Modular Scale sponsored by Typecast is a handy tool for converting type sizes from pixels to ems and beyond. Enter the size of your text in pixels and translate it using one of almost a dozen perfect ratios and the pixels will be translated into ems, ems@16 and %of 13. If you are using rems, this makes it a lot easier.
If you don’t understand any of this, you probably don’t need to. If you want an explanation, please don’t ask me. I have to look up the reasons every time I scale fonts. It’s complicated, arcane, and only of value if you code CSS.