Ems to Pixel Conversion - Why 62.5% and not 6.25%?

Question

I know that a lot of us are familiar with setting the font size on the body element in our CSS to 62.5%. This means that 1em will equal 10px and helps for keeping things pixel p

Answer 1

First of all, do not assume that 1 em will equal 10 pixels. An em unit is in direct correlation to the typography being used. If someone has a font size of 16 pixels, then 62.5% is indeed 10 pixels (16 * 0.625 = 10) but this will obviously change when someone has modified their default font size.

Secondly, this is the first I've ever heard of using 62.5% for the base body font-size. I always use a font-size of 76% as based on Sane CSS Typography by Owen Briggs.

Lastly, to answer your question, yes you could use a font-size of 6.25% and then use 12em instead of 1.2em, for example. However, I would highly discourage this methodology. In the world of typograhy, one em is intended to be the width of the capital letter 'M'. This method completely violates that common practice and will seriously confuse anyone that may maintain your CSS in the future.