Carat purity

The carat (abbreviation ct or kt) is a measure of the purity of gold alloys. In the United States and Canada, the spelling karat is used, while the spelling carat is used to refer to the measure of mass for gemstones (see Carat (mass)).
Measure

As a measure of purity, one carat is \tfrac{1}{24} purity by mass:

X = 24\,\frac{M_g}{M_m}

where

X is the carat rating of the material,
Mg is the mass of pure gold or platinum in the material, and
Mm is the total mass of the material.

Therefore 24-carat gold is fine (99.9% Au w/w), 18-carat gold is 75% gold, 12-carat gold is 50% gold, and so forth.
24 carat "Chuk Kam" (99.0% min)
22 carat (91.6%)
21 carat (87.5%), 18 carat (75.0%) in most Egypt
18 carat (75.0%)
8-18 carat (33.3 - 75.0%)