A "carat" is the internationally recognised unit of the weight of a diamond or gemstone. One carat weights 0.2 grams (200 mg). See IGS (International Gem Society) website.
Note: a "Karat" (spelt with a K) is the measure of purity of gold. 24 karats is 100% pure gold, while a mix of gold and other metals such as copper or silver to make jewellery, give a lower purity such as 18K or 14 karats. This is expressed as 18K, 14K, etc.
How is a Carat measured?
Carat weight is different across different shaped stones. The cut can also affect the weight and a deeper stone will be heavier and will therefore have more weight that the same size stone which has a shallow cut. Carat weight does not therefore, always denote size of the stone.
The most accurate way is to weigh the stone to determine it's actual weight and therefore the number of carats.
When a stone is already in a ring or jewellery setting, it makes it difficult to measure it's depth and therefore, we can only take an estimate of the number of carats by measuring the width and length of the stone.
The International Gem Society provides the charts below to help estimate the carat weight of different shaped stones.