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mm to carat conversion

Created by Aleksandra Zając, MD
Reviewed by Komal Rafay
Last updated: Jan 18, 2024

If you want to assess the price of a diamond and therefore need to perform an mm to carat conversion, the mm to carat calculator is the perfect tool! Our calculator makes every diamond mm to carat conversion easy, as it can count the weight of various shapes and sizes. It even considers the girdle thickness and weight adjustment factor in giving you the most precise results.

Diamond mm to carat conversion

Why do we need mm to ct conversion? A carat, or ct for short, is a unit of mass used exclusively for measuring gemstones and pearls. The carat got standardized:

  • 1 carat = 200 mg = 0.2 g
  • 1 carat = 0.007055 oz

The diamond's weight in carats is crucial when setting the price of a particular gem. Other factors include clarity, cut and shape, and others, but the weight is essential.

How to use mm to carat calculator?

To use the mm to ct conversion tool:

  1. Choose the diamond's shape from the drop-down list.

  2. Based on your choice, you'll see the rows with the proper dimensions. Fill in the necessary variables — length, width, and depth or just diameter and depth (in the case of a round diamond).

  3. Find out the ring girdle. You might add a few percent if the girdle is very thick (e.g., even over 6%). You don't have to change anything if it's a thin one.

  4. See if the diamond shape is a standard one. If not, you might want to add a correction factor. It might vary from 1% up to 18%. See the section below for the precise values. This doesn't apply to the round shape.

  5. You'll see the result of mm to carat conversion immediately!

  6. Remember, you can also make it work the other way around. You can give the tool carats and two known dimensions, and it will return the missing value. That way, you can find, for example, how big a round diamond of 2 mm depth should be so that it weighs one carat.

Shape correction & girdle thickness factors in mm to ct conversion

Diamonds are not always the perfect shape. If you want to consider that in your calculations, you have to know two additional factors:

  • Girdle thickness factor; and
  • Shape correction factor.

Girdle thickness refers to the outermost edge of a diamond. It can be expressed as a percentage of the average diameter. Use:

  • 0%-1% — very thin/thin girdles;
  • 1%-3% — medium;
  • 3%-6% — thick/very thick; and
  • >6% — extra thick.

Shape or weight correction factor is worth considering if your diamond is not regular. The common adjustment factors are:

Wide or fat wings, squarish or high shoulders

  • Pear: +1% to +5%; and
  • Marquise, Oval, Heart: +1% to +10%.

Large Culet

  • All Shapes: +1% to +2%.

Wide Corners

  • Emerald: -1% to -5%; and
  • Radiant: -1% to -5%.

Pavilion Bulge

  • Emerald, Radiant & Trillion: +1% to +18%.

More diamond tools

If you're looking for more on diamonds, try our diamond collection tools:


How do I convert a round diamond of diameter of 6 mm and depth of 2 mm to carats?

This diamond weighs 0.439 ct (carats).
To count it:

  1. Use the formula:

    Diameter² × Depth × 0.0061 × Girdle thickness

  2. You already know the Diameter (6 mm) and Depth (2 mm).

  3. Girdle thickness depends on the girdle, but for a standard, thin girdle, it can be just one.

  4. 0.0061 is an adjustment factor for the round-shaped diamond.

  5. Do the math:

    6² × 2 × 0.0061 × 1 = 0.439 [ct]

  6. You can always double-check with the mm to carat calculator.

How many mm are two carats?

Two carats can look like this:

  1. A round-shaped diamond of 12.5 × 2 mm dimensions;
  2. An asscher diamond with the dimensions 9 × 9 × 3 mm;
  3. A marquise-shaped diamond measuring 13 × 11 × 2.5 mm;
  4. A square emerald of 10 × 10 × 2.5 mm dimensions.
Aleksandra Zając, MD
Diamond shape
diamond illustration
Adjustments (optional)
Girdle thickness
Estimated diamond weight
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