Diamond Carat to mm Calculator Formula
Understand the math behind the diamond carat to mm calculator. Each variable explained with a worked example.
Formulas Used
Approximate Diameter
diameter_mm = 6.5 * pow(carats, 1 / 3)Weight in Milligrams
weight_mg = carats * 200Weight in Grams
weight_grams = carats * 0.2Variables
| Variable | Description | Default |
|---|---|---|
carats | Diamond Weight(carats) | 1 |
How It Works
Diamond Carat to Millimeter Conversion
Diamond size (diameter) relates to carat weight through the cube root, since volume increases with the cube of the diameter.
Formula (Round Brilliant)
Diameter (mm) = 6.5 x Carat^(1/3)
One carat equals 200 milligrams (0.2 grams). A 1-carat round diamond is approximately 6.5 mm in diameter. Doubling the carat weight only increases diameter by about 26% (cube root of 2).
Worked Example
A 1.0 carat round brilliant diamond.
- 01Diameter = 6.5 x 1.0^(1/3) = 6.5 x 1.0 = 6.5 mm
- 02Weight = 1.0 x 200 = 200 mg
- 03Weight = 1.0 x 0.2 = 0.2 grams
Frequently Asked Questions
Why does a 2-carat diamond not look twice as big?
Because diameter increases with the cube root of weight. A 2-carat round diamond is about 8.2 mm vs 6.5 mm for 1 carat, only 26% wider but covers about 59% more face-up area.
Does this work for all diamond shapes?
This formula is for round brilliant cuts. Fancy shapes (oval, pear, princess) have different proportions and may appear larger or smaller than a round of the same weight.
What is the most popular diamond size?
For engagement rings, 1.0-1.5 carats is most popular in the US. The most popular size worldwide is 0.5-0.7 carats.
Ready to run the numbers?
Open Diamond Carat to mm Calculator