A carat is a unit of weight equal to 0.2 grams, so five carats weigh one gram. Carat weight tells you how heavy a diamond is, not how big it looks, although the two are closely related for well-cut stones. It is the most objective of the diamond 4Cs because it is measured on a scale instead of judged by eye.
Where the carat came from
Traders in the ancient and medieval world weighed gems against small seeds, and the carob seed is the usual explanation for the word. The Greek name for the carob, keration, passed through Arabic and Italian into the word "carat." Seeds vary in weight, though, and for centuries the carat differed from one trading city to the next. A carat in one market could weigh noticeably more or less than a carat in another.
The modern metric carat of exactly 200 milligrams was proposed in 1907 and adopted country by country over the following years. The United States adopted it in 1913. Since then, a carat has meant the same weight everywhere.
The gem carat should not be confused with the karat used for gold. Karat (spelled with a "c" in British English) measures gold purity in 24ths, so 18K gold is 18 parts gold out of 24. The yellow gold page covers that system.
Points and how weights are written
Each carat divides into 100 points. A 0.50-carat diamond is a "50-pointer," and a 0.25-carat stone is a "25-pointer." Very small diamonds, often used in pave and halo settings, are called melee, and their weights are usually given in points or as a combined total.
Grading labs weigh diamonds on electronic balances to the thousandth of a carat and report the weight to the hundredth, such as 1.01 ct. Retailers sometimes describe weight in fractions, such as "3/4 carat." In the United States, the Federal Trade Commission's Jewelry Guides expect sellers to state weight accurately and to make clear when a fractional figure is an approximation. A decimal figure on a grading report is more precise than a fraction on a sales tag.
Magic sizes and price jumps
Diamond prices do not rise smoothly with weight. They jump at certain round numbers that buyers ask for, which the trade calls magic sizes. The most common thresholds are:
- 0.50 carat
- 0.75 carat
- 1.00 carat
- 1.50 carat
- 2.00 carat
A 1.00-carat diamond usually costs noticeably more per carat than an otherwise identical 0.95-carat stone, even though the difference in size is close to invisible. Cutters know this, and they often plan a crystal to reach a threshold weight, sometimes at the cost of ideal proportions. That is one reason stones weighing exactly 1.00 or 1.01 carats are so common in the market.
Buyers can use this to their advantage. A stone just under a magic size, sometimes called "shy" of the weight, can look nearly the same as one at the threshold and cost less. The diamond pricing guide explains why price per carat climbs with size.
Why bigger costs more per carat
Large rough diamonds are much rarer than small ones. A mine produces far more crystals that yield half-carat stones than crystals that yield three-carat stones. Because of this, the price per carat increases as weight increases. A 2-carat diamond typically costs more than twice as much as a 1-carat diamond of the same color, clarity and cut.
Cutting also affects weight. A rough crystal loses a large share of its weight during sawing, bruting and polishing, and the cutter must balance the final weight against beauty. The how diamonds are cut page covers that trade-off.
Diameter versus weight
People look at a diamond from above, so face-up diameter matters more to the eye than weight. For a well-proportioned round brilliant cut, these diameters are typical:
| Carat weight | Approximate diameter |
|---|---|
| 0.25 ct | 4.1 mm |
| 0.50 ct | 5.1 mm |
| 0.75 ct | 5.8 mm |
| 1.00 ct | 6.4 mm |
| 1.50 ct | 7.4 mm |
| 2.00 ct | 8.1 mm |
| 3.00 ct | 9.3 mm |
The numbers show something buyers often miss: doubling the weight does not double the visible size. Weight grows with volume, so a 2-carat round is only about a quarter wider than a 1-carat round, though its face-up area is noticeably larger.
Fancy shapes carry weight differently. Elongated shapes such as marquise, oval and pear tend to look larger face-up than a round of the same weight, while compact, deep shapes such as cushions and Asschers can look smaller.
Spread and hidden weight
Spread describes how large a diamond looks for its weight. A stone with good spread puts its weight where you can see it. A stone with poor spread hides weight below the girdle or in the girdle itself.
Common places for hidden weight include:
- A deep pavilion, which adds weight to the bottom of the stone and can also make it look darker.
- A thick girdle, which adds a band of weight around the widest point that contributes nothing to brightness.
- A high crown, which adds height without adding diameter.
Two 1-carat diamonds can differ by several tenths of a millimeter in diameter for these reasons. On a grading report, the measurements line (minimum diameter, maximum diameter and depth in millimeters) tells you how the weight is distributed. The diamond certificates guide explains how to read it, and the optical properties page explains why a deep stone loses light.
Total carat weight
Jewelry with more than one diamond is often described by total carat weight, written as "ctw," "tcw" or "t.w." A pair of earrings marked 1 ctw holds two stones of about half a carat each. A ring with a center stone and many small side diamonds might also carry a total weight.
Total weight is useful for comparing the overall amount of diamond in a piece, but it says nothing about how that weight is split. A 1 ctw ring with a single 0.90-carat center stone and a 1 ctw ring with dozens of tiny melee are very different pieces and carry very different prices. Ask for the weight of the center stone separately.
Carat weight and the other Cs
Larger diamonds show color and inclusions more readily. A slight yellow tint that disappears in a small stone can show in a large one, and an inclusion that hides in a small stone is easier to find in a big one. Buyers stepping up in size often need to be a little stricter on diamond color and diamond clarity than they would be for a small stone.
Frequently asked questions
How many grams is a carat?
One carat equals 0.2 grams, or 200 milligrams. Five carats make one gram.
How big is a 1-carat diamond?
A well-cut 1-carat round brilliant measures about 6.4 to 6.5 mm across. Fancy shapes of the same weight have different dimensions.
Is carat the same as karat?
No. Carat measures the weight of gemstones, while karat measures the purity of gold in parts per 24.
What does ctw mean on jewelry?
It stands for carat total weight, the combined weight of all the diamonds in a piece.