Diamond Carat Weight

Carat, Not Karat, Not Carrot

Diamonds are weighed on a carat scale (not to be confused with Karat, a unit of gold purity). The name “carat” is derived from the carob seed – a measuring unit used in the Middle Ages.

There are 100 points in one carat. For example, a half-carat diamond weighs 50/100’s of a carat or fifty points. The relationship between a diamond’s size and weight is not linear. A two-carat diamond is not twice the size of a one-carat diamond, only twice as heavy.

In the absence of the other three Cs – cut, clarity, and color, the increase in carat size is the most objective and definitive component of the 4Cs that makes the diamond rarer and more valuable.

Diamond’s Size in Millimeters

The size alone is not enough to determine a diamond’s value. We have to consider its cut, proportions, and polish. Even a large diamond has its value plummet when it is poorly cut. When measuring small stones (melee or stones less than 0.05 carats in weight), we stop speaking of diamond weight and speak in terms of the diameter of a stone when referring to its size.

A single tiny stone is impossible to weigh. Jewelers weigh small diamonds in parcels to get an accurate reading. Center stones are usually weighed up to 100th of a carat. Recently, gem labs started issuing certificates that state a diamond weighs up to one-thousandth of a carat.

A round diamond’s diameter measured in millimeters can be used to estimate its weight in carats by using the following formula:

[Average diameter in mm] x [Depth in mm] x 0.0061

Keystones

Sidestones

Melee

Diameter	Carat
6.5 mm	1.00 ct
6.8 mm	1.25 ct
7.3 mm	1.50 ct
7.75 mm	1.75 ct
8.00 mm	2.00 ct
8.40 mm	2.25 ct
8.70 mm	2.50 ct
9.00 mm	2.75 ct
9.10 mm	3.00 ct
9.70 mm	3.50 ct
10.1 mm	4.00 ct
11.1 mm	5.00 ct

Diameter	Carat
3.10 mm	0.12 ct
3.20 mm	0.13 ct
3.30 mm	0.14 ct
3.40 mm	0.15 ct
3.50 mm	0.16 ct
3.60 mm	0.17 ct
3.70 mm	0.18 ct
3.80 mm	0.20 ct
3.90 mm	0.22 ct
4.00 mm	0.25 ct
4.10 mm	0.27 ct
4.20 mm	0.28 ct
4.30 mm	0.30 ct
4.40 mm	0.31 ct
4.50 mm	0.34 ct
4.60 mm	0.40 ct
4.80 mm	0.44 ct
5.00 mm	0.49 ct
5.30 mm	0.55 ct
5.50 mm	0.63 ct
5.70 mm	0.70 ct
5.80 mm	0.75 ct
6.00 mm	0.82 ct

Diameter	Carat
0.80 mm	0.0025 ct
1.00 mm	0.0050 ct
1.10 mm	0.0067 ct
1.20 mm	0.0090 ct
1.25 mm	0.010 ct
1.30 mm	0.012 ct
1.40 mm	0.013 ct
1.50 mm	0.014 ct
1.60 mm	0.016 ct
1.75 mm	0.019 ct
1.80 mm	0.02 ct
1.90 mm	0.025 ct
2.00 mm	0.03 ct
2.10 mm	0.035 ct
2.20 mm	0.04 ct
2.30 mm	0.045 ct
2.40 mm	0.05 ct
2.50 mm	0.06 ct
2.60 mm	0.07 ct
2.70 mm	0.08 ct
2.80 mm	0.09 ct
2.90 mm	0.10 ct
3.00 mm	0.11 ct