Pixels And REM
This calculator converts pixels to the CSS unit REM. The conversion is based on the default font-size of 16 pixel, but can be changed.
Inside a sinlge document, the length of a REM unit is everywhere the same, it can just differ between documents. EM on the other side can differ between every element, because it is relative to the elements own font-size (excpetion is the font-size itself, in it EM is relative to the parent). REM is the newer unit, older browsers don't support it.