px);
  height:    calc(100vh - 20px);
  padding:   calc(1vw + 5px);
}

It could be used for only part of a property too, for example:

.el {
  margin: 10px calc(2vw + 5px);
  border-radius: 15px calc(15px / 3) 4px 2px;
  transition: transform calc(1s - 120ms);
}

It can even be a part of another function that forms a part of a property! For example, here’s calc() used within the color stops of a gradient

.el {
  background: #1E88E5 linear-gradient(
    to bottom,
    #1E88E5,
    #1E88E5 calc(50% - 10px),
    #3949AB calc(50% + 10px),
    #3949AB
  );
}

calc() is for lengths and other numeric things

Notice all the examples above are essentially numbers-based. We’ll get to some of the caveats of how the numbers can be used (because sometimes you don’t need a unit), but this is for number math, not strings or anything like that.

.el {
  /* Nope! */
  counter-reset: calc("My " + "counter");
}
.el::before {
  /* Nope! */
  content: calc("Candyman " * 3);
}

There are many lengths of CSS though, and they can all be used with calc():

• px
• %
• em
• rem
• in
• mm
• cm
• pt
• pc
• ex
• ch
• vh
• vw
• vmin
• vmax

Unit-less numbers are acceptable, too. For example line-height: calc(1.2 * 1.2); as well as angles like transform: rotate(calc(10deg * 5));.

You can also not perform any calculation and it is still valid:

.el {
  /* Little weird but OK */
  width: calc(20px);
}

Nope on media queries

When calc() is used correctly (length units used as a value to a property), sadly, calc() won’t work when applied to media queries.

@media (max-width: 40rem) {
  /* Narrower or exactly 40rem */
}

/* Nope! */
@media (min-width: calc(40rem + 1px)) {
  /* Wider than 40rem */
}

It would be cool someday because you could do mutually exclusive media queries in a fairly logical way (like above).

Mixing units 🎉

This is perhaps the most valuable feature of calc()! Almost every example above has already done this, but just to put a point on it, here it is mixing different units:

/* Percentage units being mixed with pixel units */
width: calc(100% - 20px);

That’s saying: As wide as the element is, minus 20 pixels.

There is literally no way to pre-calculate that value in pixels alone in a fluid width situation. In other words, you can’t preprocess calc() with something like Sass as an attempt to complete a polyfill. Not that you need to, as the browser support is fine. But the point is that it has to be done in the browser (at “runtime”) when you mix units in this way, which is most of the value of calc().

Here’s some other examples of mixing units:

transform: rotate(calc(1turn + 45deg));

animation-delay: calc(1s + 15ms);

Those probably could be preprocessed as they mix units that aren’t relative to anything that is determined at runtime.

Comparison to preprocessor math

We just covered that you can’t preprocess the most useful things that calc() can do. But there is a smidge of overlap. For example, Sass has math built into it, so you can do things like:

$padding: 1rem;

.el[data-padding="extra"] {
  padding: $padding + 2rem; // processes to 3rem;
  margin-bottom: $padding * 2; // processes to 2rem; 
}

Even math with units is working there, adding same-unit values together or multiplying by unit-less numbers. But you can’t mix units and it has similar limitations to calc() (e.g. like multiplying and dividing must be with unit-less numbers).

Show the math

Even you aren’t using a feature that is uniquely possible only with calc(), it can be used to “show your work” inside CSS. For example, say you need to calculate exactly ¹⁄₇th the width of an element…

.el {
  /* This is easier to understand */
  width: calc(100% / 7);

  /* Than this is */
  width: 14.2857142857%;
}

That might pan out in some kind of self-created CSS API like:

[data-columns="7"] .col { width: calc(100% / 7); }
[data-columns="6"] .col { width: calc(100% / 6); }
[data-columns="5"] .col { width: calc(100% / 5); }
[data-columns="4"] .col { width: calc(100% / 4); }
[data-columns="3"] .col { width: calc(100% / 3); }
[data-columns="2"] .col { width: calc(100% / 2); }

The Math operators of calc()

You’ve got +, -, *, and /. But they differ in how you are required to use them.

Addition (+) and subtraction (-) require both numbers to be lengths

.el {
  /* Valid 👍 */
  margin: calc(10px + 10px);

  /* Invalid 👎 */
  margin: calc(10px + 5);
}

Invalid values invalidate the whole individual declaration.

Division (/) requires the second number to be unitless

.el {
  /* Valid 👍 */
  margin: calc(30px / 3);

  /* Invalid 👎 */
  margin: calc(30px / 10px);

  /* Invalid 👎 (can't divide by 0) */
  margin: calc(30px / 0);
}

Multiplication (*) requires one of the numbers to be unitless

.el {
  /* Valid 👍 */
  margin: calc(10px * 3);

  /* Valid 👍 */
  margin: calc(3 * 10px);

  /* Invalid 👎 */
  margin: calc(30px * 3px);
}

Whitespace matters

Well, it does for addition and subtraction.

.el {
  /* Valid 👍 */
  font-size: calc(3vw + 2px);

  /* Invalid 👎 */
  font-size: calc(3vw+2px);

  /* Valid 👍 */
  font-size: calc(3vw - 2px);

  /* Invalid 👎 */
  font-size: calc(3vw-2px);
}

Negative numbers are OK (e.g. calc(5vw - -5px)), but that’s an example of where the whitespace is not only required, but helpful too.

Tab Atkins tells me that the reason for the required spacing around + and - is actually because of parsing concerns. I can’t say I fully understand it, but for example, 2px-3px is parsed as the number “2” and the unit “px-3px” which isn’t doing anybody any good, and the + has other issues like getting “consumed by the number syntax.” I would have guessed the whitespace would have had to do with the -- syntax of custom properties, but nope!

Multiplication and division do not need the whitespace around the operators. But I’d think good general advice is to include the space for readability and muscle memory for the other operators.

Whitespace around the outsides doesn’t matter. You can even do line breaks if you’d like:

.el {
  /* Valid 👍 */
  width: calc(
    100%     /   3
  );
}

Careful about this, though: no spaces between calc() and the opening paren.

.el {
  /* Invalid 👎 */
  width: calc (100% / 3);
}

Nesting calc(calc());

You can but it’s never necessary. It’s the same as using an extra set of parentheses without the calc() part. For example:

.el {
  width: calc(
    calc(100% / 3)
    -
    calc(1rem * 2)
  );
}

You don’t need those inside calc() because the parens work alone:

.el {
  width: calc(
   (100% / 3)
    -
   (1rem * 2)
  );
}

And in this case, the “order of operations” helps us even without the parentheses. That is, division and multiplication happen first (before addition and subtraction), so the parentheses aren’t needed at all. It could be written like this:

.el {
  width: calc(100% / 3 - 1rem * 2);
}

But feel free to use the parens if you feel like it adds clarity. If the order of operations doesn’t work in your favor (e.g. you really need to do the addition or subtraction first), you’ll need parens.

.el {
  /* This */
  width: calc(100% + 2rem / 2);

  /* Is very different from this */
  width: calc((100% + 2rem) / 2);
}

CSS custom properties and calc() 🎉

Other than the amazing ability of calc() to mix units, the next most awesome thing about calc() is using it with custom properties. Custom properties can have values that you then use in a calculation:

html {
  --spacing: 10px;
}

.module {
  padding: calc(var(--spacing) * 2);
}

I’m sure you can imagine a CSS setup where a ton of configuration happens at the top by setting a bunch of CSS custom properties and then letting the rest of the CSS use them as needed.

Custom properties can also reference each other. Here’s an example where some math is used (note the lack of a calc() function at first) and then later applied. (It ultimately has to be inside of a calc().)

html {
  --spacing: 10px;
  --spacing-L: var(--spacing) * 2;
  --spacing-XL: var(--spacing) * 3;
}

.module[data-spacing="XL"] {
  padding: calc(var(--spacing-XL));
}

You may not like that, as you need to remember the calc() where you use the property then, but it’s possible and potentially interesting from a readability perspective.

Custom properties can come from the HTML, which is a pretty darn cool and useful thing sometimes. (See how Splitting.js adds indexes to words/characters as an example.)

<div style="--index: 1;"> ... </div>
<div style="--index: 2;"> ... </div>
<div style="--index: 3;"> ... </div>

div {
  /* Index value comes from the HTML (with a fallback) */
  animation-delay: calc(var(--index, 1) * 0.2s);
}

Adding units later

In case you’re in a situation where it’s easier to store numbers without units, or do math with unit-less numbers ahead of time, you can always wait until you apply the number to add the unit by multiplying by 1 and the unit.

html {
  --importantNumber: 2;
}

.el {
  /* Number stays 2, but it has a unit now */
  padding: calc(var(--importantNumber) * 1rem);
}

Messing with colors

Color format like RGB and HSL have numbers you can mess with using calc(). For example, setting some base HSL values and then altering them forming a system of your own creation (example):

html {
  --H: 100;
  --S: 100%;
  --L: 50%;
}

.el {
  background: hsl(
    calc(var(--H) + 20),
    calc(var(--S) - 10%),
    calc(var(--L) + 30%)
  )
}

You can’t combine calc() and attr()

The attr() function in CSS looks appealing, like you can pull attribute values out of HTML and use them. But…

<div data-color="red">...</div>

div {
  /* Nope */
  color: attr(data-color);
}

Unfortunately, there are no “types” in play here, so the only thing attr() is for are strings in conjunction with the content property. That means this works:

div::before {
  content: attr(data-color);
}

I mention this because it might be tempting to try to pull a number in that way to use in a calculation, like:

<div class="grid" data-columns="7" data-gap="2">...</div>

.grid {
  display: grid;

  /* Neither of these work */
  grid-template-columns: repeat(attr(data-columns), 1fr);
  grid-gap: calc(1rem * attr(data-gap));
}

Fortunately, it doesn’t matter much because custom properties in the HTML are just as useful or more!

<div class="grid" style="--columns: 7; --gap: 2rem;">...</div>

.grid {
  display: grid;

  /* Yep! */
  grid-template-columns: repeat(var(--columns), 1fr);
  grid-gap: calc(var(--gap));
}

Browser tooling

Browser DevTools will tend you show you the calc() as you authored it in the stylesheet.

If you need to figure out the computed value, there is a Computed tab (in all browser DevTools, at least the ones I know about) that will show it to you.

Browser support

If you really needed to support super far back (e.g. IE 8 or Firefox 3.6), the usual trick is to add another property or value before the one that uses calc():

.el {
  width: 92%; /* Fallback */
  width: calc(100% - 2rem);
}

There are quite a few known issues for calc() as well, but they are all for old browsers. Can I use… lists 13 of them. Here’s a handful:

• Firefox <59 does not support calc() on color functions. Example: color: hsl(calc(60 * 2), 100%, 50%).
• IE 9-11 will not render the box-shadow property when calc() is used for any of the values.
• Neither IE 9-11 nor Edge support width: calc() on table cells.

Use-case party

I asked some CSS developers when they last used calc() so we could have a nice taste here for how others use it in their day-to-day work.

I used it to create a full-bleed utility class: .full-bleed { width: 100vw; margin-left: calc(50% - 50vw); } I’d say calc() is in my top 3 CSS things.

I used it to make space for a sticky footer.

I used it to set some fluid type / dynamic typography… a calculated font-size based on minimums, maxiums, and a rate of change from viewport units. Not just the font-size, but line-height too.

If you’re using calc() as part of a fluid type situation that involves viewport units and such, make sure that you include a unit that uses rem or em so that the user still has some control over bumping that font up or down by zooming in or out as they need to.

One I really like is having a “content width” custom property and then using that to create the spacing that I need, like margins: .margin { width: calc( (100vw - var(--content-width)) / 2); }

I used it to create a cross-browser drop-cap component. Here’s a part of it:

.drop-cap { --drop-cap-lines: 3; font-size: calc(1em * var(--drop-cap-lines) * var(--body-line-height)); }

I used it to make some images overflow their container on an article page.

I used it to place a visualization correctly on the page by combining it with padding and vw/vh units.

I use it to overcome limitations in background-position, but expecially limitations in positioning color stops in gradients. Like “stop 0.75em short of the bottom”.

Other trickery

• A two-up grid that breaks into a single column with no media query
• An aspect-ratio-ish hero component
• Enforcing high-contrast colors
• Help with the coordinates of a percentage-based clip-path