BODY {font-size:62.5%}
This takes 16px down to 10px. From now on it’s easy to think in pixels but still set sizes in terms of ems: 1em is 10px, 0.8em is 8px, 1.6em is 16px, etc.
.element {
font-size: 20px;
width: 4em;
height: 4em;
}
Then that means that the width and height of the element (defined here as 4em x 4em) would compute to 80px x 80px (20px * 4 = 80px).