You have a diamond with a rectangular profile (radiant, princess, cushion, or emerald cut) and you want to find out the stone’s carat weight. The only problem is that you don’t have a scale to weigh your diamond with. The good news is that it is actually possible to estimate the weight of your stone if you know its measurements, even if the value is not going to be precise. Let’s see how this is done.
Diamond Millimeter-to-Carat Conversion Formula: Emerald, Radiant, and Princess Cuts
You can use a formula to estimate the carat weight of a diamond with an emerald, radiant, or princess cut.
All you need to know are the stone’s length, width, and depth (distance from its top to bottom), and you can input them in the following formula:
Carat Weight = Length x Width x Depth x Adjustment Coefficient*
*The value of the Adjustment Coefficient used depends on the length-to-width ratio of the measured stone.
To calculate the length-to-width ratio, simply divide the length of the stone by its width, and find the resulting ratio in the table below to determine the Adjustment Coefficient you will need to use:
L/W Ratio = 1.25 –> Adjustment Coefficient = 0.0080
L/W Ratio = 1.50 –> Adjustment Coefficient = 0.0090
L/W Ratio = 2.00 –> Adjustment Coefficient = 0.0100
L/W Ratio = 2.50 –> Adjustment Coefficient = 0.0105
(If the value of the Adjustment Coefficient you calculated is not exactly equal to one of the listed in the table, use one of the above values that is closest to your calculation.)
For example, if an emerald-cut diamond has a length of 6.25 mm, width equal to 5 mm, and depth of 3.5 mm, its length-to-width ratio will be 1.25 (6.25 mm divided by 5 mm), and the coefficient you need to use will be 0.0080.
Thus, the estimated carat weight of this diamond will be equal to 6.25 x 5 x 3.5 x 0.0080 = 0.875 carats.
Keep in mind that the results obtained through this method are not likely to be very precise, so factor in a margin of error of at least 10-15%.
Conversion Table: Millimeters to Carat Weight for Emerald-Cut Diamonds
Here is a conversion table containing carat estimates for some of the common emerald cut length-width combinations.
Measure the length and width of your stone, and find the length-width pair on the left that is closest to the measurement you’ve taken; on the right is the corresponding estimated carat weight:
4.0 x 3.0 mm – 0.20 ct
4.5 x 3.0 mm – 0.25 ct
5.5 x 4.0 mm – 0.50 ct
6.0 x 4.5 mm – 0.75 ct
6.5 x 5.0 mm – 1.00 ct
7.2 x 5.3 mm – 1.25 ct
7.5 x 5.5 mm – 1.50 ct
8.5 x 6.0 mm – 2.00 ct
9.0 x 7.0 mm – 2.50 ct
9.5 x 7.0 mm – 3.00 ct
11.0 x 9.0 mm – 4.00 ct
12.0 x 10.0 mm – 5.00 ct
Keep in mind that the actual carats of your stone are likely to differ, sometimes significantly.
Conversion Table: Millimeters to Carat Weight for Radiant-Cut Diamonds
Below is a conversion table showing some of the most common length-width combinations for radiant-cut diamonds and their carat equivalents:
3.5 x 3.0 mm – 0.25 ct
5.0 x 4.5 mm – 0.50 ct
5.5 x 5.0 mm – 0.75 ct
6.0 x 5.5 mm – 1.00 ct
6.6 x 5.7 mm – 1.25 ct
7.0 x 6.0 mm – 1.50 ct
7.5 x 7.0 mm – 2.00 ct
8.5 x 7.5 mm – 3.00 ct
Conversion Table: Millimeters to Carat Weight for Cushion-Cut Diamonds
Here you can some of the popular cushion cut measurements along with their corresponding carat weight values:
4.0 x 3.5 mm – 0.25 ct
5.0 x 4.5 mm – 0.50 ct
6.0 x 5.0 mm – 0.75 ct
6.5 x 5.5 mm – 1.00 ct
7.0 x 6.0 mm – 1.25 ct
7.5 x 6.5 mm – 1.50 ct
8.0 x 7.0 mm – 2.00 ct
9.0 x 8.0 mm – 3.00 ct
Note: Keep in mind that there are different variations of the cushion cut, and depending on which one your stone is cut into, its actual carat weight may differ significantly from the estimated value in the above table.
Where to Buy Diamonds and Diamond Jewelry?
For diamond jewelry, we highly recommend James Allen because it shows real photos and videos for each diamond so you can take a 360-degree look at any stone before buying it.
For colored diamonds, we recommend Leibish & Co., which specializes in fancy color diamonds (use code JewelryNotes200 at checkout to get a 5% discount).