The karat or carat (symbol: K or kt) is a unit of purity for gold alloys.

Karat purity is measured as 24 times the purity by mass:

where

Therefore, 24-Karat gold is fine (99.9% Au w/w), 18-Karat gold is 18 parts gold 6 parts another metal (forming an alloy), 12-Karat gold is 12 parts gold (12 parts another metal), and so forth.

In England, the carat was divisible into four grains, and the grain was divisible into four quarts. For example, a gold alloy of Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{381}{384}

The Karat system is increasingly being complemented or superseded by the millesimal fineness system, in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.

"Karat" is a variant of "carat". First attested in English in the mid-15th century, the word "carat" came from Middle French carat, in turn from Italian carato, which came from Arabic qīrāṭ (قيراط), which came from Greek kerátion (κεράτιον) meaning carob seed (literally "small horn") (diminutive of κέρας - keras, "horn") and was a unit of weight though it was not likely used to measure gold in classical times.

Purity is the absence of impurity or contaminants in a substance. The term also applies to the absence of vice in human character.

Purity may also refer to: