The round(<rounding-strategy>, valueToRound, roundingInterval) function specifies an optional rounding strategy, a value (or mathematical expression) to be rounded and a rounding interval (or mathematical expression). The valueToRound is rounded according to the rounding strategy, to the nearest integer multiple of roundingInterval.

<rounding-strategy>

The rounding strategy. This may be one of the following values:

up

Round valueToRound up to the nearest integer multiple of roundingInterval (if the value is negative, it will become "more positive"). This is equivalent to the JavaScript Math.ceil() method.

down

Round valueToRound down to the nearest integer multiple of roundingInterval (if the value is negative, it will become "more negative"). This is equivalent to the JavaScript Math.floor() method.

nearest (default)

Round valueToRound to the nearest integer multiple of roundingInterval, which may be either above or below the value. If the valueToRound is half way between the rounding targets above and below (neither is "nearest"), it will be rounded up. Equivalent to JavaScript Math.round().

to-zero

Round valueToRound to the nearest integer multiple of roundingInterval closer to/towards zero (a positive number will decrease, while a negative value will become "less negative"). This is equivalent to the JavaScript Math.trunc() method.

valueToRound

The value to be rounded. This must be a <number>, <dimension>, or <percentage>, or a mathematical expression that resolves to one of those values.

roundingInterval

The rounding interval. This is a <number>, <dimension>, or <percentage>, or a mathematical expression that resolves to one of those values.

The value of valueToRound, rounded to the nearest lower or higher integer multiple of roundingInterval, depending on the rounding strategy.

• If roundingInterval is 0, the result is NaN.
• If valueToRound and roundingInterval are both infinite, the result is NaN.
• If valueToRound is infinite but roundingInterval is finite, the result is the same infinity.
• If valueToRound is finite but roundingInterval is infinite, the result depends on the rounding strategy and the sign of A:
  • up - If valueToRound is positive (not zero), return +∞. If valueToRound is 0⁺, return 0⁺. Otherwise, return 0⁻.
  • down - If valueToRound is negative (not zero), return −∞. If valueToRound is 0⁻, return 0⁻. Otherwise, return 0⁺.
  • nearest, to-zero - If valueToRound is positive or 0⁺, return 0⁺. Otherwise, return 0⁻.
• The argument calculations can resolve to <number>, <dimension>, or <percentage>, but must have the same type, or else the function is invalid; the result will have the same type as the arguments.
• If valueToRound is exactly equal to an integer multiple of roundingInterval, round() resolves to valueToRound exactly (preserving whether valueToRound is 0⁻ or 0⁺, if relevant). Otherwise, there are two integer multiples of roundingInterval that are potentially "closest" to valueToRound, lower roundingInterval which is closer to −∞ and upper roundingInterval which is closer to +∞.

<round()> = 
  round( <rounding-strategy>? , <calc-sum> , <calc-sum> )  

<rounding-strategy> = 
  nearest  |
  up       |
  down     |
  to-zero  

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

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

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

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

This example demonstrates how the round() function's rounding strategies work for positive values.

Of the five boxes below, the round() function is used to set the height of the last four. The value to be rounded is between 100 px and 125 px in each case, and the rounding value is 25px in all cases. The height of the boxes is therefore either rounded up to 125 px or down to 100 px.

HTML

The HTML defines 5 div elements that will be rendered as boxes by the CSS. The elements contain text indicating the rounding strategy, initial value, and expected final height of the box (in brackets).

<div class="box box-1">height: 100px</div>
<div class="box box-2">up 101px (125px)</div>
<div class="box box-3">down 122px (100px)</div>
<div class="box box-4">to-zero 120px (100px)</div>
<div class="box box-5">nearest 117px (125px)</div>

CSS

The CSS that is applied to all boxes is shown below. Note that we apply a custom CSS property named --rounding-interval, that we will use for the rounding interval.

div.box {
  width: 100px;
  height: 100px;
  background: lightblue;
  padding: 5px;
  --rounding-interval: 25px;
}

The first div from the left isn't targeted with specific CSS rules, so it will have a default height of 100px. The CSS for div two, three, and four is shown below, which round, up, down, and to-zero, respectively.

div.box-2 {
  height: round(up, 101px, var(--rounding-interval));
}
div.box-3 {
  height: round(down, 122px, var(--rounding-interval));
}
div.box-4 {
  height: round(to-zero, 120px, var(--rounding-interval));
}

Notice how above we indicate the rounding interval using var() and the custom CSS property --rounding-interval.

The last box is set without specifying a rounding strategy, and hence defaults to nearest. In this case, the nearest interval to 117 px is 125px, so it will round up. Just for contrast, here we specified hard coded values for both the rounding value and interval. While this is allowed, you wouldn't do this normally because there is no point rounding a number when you already know what the result must be.

div.box-5 {
  height: round(117px, 25px);
}

Result

If the browser supports the CSS round() function, you should see five columns with heights that are rounded as indicated by their contained text.

• mod()
• rem()