calc() Function

The calc() function allows mathematical expressions with addition (+), subtraction (-), multiplication (*), division (/) to be used as component values. Percentages are solved relative to the dimensions of the containing parent block. It can be used when length values are accepted:

elem {
	width: calc(100% - 1em);
}

For more information, see: https://drafts.csswg.org/css-values-3/#calc-notation