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calc()

The calc() CSS function lets you perform calculations when specifying CSS property values. It can be used anywhere a <length>, <frequency>, <angle>, <time>, <percentage>, <number>, or <integer> is allowed.

Syntax

/* property: calc(expression) */
width: calc(100% - 80px);

The calc() function takes a single expression as its parameter, with the expression's result used as the value. The expression can be any simple expression combining the following operators, using standard operator precedence rules:

+
Addition.
-
Subtraction.
*
Multiplication. At least one of the arguments must be a <number>.
/
Division. The right-hand side must be a <number>.

The operands in the expression may be any <length> syntax value. You can use different units for each value in your expression, if you wish. You may also use parentheses to establish computation order when needed.

Notes

  • The + and - operators must be surrounded by whitespace. For instance, calc(50% -8px) will be parsed as a percentage followed by a negative length — an invalid expression — while calc(50% - 8px) is a percentage followed by a subtraction operator and a length. Likewise, calc(8px + -50%) is treated as a length followed by an addition operator and a negative percentage.
  • The * and / operators do not require whitespace, but adding it for consistency is both allowed and recommended.
  • Division by zero results in an error being generated by the HTML parser.
  • Math expressions involving percentages for widths and heights on table columns, table column groups, table rows, table row groups, and table cells in both auto and fixed layout tables may be treated as if auto had been specified.
  • It is permitted to nest calc() functions, in which case the inner ones are treated as simple parentheses.

Formal syntax

calc( <calc-sum> )

where
<calc-sum> = <calc-product> [ [ '+' | '-' ] <calc-product> ]*

where
<calc-product> = <calc-value> [ '*' <calc-value> | '/' <number> ]*

where
<calc-value> = <number> | <dimension> | <percentage> | ( <calc-sum> )

Accessibility concerns

When calc() is used for controlling text size, be sure that one of the values includes a relative length unit, for example:

h1 {
  font-size: calc(1.5rem + 3vw);
}

This ensures that text size will scale if the page is zoomed.

Usage with integers

When calc() is used where an <integer> is expected, the value will be rounded to the nearest integer. For example:

.modal {
  z-index: calc(3 / 2);
}

This will give .modal a final z-index value of 2.

Note: The Chrome browser currently won’t accept some values returned by calc() when an integer is expected. This includes any division, even if it results in an integer. ie. z-index: calc(4 / 2); will not be accepted.

Examples

Positioning an object on screen with a margin

calc() makes it easy to position an object with a set margin. In this example, the CSS creates a banner that stretches across the window, with a 40-pixel gap between both sides of the banner and the edges of the window:

.banner {
  position: absolute;
  left: 40px;
  width: calc(100% - 80px);
  border: solid black 1px;
  box-shadow: 1px 2px;
  background-color: yellow;
  padding: 6px;
  text-align: center;
  box-sizing: border-box;
}
<div class="banner">This is a banner!</div>

Automatically sizing form fields to fit their container

Another use case for calc() is to help ensure that form fields fit in the available space, without extruding past the edge of their container, while maintaining an appropriate margin.

Let's look at some CSS:

input {
  padding: 2px;
  display: block;
  width: calc(100% - 1em);
}

#formbox {
  width: calc(100% / 6);
  border: 1px solid black;
  padding: 4px;
}

Here, the form itself is established to use 1/6 of the available window width. Then, to ensure that input fields retain an appropriate size, we use calc() again to establish that they should be the width of their container minus 1em. Then, the following HTML makes use of this CSS:

<form>
  <div id="formbox">
  <label>Type something:</label>
  <input type="text">
  </div>
</form>

Nested calc() with CSS Variables

You can also use calc() with CSS variables. Consider the following code:

.foo {
  --widthA: 100px;
  --widthB: calc(var(--widthA) / 2);
  --widthC: calc(var(--widthB) / 2);
  width: var(--widthC);
}

After all variables are expanded, widthC's value will be calc( calc( 100px / 2) / 2), then when it's assigned to .foo's width property, all inner calc()s (no matter how deeply nested) will be flattened to just parentheses, so the width property's value will be eventually calc( ( 100px / 2) / 2), i.e. 25px. In short: a calc() inside of a calc() is identical to just parentheses.

Specifications

Specification Status Comment
CSS Values and Units Module Level 3
The definition of 'calc()' in that specification.
Candidate Recommendation Initial definition

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
<calc()> 26
26
19
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
12 16
16
Before Firefox 57 calc(1*2*3) is not parsed successfully.
Firefox 57 increased the number of places calc() could substitute another value. See bug 1350857.
4 — 53
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
9 15 7
7
6
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
<color> value support No No 59 No No 6
Gradient color stops support 19 12 19 9 15 6
Nested calc() support 51 16 48 No 38 11
<number> value support 31 12 48 9 18 6
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
<calc()> ≤37
≤37
≤37
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
28 16
16
Before Firefox 57 calc(1*2*3) is not parsed successfully.
Firefox 57 increased the number of places calc() could substitute another value. See bug 1350857.
4 — 53
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
14 7
7
6
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
1.5
<color> value support No No 59 No 6 No
Gradient color stops support 37 28 19 15 6 1.5
Nested calc() support 51 51 48 41 11 5.0
<number> value support 4.4.3 31 48 18 6 2.0

See also

© 2005–2020 Mozilla and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/calc()