Gemstones

Guide : How to Determine Diamond Carat Weight by Measurements

Soualy Guide : How to Determine Diamond Carat Weight by Measurements

We all know that the best way to tell the carat weight of a diamond is to weigh it, but what should you do when you cannot do so? It is actually possible to infer the carat weight of a diamond by measuring its size. Keep in mind that an estimate made this way is by no means 100% precise, but it will at least give you an approximate idea of how much a diamond weighs.

Let’s take a look at how the size of a diamond converts to carat weight for some of the most common diamond cuts – the round cut, the princess cut, and the Asscher cut.

diamond-carat-weight-measurements

Conversion Table: Millimeters to Carat Weight (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.

Conversion Table: Millimeters to Carats (Princess-Cut Diamonds)

Below is a millimeter-to-carat conversion table for 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

Conversion Table: Millimeters to Carat Weight (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

Note: 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 diamond, as they will have different densities and hence, different weights.

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