Hong Life CalculatorsUnderstanding the diamond mm to carat calculator — concept and principle
This calculator estimates the carat weight of a round brilliant diamond from its diameter and depth, or the diameter a stone of a given carat weight should have. Buyers use it to sanity-check a listing, compare stones of different cuts, or estimate a mounted stone that cannot be weighed.
How it is calculated
Carat ≈ 0.0061 × depth% × d³, which is the trade formula carat ≈ d² × depth (mm) × 0.0061 with depth written as a fraction of the diameter d.
- In reverse: d = ∛(carat ÷ (0.0061 × depth%)).
- Total depth (mm) = d × depth%.
- A thick-girdle correction multiplies the weight by (1 + correction %).
- 1 carat = 200 mg exactly; 1 carat = 100 points.
The principle
Weight is volume times density, and for a fixed shape volume grows with the cube of size. A round brilliant has a standard outline, so its volume is a nearly constant fraction of diameter² × depth. The factor 0.0061 combines that shape fraction with the density of diamond and the conversion to carats. Because of the cube, a small change in diameter is a large change in weight: a stone 10% wider is about 33% heavier at the same proportions. Depth matters too: a deeper stone hides weight below the girdle without looking larger from the top.
Worked example
Default inputs: carat from diameter, 6.5 mm, depth 61%, no girdle correction.
- d³ = 6.5³ = 274.625.
- Carat = 0.0061 × 0.61 × 274.625 = 1.022, shown as 1.02 ct.
- Total depth = 6.5 × 0.61 = 3.965, shown as 3.96 mm.
- Weight = 1.022 × 200 = 204 mg, or 102 points.
| Average girdle diameter (mm) | Carat weight (ct) |
|---|---|
| 1.62 | 0.02 |
| 3.25 | 0.13 |
| 4.88 | 0.43 |
| 6.5 | 1.02 |
| 8.12 | 2 |
| 9.75 | 3.45 |
| 13 | 8.18 |
How to read the result
A well-cut 1 ct round is typically about 6.4–6.5 mm across. If a listed stone weighs much more than its diameter suggests, it may be cut deep or have a thick girdle; if it weighs less, it may be shallow. A stone that is a little under a "magic" size such as 1.00 ct can cost noticeably less while looking almost the same.
Common mistakes
- Using one diameter reading; measure several and average them.
- Applying the round-brilliant formula to ovals, pears or princess cuts.
- Forgetting the girdle correction for thick girdles.
- Treating the estimate as the stone's certified weight.
Frequently asked questions
How many mm is a 1 carat round diamond?
About 6.4–6.5 mm for typical proportions.
How many milligrams is a carat?
Exactly 200 mg. One carat is also 100 points.
Why do two 1 ct diamonds look different in size?
Cut proportions differ. A deep stone puts more weight below the girdle and looks smaller from above.
Sources: M. T. Cartwright, "Zeroing in on diamond weight calculations", Jewellery Business (1 July 2013) · Metric carat = 200 mg (adopted internationally; e.g. NIST Handbook 44 Appendix C)