Carat is a measure of weight, not size, so a stone’s dimensions in millimeters are what decide how large it looks on a finger. This tool renders a diamond at true millimeter scale on a photograph of your own hand, calibrated against something whose size is already known — your ring size, or a bank card in the shot — and tells you whether the stone will physically fit a given setting.

Diamond size guidesCarat to millimetersHow big is a 1 carat diamond?Ring size to finger widthSetting styles