It’s easy to determine the weight of a princess-cut diamond when you have a scale, but can you calculate the carats of such a stone without weighing it? Let’s see how you can estimate the weight of a princess-cut diamond by its measurements.
Princess Cut Shape and Important Measurements
Princess-cut diamonds have a rectangular shape, and most such stones are square (or approximately square), i.e., all their sides are equal.
The most important dimensions of a princess-cut diamond you should be familiar with are its length, width, and depth:
- The length and the width of a princess-cut stone are measured along the girdle, which is the widest part of the diamond.
- The depth of a princes-cut diamond is the vertical distance from its flat top (the table) to its bottom.
These are the measurements you need to know in order to estimate the weight of a princess-cut diamond. Let’s see how this is done.
Formula for Estimating the Weight of a Princess-Cut Diamond
The formula for calculating the carat weight of princess-cut diamonds is the following:
Weight (ct) = Length (mm) x Width (mm) x Depth (mm) x Coefficient
You already know how the length, width, and depth of a princess-cut stone are defined and how to measure them.
What you also need to know is how to calculate the value of the coefficient, which depends on the length-to-width ratio of the diamond.
To figure out the coefficient value, divide the length of the diamond by its width, and then find below the value that is closest to the result you got:
- Ratio closest to 1.25: Coefficient = 0.0080
- Ratio closest to 1.50: Coefficient = 0.0090
- Ratio closest to 2.00: Coefficient = 0.0100
- Ratio closest to 2.50: Coefficient = 0.0105
After you’ve determined which of the ratios above is closest to the length-to-width ratio of your diamond, use the corresponding coefficient in the formula to estimate the stone’s weight.
A worked example: Suppose you have a princess-cut diamond with a length of 5.5 mm, width of 5 mm, and depth of 4 mm.
Its length-to-width ratio will be equal to 1.1 (5.5/5), and the coefficient value will be equal to 0.008. Using the formula, this is how the carat weight of the stone will be calculated:
Weight = 5.5 x 5 x 4 x 0.008 = 0.88 carats
Most princess-cut diamonds have square dimensions, or ones that are very close to square, so in most cases, their length-to-width ratios will be close to 1, and the coefficient you will need to use will be equal to 0.008.
Don’t forget that this formula gives you only an approximation of a diamond’s carat weight, so be aware that you will need to factor in some margin of error when interpreting the result.
Millimeter-to-Carat Conversion Table for Princess-Cut Diamonds
If you want a quick way of estimating the weight of a princess-cut diamond, you can reference a millimeter-to-carat conversion table.
The one below contains the most common diamond carat weights and shows what side length corresponds to each carat value.
These values are for square princess-cut diamonds, so it is assumed that all their sides are equal in length:
3.5 mm – 0.25 ct
4.4 mm – 0.50 ct
5.0 mm – 0.75 ct
5.5 mm – 1.00 ct
6.0 mm – 1.25 ct
6.4 mm – 1.50 ct
7.0 mm – 2.00 ct
8.0 mm – 3.00 ct
The values in this table are calculated assuming a certain proportion of the depth of the stone to its length, but your diamond won’t necessarily have the same dimensions.
So, use these values only as a general guideline – they are not meant to replace a more precise calculation.
Additional resources: Diamond carat size chart and weight estimation formulas.
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