If you are familiar with concept of Responsive Web Design, you should also understand fluid layouts. Sass, the CSS pre-processing language, has a few features to assist you with calculating values for fluid layouts.
This tutorial shows you how to calculate percentages with the percentage function in Sass and also how to write a custom function in Sass to convert pixel values to ems.
You should be using Sass already because it supports variables, functions and nesting. But if you’ve been holding out for some reason, now you have another reason to use it. This is the future of CSS.