Calculating Distances in CSS

I was originally planning this week to write a short introduction to Entity Framework, when I saw that I had received a seventh upvote on my answer to this Stack Overflow question.

What the OP wanted to do was simple: calculate a distance in CSS as a percentage value minus a set number of pixels. While this would seem to be a simple enough task, all the other answers provided a wide variety of tricks and hacks to get the job done. Before CSS 3 came along, these methods were indeed necessary. Now that we have CSS 3 however, there is a much, much simpler solution:

height: calc(XX% - XXpx);

The calc function in CSS 3 allows you to multiply, divide, add and subtract virtually any combination of valid CSS units. You can even nest calcs inside one another should the need arise. The only real drawback to calc is that at the current time it isn’t well supported enough to allow its adoption as a de-facto solution. Until that day comes, you should take advantage of the way in which CSS applies equivalent-priority style rules (last rule is applied) and use the following general syntax:

width: 90%; /* An alternative for non-compliant browsers */
width: -moz-calc(95% - 5em); /* Firefox 4+ */
width: -webkit-calc(95% - 5em); /* Chrome 19+ */
width: calc(95% - 5em); /*IE9+, Firefox 16+ */

I recommend that you look at the aforementioned Stack Overflow question for non-compliant browser alternatives.

The one other thing worth mentioning is that rules containing calculated distances will obviously take more time to apply than their static counterparts. In my own experience I have never had any performance problems when using calc, but code like this

width: calc(100% - 73%);

should evidently be avoided.

I have to say that I normally wouldn’t have considered this subject to be worthy of a blog post, but given the relatively high amount of attention that my very late-to-the-game S.O. answer has received, it appears that this CSS 3 feature isn’t as well-known as it should be. It is indispensable when creating full-screen page layouts, and I consider it a much needed addition to CSS.