PostCSS Calc lets you reduce calc() references whenever it's possible. When multiple units are mixed together in the same expression, the calc() statement is left as is, to fallback to the W3C calc() implementation.

npm install postcss-calc

// dependencies
var fs = require("fs")
var postcss = require("postcss")
var calc = require("postcss-calc")
// css to be processed
var css = fs.readFileSync("input.css", "utf8")
// process css
var output = postcss()
  .use(calc())
  .process(css)
  .css

Using this input.css:

h1 {
  font-size: calc(16px * 2);
  height: calc(100px - 2em);
  width: calc(2*var(--base-width));
  margin-bottom: calc(16px * 1.5);
}

you will get:

h1 {
  font-size: 32px;
  height: calc(100px - 2em);
  width: calc(2*var(--base-width));
  margin-bottom: 24px
}

Checkout tests for more examples.

Allow you to define the precision for decimal numbers.

var out = postcss()
  .use(calc({precision: 10}))
  .process(css)
  .css

Allow you to preserve calc() usage in output so browsers will handle decimal precision themselves.

var out = postcss()
  .use(calc({preserve: true}))
  .process(css)
  .css

Adds warnings when calc() are not reduced to a single value.

var out = postcss()
  .use(calc({warnWhenCannotResolve: true}))
  .process(css)
  .css

Allows calc() usage as part of media query declarations.

var out = postcss()
  .use(calc({mediaQueries: true}))
  .process(css)
  .css

Allows calc() usage as part of selectors.

var out = postcss()
  .use(calc({selectors: true}))
  .process(css)
  .css

Example:

div[data-size="calc(3*3)"] {
  width: 100px;
}

To replace the value of CSS custom properties at build time, try PostCSS Custom Properties.

Work on a branch, install dev-dependencies, respect coding style & run tests before submitting a bug fix or a feature.

git clone git@github.com:postcss/postcss-calc.git
git checkout -b patch-1
npm install
npm test