Lay the O-ring on your screen. Calibrate the screen once with a ruler or a bank card, then size the drawing until it matches the ring, and the closest standard sizes appear with their designations in each standard.
You can also browse every size of nine standards and Parker’s series, find a size by its number, and look up the O-rings the diving equipment makers fit. A screen can’t tell apart sizes that differ by a few tenths of a millimetre, so the neighbouring sizes are listed too..