Set the DPI, then type a value in any unit field. The tool converts everything through pixels to update pt, em, rem, mm and cm.
At 96 DPI, 12pt equals 16px using px = pt × DPI / 72.
Point-to-pixel conversion depends on resolution; changing DPI rescales every point and metric result.
Both use a 16px root by default, so 1em = 1rem = 16px unless your root font-size differs.