Setting width/height as percentage minus pixels

I’m trying to create some re-usable CSS classes for more consistency and less clutter on my site, and I’m stuck on trying to standardize one thing I use frequently.

I have a container <div> that I don’t want to set the height for (because it will vary depending on where on the site it is), and inside it is a header <div>, and then an unordered list of items, all with CSS applied to them.

It looks a lot like this:

I want the unordered list to take up the remaining room in the container <div>, knowing that the header <div> is 18px tall. I just don’t know how to specify the list’s height as “the result of 100% minus 18px“.

I’ve seen this question asked in a couple other contexts on SO, but I thought it would be worth asking again for my particular case. Does anyone have any advice in this situation?

You can use calc:

height: calc(100% - 18px);

Note that some old browsers don’t support the CSS3 calc() function, so implementing the vendor-specific versions of the function may be required:

/* Firefox */
height: -moz-calc(100% - 18px);
/* WebKit */
height: -webkit-calc(100% - 18px);
/* Opera */
height: -o-calc(100% - 18px);
/* Standard */
height: calc(100% - 18px);