This value allows you to perform a calculation in CSS.
In the example we have a div with 150px width and 5em padding. We want another div to have the same width, but without any padding. Its width should thus be 150px+10em.
calc() allows us to specify that.
width: calc(150px + 10em);
The test div should be as wide as the comparison div.
width: 150px; padding: 5px 5em;