What Is a Carat?

The short version

The carat (ct) is the standard unit for weighing gemstones. One carat is 200 milligrams. That is a fixed, legal definition, so 1 ct is 0.2 grams everywhere in the world, and 5 ct is exactly 1 gram.

Carats are divided into 100 points. Jewellers say "a 50 pointer" for a half carat stone, and small stones in pavé or halos are often described in points: 1 pt, 2 pt, 5 pt.

Where the carat came from

Gem traders have weighed stones in "carats" for centuries, but the unit was not always the same size from one market to another. The modern metric carat of exactly 200 mg was agreed internationally in the early twentieth century and then adopted country by country. Today any reputable scale, lab report or invoice uses the metric carat.

The word is often linked to the carob seed, once used as a counterweight on balance scales. That story is popular, and it makes a nice image, but it does not change anything about the modern unit, which is defined in milligrams.

Carat weight is not size

This is the single most useful idea on this site. A carat tells you how heavy a stone is. It does not tell you how big it looks. Three things decide the size of a stone of a given weight:

1. Density. Gemologists call it specific gravity, or SG. Denser materials pack more weight into less space. Sapphire is denser than diamond, so a 1 ct sapphire is smaller. Emerald is lighter, so a 1 ct emerald is bigger.
2. Shape. At the same weight, an oval or marquise spreads across more of the finger than a round. A princess cut carries weight in its corners and looks smaller face up.
3. Proportions. A deep stone hides weight underneath. A shallow stone spreads it across the top. Two 1 ct rounds can differ by a few tenths of a millimetre in diameter.

Our guide on why the same carat weight looks different works through each of these with numbers.

Carat, points and grams at a glance

• 1 carat = 100 points = 200 mg = 0.2 g
• 0.50 ct = 50 points = 100 mg = 0.1 g
• 0.01 ct = 1 point = 2 mg
• 1 gram = 5 carats

For grains, pennyweights and ounces, use the carat to gram converter.

Carat and karat are different things

Gold purity is measured in karats, out of 24. 18 karat gold is 18 parts gold in 24. In American English it is usually spelled with a k (18k), and in British English often with a c (18ct). The two words share a history but measure completely different things. A "1 ct, 18 ct gold ring" is a ring with a 1 carat stone in 18 karat gold.

Words to watch for in listings

• ct or cttw. "ct" alone usually means the weight of one stone. "cttw", "TW" or "TCW" means the total of all stones. See the total carat weight calculator.
• DEW. Diamond equivalent weight, used for moissanite and some simulants. It describes size, not real weight. See the DEW calculator.
• "Approx." or "est." The weight was estimated from measurements, usually because the stone is set. That is normal and honest, but it is not a weighed figure.
• Fractions. "3/4 ct" is a rounded label. Trade rules about how much rounding is allowed vary by country, so ask for the exact decimal weight.

How gems are weighed

Loose stones are weighed on a carat scale, a small electronic balance that reads in carats, usually to two or three decimal places. Scales should be checked regularly with calibration weights. A stone in a setting cannot be weighed on its own, so its weight is estimated from measurements, as described in our guide to estimating gem weight in a setting.

Grading reports from gem labs state the weight to two decimal places, along with the measurements. When you have a report, it beats any estimate, including ours.

Why weight drives price

Weight is the one property every stone shares, so the trade prices gems as a rate per carat multiplied by weight. The rate depends on quality, size and the market, and larger stones are rarer, so the rate often rises with weight. This site does not give prices, but the price per carat calculator does the arithmetic.