css types 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.

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

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:

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.

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);
}

#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:

<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:

.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.

See also