What Is a Carat? Diamond Carat Weight and Size in mm

Short answer: A carat is the unit of weight for precious stones. One carat is 0.2 grams, or 200 milligrams. The more carats a diamond has, the bigger it is; but how large it looks on the finger depends not only on the carat but also on the shape and proportions of the cut.

What does carat mean?

A carat is a measure of weight, not size. A five-carat stone weighs exactly 1 gram. The name is believed to come from carob seeds, once used as weights on jewellers’ scales; today it is fixed at the same value everywhere in the world: 1 carat = 0.2 grams.

Jewellers often talk in “points”. One carat is 100 points, so a “50-point diamond” is half a carat and a “25-point diamond” is a quarter carat.

Are carat and karat the same thing?

No, although the two are often confused. Carat is the weight of the stone; karat is the purity of the gold.

So a “14 karat ring with a 1-carat diamond” has a stone weighing 0.2 grams and a band that is 58.5% pure gold.

How many mm is a 1-carat diamond?

For a well-cut round diamond, the relationship between carat and diameter is roughly as follows:

One thing stands out in the table: when the carat doubles, the diameter does not. A 2-carat stone is only about a quarter wider than a 1-carat stone, because the weight is spread not only across the face you see from above but also through the depth of the stone.

If you would rather see the difference than read numbers, in our ring designer you can compare stones from 0.50 to 3 carats on the same finger, at true scale.

Which cut looks bigger at the same carat?

Two stones of the same weight look different in size if their cuts differ. Elongated cuts spread their weight across the surface; deep cuts hide part of the weight underneath.

• Look bigger: Marquise, pear and oval. A 1-carat oval measures about 7.7 × 5.9 mm and also makes the finger look longer.
• Balanced: Round. About 6.4 mm at 1 carat.
• Look smaller: Princess, cushion and emerald cuts. More of their weight sits in the depth of the stone; a 1-carat princess measures about 5.5 mm on each side.

How does carat affect the price?

Price rises with carat, but not in a straight line. As the stone gets bigger, the price per carat goes up as well. That is why a single 2-carat stone is worth more than two 1-carat stones of the same quality.

There is a well-known way to make the most of a budget: weights just below the round numbers. The size difference between a 0.90-carat stone and a 1-carat stone, for example, is hard to see with the eye.

Frequently asked questions