calc()

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

Try it

Syntax

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

The calc() function takes a single expression as its parameter, and the expression's result is used as the value for a CSS property. In this expression, the operands can be combined using the operators listed below. When the expression contains multiple operands,calc() uses the standard operator precedence rules:

+

Adds the specified operands.

-

Subtracts the second operand from the first operand.

*

Multiplies the specified operands.

/

Divides the left-side operand (dividend) by the right-side operand (divisor).

All operands, except those of type <number>, must be suffixed with an appropriate unit string, such as px, em, or %. You can use a different unit with each operand in your expression. You may also use parentheses to establish computation order when needed.

Notes

Serializing the arguments inside calc() follows the IEEE-754 standard for floating point math which means there's a few cases to be aware of regarding the infinity and NaN constants. For more details on how constants are serialized, see the calc-constant page.

In addition, the following notes apply:

  • The + and - operators must be surrounded by whitespace. For instance, calc(50% -8px) will be parsed as "a percentage followed by a negative length" — which is 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 recommended.
  • 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.
  • For lengths, you can't use 0 to mean 0px (or another length unit); instead, you must use the version with the unit: margin-top: calc(0px + 20px); is valid, while margin-top: calc(0 + 20px); is invalid.
  • The calc() function cannot directly substitute the numeric value for percentage types; for instance calc(100 / 4)% is invalid, while calc(100% / 4) is valid.

Formal syntax

<calc()> = 
calc( <calc-sum> )

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

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

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

<calc-keyword> =
e |
pi |
infinity |
-infinity |
NaN

Accessibility concerns

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

css
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:

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

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

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:

css
.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;
}
html
<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:

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

#form-box {
  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:

html
<form>
  <div id="form-box">
    <label for="misc">Type something:</label>
    <input type="text" id="misc" name="misc" />
  </div>
</form>

Nested calc() with CSS Variables

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

css
.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
CSS Values and Units Module Level 4
# calc-func

Browser compatibility

BCD tables only load in the browser

See also