How to Calculate Diamond Carat Weight by Size Measurements

What Is a Carat and How Much Does It Weigh?

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.

Diamond Carats and Points

Points, just like carats, are units of mass commonly used to measure the weight of diamonds and other gemstones. One point is equal to 1/100^th of a carat. So, it follows that one point equals 2 milligrams, or 0.002 grams.

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.

Check out this diamond search engine, where you can filter loose diamonds by carat, price, cut, color, and clarity.

To measure the weight of the diamond more precisely, it’s best if you use a sensitive scale that can measure milligrams. Then simply divide the weight of the stone in milligrams by 200. For example, if the scale shows 263 milligrams, this is equal to 1.315 carats (263/200). Dividing the number after the decimal point by 100 will give you the points of the diamond. You can see that the number after the decimal point is 315, and dividing it by 100 yields 31.5 points. From this example, the mass of a diamond that weighs 1.315 carats can be expressed as 1 carat and 31.5 points.

Conversely, to calculate the weight of a diamond in milligrams from carats, multiply the carat number of the diamond by 200 to obtain its weight in milligrams. So if the diamond weighs 1.62 carats, this results in 324 milligrams (1.62 carats x 200), or 0.324 grams.

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

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 value of the coefficient depends on the length-to-width ratio of the stone, and we’ll see what these values can be in a more detailed explanation further below.

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 need to weigh it on a scale, preferably one that shows milligrams.

Now, let’s take a look at how the size of a diamond converts to carat weight for some of the most common diamond cuts.

Conversion Table: Millimeters to Carat Weight for Round-Cut Diamonds

Here is a conversion table showing the estimated carat weight for the most commonly sold round diamond sizes.

To use the table, first measure the diameter of your round diamond, and then find the value to the left that is closest to the diameter you’ve measured in millimeters; the value to the right is the approximate carat weight corresponding to a diamond of the respective size:

• 4.1 mm – 0.25 ct
• 4.4 mm – 0.33 ct
• 5.1 mm – 0.50 ct
• 5.8 mm – 0.75 ct
• 6.4 mm – 1.00 ct
• 6.9 mm – 1.25 ct
• 7.4 mm – 1.50 ct
• 7.8 mm – 1.75 ct
• 8.1 mm – 2.00 ct
• 8.8 mm – 2.50 ct
• 9.3 mm – 3.00 ct
• 9.7 mm – 3.50 ct
• 10.4 mm – 4.00 ct

*(1 ct = 200 mg)

Estimating the Carat Weight of a Round Diamond by using a Formula

You can also calculate the approximate carats of a round-cut diamond with the following formula:

Carat Weight = Diameter x Diameter x Depth x 0.006

The diameter of a round stone is the length of an imaginary line that goes through the stone’s center (as seen when looking at the diamond from the top) and connects two points that are across from each other on the stone’s girdle (edge).

The depth of a round-cut diamond is measured as the distance from the stone’s top facet (the table) to the point at the diamond’s bottom (if there is a facet there, it is called “the culet”).

For example, if you have a round diamond with a diameter of 6 mm and depth of 3.5 mm, then the weight of the stone can be estimated in the following way:

6 mm x 6 mm x 3.5 mm x 0.006 = 0.76 ct

This result should be viewed simply as an approximation, as the actual carat weight of the stone may vary by at least 5-10% in either direction, i.e., the stone’s real weight can be anywhere from 0.70 ct to 0.80 ct.

Estimating the Weight of a Princess-Cut Diamond with a Formula

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.

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 that have a square outline, i.e., their length-to-width ratio is equal to 1.

Just measure the side of your square princess-cut stone, and find in the table below the carat value (on the right) that corresponds to the millimeter size (on the left) closest to your measurement:

• 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.

Estimating the Weight of Emerald and Radiant Cuts by Using a Formula

As with the princess cut, you can use a formula to estimate the carat weight of a diamond with an emerald or radiant cut. The process is pretty much the same as with the 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*

*As already mentioned, 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 Asscher-Cut Diamonds

The Asscher cut is a square variation of the emerald cut. To determine the approximate carat weight of an Asscher-cut diamond, measure its side, and find the closest millimeter measurement in the table below:

• 3.7 mm – 0.25 ct
• 4.4 mm – 0.50 ct
• 5.0 mm – 0.75 ct
• 5.5 mm – 1.00 ct
• 5.9 mm – 1.25 ct
• 6.4 mm – 1.50 ct
• 7.0 mm – 2.00 ct
• 8.1 mm – 3.00 ct

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.

Use all the above conversion values only as rough guidelines, and don’t forget that the weight of individual stones can deviate from the estimated weight because cut proportions inevitably vary among stones.

These tables are also not useful for determining the weight of stones other than diamonds, as they will have different densities and hence, different weights.