css percentrage minus px

1You can use calc:
2
3height: calc(100% - 18px);
4Note that some old browsers don't support the CSS3 calc() function, so implementing the vendor-specific versions of the function may be required:
5
6/* Firefox */
7height: -moz-calc(100% - 18px);
8/* WebKit */
9height: -webkit-calc(100% - 18px);
10/* Opera */
11height: -o-calc(100% - 18px);
12/* Standard */
13height: calc(100% - 18px);