How it works
Everything goes through pixels
The value is turned into pixels, then out into every other unit. Physical units use the resolution: at the CSS default of 96 pixels per inch, 16px is 12pt, 1 pica, about 4.23mm.
Em and rem use the base font size instead of the resolution, so changing the resolution leaves them alone.
px = in × ppi; pt = in × 72; pc = pt ÷ 12; ag = in × 14; mm = in × 25.4; cicero = 4.512 mm; em = rem = px ÷ base
Picking a unit or a row
Choosing a unit under the value reads the same number in the new unit. Picking a row in the results converts instead: the value becomes that row's figure in that row's unit.
The minus and plus buttons, and the arrow keys, step the way a typesetter would: 1 for pixels, points, agates and millimetres, half for picas and ciceros, a tenth of a centimetre, an eighth of an inch or em.
Rounding
Figures keep more decimals the smaller they are (six under 1, two over 1,000), so small values convert back without drifting. Anything under 0.001 shows as 0.
