When you work with responsive designs, and not only then, you need to convert various pixel sizes to em. A very useful tool is PXtoEM.com that converts px to em, or em to px, based on a base pixel size.
The website has three sections:
Has tables with conversion based on 16px body size and your custom body size, and a calculator for custom conversions.
2. Grab CSS
Generates normalize.css customazied for you body size that you selected on previous section.
This section tells you what px, em, pt and % is, regarding typography.