calc() lets you mix CSS units. Ever wanted to give an element "the container's width minus 20px on each side"? Here you go:

.foo {
  width: calc(100% - (20px * 2));
}

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When using Sass, you need to interpolate Sass expressions:

$margin: 20px * 2

.foo
  width: calc(100% - #{$margin})

Supported Show archive.org snapshot by all modern browsers and IE9+.