Caliper.
A live CSS unit converter. Type a value into any field below — every other unit recalculates instantly, using the calibration settings you set.
Calibration — root, context & viewport
rem is relative to the root font-size; em is relative to the parent element's font-size; % is relative to a parent box width; vw/vh/vmin/vmax are relative to the viewport. Adjust these to match your real layout.
Conversions use the CSS reference: 96px = 72pt = 1 physical inch.