Diamond Carat Weight

Diamond carat sizes on hand illustration Leon Mege

Carat, Not Karat, Not Carrot

Diamonds are weighed on a carat scale (not to be confused with Karat, a unit of gold purity). The weight unit’s name “carat” is derived from the carob seed – a measuring unit used in the Middle Ages. There are 100 points in one carat. Therefore, weight can also be understood as a ratio (i.e., points/ 100). For example, a fifty-point diamond or half a carat is 50/100’s of a carat.

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 (where “x” stands for “multiply” and 0.0061 is the Size/Weight conversion factor). However, the relationship between size and weight is not linear (i.e., a half-carat diamond is not twice the size of a twenty-five-point diamond, only twice as heavy).

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 plummeted 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 weight up to one-thousandth of a carat.

Keystones

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

Melee

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

Sidestones

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
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