Sorry to tell you, but the whole 62.5% bullshit is that. 100% grade A farm fresh priarie pies!

Why? Because the default font size isn't always 16px. In fact the entire reason for using EM or REM is the fact the user can change that size. Every PC I've owned since Windows 3.1 has used the 20px -- aka 8514 / SVGA / 125% / 120dpi / Windows XP/earlier "Large" / Windows Vista/later "Medium" / Pick a freaking name already -- setting as a minimum. My workstation is set to 24px. I'm chillin' on the sofa right now at my media center with the 32px ten foot interface since it's a 4k display.

Thus on this machine your 62.5% is delivering 20px. On my workstation it's 15px. On my laptop it's 12px.

If you are even thinking in pixels, and "need" that conversion, you have utterly, totally, and completely failed to grasp what EM and REM are even for and why you're not suppoed to declare anything in pixels!