calc()

/* calc(expression) */
calc(100% - 80px)

/* Expression with a CSS function */
calc(100px * sin(pi / 2))

/* Expression containing a variable */
calc(var(--hue) + 180)

/* Expression with color channels in relative colors */
lch(from aquamarine l c calc(h + 180))

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.

There's a few points to keep in mind about calc():

• 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-keyword page.
• 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 is specified.
• 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.
• When calc() is used where an <integer> is expected, the value will be rounded to the nearest integer. So, calc(1.4) will result in a value of 1. If the fractional part of the value is exactly 0.5, the value will be rounded up. For example, calc(1.5) will result in a value of 2, while calc(-1.5) will round to -1.

• 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.
• 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.
• Current implementations require that for the * and / operators, one of the operands has to be unitless. For /, the right operand must be unitless. For example font-size: calc(1.25rem / 1.25) is valid but font-size: calc(1.25rem / 125%) is invalid.

The calc() function can be used to manipulate color channels directly within the context of relative colors. This allows for dynamic adjustments of color channels in color models such as rgb(), hsl(), and lch().

The relative color syntax defines a number of color-channel keywords, each of which represents the value of the color channel as a <number> (see Channel values resolve to <number> values for more information). The calc() function can use these color-channel keywords to perform dynamic adjustments on the color channels, for example, calc(r + 10).

<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        

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

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>

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>

You can 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). When it's assigned to .foo's width property, all inner calc() functions (no matter how deeply nested) will be flattened to just parentheses. Therefore, the width property's value will eventually be calc((100px / 2) / 2), which equals 25px. In short, a calc() inside of a calc() is identical to using parentheses.

The calc() function can be used to adjust individual color channels in relative colors without the need for storing color channel values as variables.

In the example below, the first paragraph uses a <named-color>. In the paragraphs that follow, calc() is used with the rgb() and hsl() functions to adjust the values of each color channel relative to the original named color.

<p class="original">Original text color in rebeccapurple</p>
<p class="increase-hue">Hue increased by 80</p>
<p class="increase-lightness">Lightness increased by 20</p>
<p class="decrease-lightness">Lightness decreased by 10</p>

p {
  font-family: monospace;
  font-size: 16px;
}

.original {
  color: rebeccapurple;
}

.increase-hue {
  color: lch(from rebeccapurple l c calc(h + 80));
}

.increase-lightness {
  color: lch(from rebeccapurple calc(l + 20) c h);
}

.decrease-lightness {
  color: lch(from rebeccapurple calc(l - 10) c h);
}

For another example of using the calc() function to derive relative colors, see the Using math functions section on the Using relative colors page.

• <calc-keyword>
• CSS functions
• A Complete Guide to calc() in CSS (CSS-Tricks)