calc() - CSS: Cascading Style Sheets 编辑

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.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

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.

• MDN Understanding WCAG, Guideline 1.4 explanations
• Understanding Success Criterion 1.4.4 | W3C Understanding WCAG 2.0

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 compatibility

BCD tables only load in the browser

See also

• Firefox 4: CSS3 calc() ✩ Mozilla Hacks – the Web developer blog