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

The conversion may be simpler, but an em wouldn't mean what it is supposed to mean.

1em is supposed to be equal to the width if a capitalized "M" in a given font. If the width of the letter M is 1 pixel, your font is going to be unreadable.

http://en.wikipedia.org/wiki/Em_(typography)