Carat is the unit used to weigh gemstones. One carat equals 0.2 grams and is divided into 100 points, so a 0.50 carat diamond can also be described as a "fifty-pointer". Carat measures weight, not size: two diamonds of the same weight can look different depending on how they are cut.
Carat and karat
The two words sound alike but measure different things. Carat is the weight of a gemstone. Karat, written K, is the purity of gold. Read Gold karats explained.
Approximate sizes of round brilliant diamonds
| Carat weight | Approximate diameter |
|---|---|
| 0.25 ct | 4.1 mm |
| 0.50 ct | 5.2 mm |
| 0.75 ct | 5.8 mm |
| 1.00 ct | 6.5 mm |
| 1.50 ct | 7.4 mm |
| 2.00 ct | 8.2 mm |
| 3.00 ct | 9.3 mm |
These figures assume well-proportioned round brilliants. A diamond cut deep hides weight below the girdle and can look smaller than its weight suggests. Moissanite is lighter than diamond, so compare it by millimetres; read What is moissanite?.
Total carat weight
For pieces with several stones, such as a halo ring or a tennis bracelet, the weight is often given as total carat weight (ctw or TCW): the combined weight of all the stones. A "1 ctw" ring may have a much smaller centre stone.
Weight and price
Larger diamonds are rarer, so price per carat usually rises with weight. Prices also tend to step up at popular weights such as 0.50, 1.00 and 1.50 carats. A stone just below one of these thresholds can look almost identical and cost noticeably less.
Choosing a weight
Decide on size together with cut and the rest of the 4Cs, and in proportion to the hand and the setting. A smaller, beautifully cut diamond often gives more pleasure than a larger, lifeless one.
Face-up size and spread
What you see when a diamond is set is its face-up size: its diameter or outline from above. Two stones of the same weight can differ noticeably in face-up size depending on their proportions. A stone cut too deep carries weight you cannot see; a stone cut too shallow looks large but can lose brilliance. Measurements on a report tell you more about visible size than weight alone.
Weight in pairs and sets
Earrings are usually described by the combined weight of the pair: "1 ct total" studs are normally two stones of about half a carat each. Bracelets and bands with many stones are described by total carat weight, and the quality of small stones is often given as a range.
Weight and the hand
The same stone looks larger on a slender finger than on a broad one, and a thin band makes a centre stone appear bigger. Consider proportion as well as weight.
Sources: Gemological Institute of America (gia.edu), the 4Cs of diamond quality and GIA grading scales.
Frequently asked questions
Is carat the same as karat?
No. Carat measures the weight of gemstones; karat (K) measures the purity of gold.
What does ctw or TCW mean?
Total carat weight: the combined weight of all the stones in a piece, not the weight of the largest stone.
How is a carat divided?
Into 100 points. A 0.75 carat diamond is a 75-point stone.
Does a 1 carat moissanite look like a 1 carat diamond?
Not exactly. Moissanite is lighter, so a moissanite of the same diameter as a 1 carat diamond weighs less. Compare by millimetres.
