`log()` CSS function

The log() CSS function is an exponential function that returns the logarithm of a number.

Logarithm is the inverse of exponentiation. It is the number that a fixed base has to be raised to in order to yield the number passed as the first parameter.

In CSS, when a single parameter is passed, the natural logarithm e, or approximately 2.7182818, is used, though the base can be set to any value with an optional second parameter.

Syntax

css

/* A <number> value */
width: calc(100px * log(7.389)); /* 200px */
width: calc(100px * log(8, 2)); /* 300px */
width: calc(100px * log(625, 5)); /* 400px */

Parameters

The log(value [, base]?) function accepts two comma-separated values as its parameters.

value: A calculation which resolves to a <number> greater than or equal to 0. Representing the value to be taken the log of.
base: Optional. A calculation which resolves to a <number> greater than or equal to 0. Representing the base of the logarithm. If not defined, the default logarithmic base e is used.

Return value

The logarithm of value, when base is defined.

The natural logarithm (base e) of value, when base is not defined.

Formal syntax

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

Examples

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Using the `log()` function on a logarithmic scale

This example illustrates how the log() function can be used to visualize data values by using a logarithmic scale. The width of each bar in this example is relative to its data value on a logarithmic scale with base 10. On each element, its value is assigned to a CSS custom property named --value, which is then used by the .bar class to calculate its width.

HTML

html

<div class="bar" style="--value: 50">50</div>
<div class="bar" style="--value: 100">100</div>
<div class="bar" style="--value: 500">500</div>
<div class="bar" style="--value: 10000">10,000</div>
<div class="bar" style="--value: 2000000">2,000,000</div>

CSS

css

.bar {
  width: calc(log(var(--value), 10) * 2em);
}

.bar {
  height: 2em;
  background-color: tomato;
  color: white;
  display: flex;
  align-items: center;
  justify-content: center;
  margin: 0.25em 0;
}

Result

Specifications

CSS Values and Units Module Level 4

log() CSS function