the ratio of pure gold in 24 carat gold is
Fineness Various ways of expressing fineness have been used and two remain in common use: millesimal fineness expressed in units of parts per 1,000[1] and karats used only for gold. Karats measure the parts per 24, so that 18 karat =  18⁄24 = 75% and 24 karat gold is considered 100% gold.[2]