Diamond carat is one of the main determinants of a stone’s value, but what exactly is the meaning of carats, and how are they measured? Let’s see how you can calculate the carat weight of a particular diamond or estimate its carats by looking at its measurements.
What Is a Carat?
Carat is a unit of weight, commonly abbreviated as “ct”. It is often used to express the weight of diamonds and other gemstones.
One carat is equal to 200 milligrams, or 0.2 grams.
An important fact to keep in mind is that carat weight is not the same as size or volume.
Different gemstones have different densities, and this is why a 1-carat diamond will not necessarily be as big as another gemstone that is also 1 carat in weight.
Calculating a Diamond’s Weight in Carats
To calculate the carats of a diamond, simply weigh the stone, and record its weight in grams.
Then, divide that number by 0.2 to obtain the carat weight of the diamond.
For example, if a diamond weighs 0.1 grams, then dividing this number by 0.2 yields 0.5, meaning that the stone weighs half a carat.
Estimating Diamond Carat Weight by Measurements
Converting grams and milligrams to carats is straightforward, but what if you don’t have a scale? Actually, if you know some key measurements of a diamond, you can estimate its carat weight.
For round diamonds, you can use the following formula to estimate their carat weight:
Weight in carats = Diameter in mm x Diameter in mm x Depth in mm x 0.006
The diameter of a round stone is measured as a straight line that goes through the stone’s center and connects two points on the diamond’s girdle (outer edge).
The depth (or total height) of a round diamond is simply the vertical distance from its top facet (the table) to the bottom end of the stone.
For example, if your round diamond has a girdle diameter of 4 mm and its depth is 2 mm, then the stone’s weight in carats could be estimated to be around 0.2 ct ( = 4 x 4 x 2 x 0.006).
You can use the same formula to estimate the carat weight of oval diamonds as well.
For square and rectangular cuts, such as the emerald cut, the Asscher cut, the princess cut, the radiant cut, you can use the following formula:
Weight in carats = Length x Width x Depth x Coefficient
The length and width should be measured along the outer edge of the stone (its girdle).
The depth is defined the same way as for round diamonds – the distance from the top flat facet to the bottom end (or bottom facet, if the lower end is faceted).
The value of the coefficient depends on the length-to-width ratio of the stone, and the possible values are as follows:
L/W Ratio 1.25: Coefficient = 0.0080
L/W Ratio 1.50: Coefficient = 0.0090
L/W Ratio 2.00: Coefficient = 0.0100
L/W Ratio 2.50: Coefficient = 0.0105
If the length-to-width ratio of the stone you’re measuring is not exactly equal to one of the values above, use the closest value.
For example, if you have an emerald-cut diamond that is 2.5 mm deep, 5 mm long and 3.5 mm wide, then its length-to-width ratio is 5/3.5 = 1.43.
In the table above, the L/W ratio closest to your calculated value is 1.50, so you can use 0.009 for the coefficient.
Now, let’s plug the values we have in the formula to calculate the carat weight:
Carat weight = 4 x 3.5 x 2.5 x 0.009 = 0.315 ct
Keep in mind that all these formulas will give you estimates that are only approximate.
If you want to get an exact value for the carat weight of a diamond, you should weigh it with a scale, preferably one that shows milligrams.