Introduction

Millimeters (mm) and carats (ct) are two different units of measurement used for diamonds. Millimeters indicate the physical dimensions or size of the diamond, while carats refer to the weight of the diamond.

To convert millimeters to carats, you would need to know the specific gravity (density) of the diamond, which is 3.52 grams per cubic centimeter (g/cm³), as well as the diamond’s dimensions (length, width, and depth in millimeters).

Here’s the general process for estimating the carat weight of a round brilliant cut diamond from its dimensions:

Measure the diameter (in millimeters) of the diamond across its widest part. For a round diamond, the diameter is equal to both the length and the width.

Measure the depth (in millimeters) of the diamond from the top (table) to the bottom (culet).

Calculate the average diameter by adding the smallest and largest diameters and dividing by 2.

Calculate the diamond’s volume using the formula for the volume of a cone: V = (1/3) × π × (average diameter/2)² × depth.
Multiply the volume (V) by the specific gravity (3.52 g/cm³) to find the weight in grams.

Convert the weight in grams to carats by multiplying by 5 (1 carat = 0.2 grams).
Keep in mind that this method is only an estimation and may not be completely accurate, especially for diamonds with irregular shapes or non-standard proportions. For the most accurate carat weight measurement, it’s best to use a calibrated diamond scale.