Moissanite carat size chart: how big is 1, 2 or 3 carats in mm?
A 1 carat round moissanite is about 6.5 mm across, the same as a 1 carat diamond, because moissanite is usually sold by “diamond equivalent weight” (DEW): the carat weight a diamond of that size would have. Moissanite is lighter (specific gravity 3.22, against 3.52 for diamond), so the stone itself weighs less: a Charles & Colvard stockist lists a 6.5 mm round as 0.88 carat actual weight and 1.00 carat DEW. The chart below gives approximate round and oval sizes, worked out with the International Gem Society’s weight formulas; real stones vary with their cut.
Why moissanite carats are “diamond equivalent”
Carat measures weight, not size: GIA defines a metric carat as 200 milligrams. Moissanite is about 9% less dense than diamond (specific gravity 3.22 against 3.52), so a moissanite of the same diameter weighs less. Sellers quote the diamond equivalent so shoppers can compare sizes directly; a “2 carat” moissanite is an 8 mm round that looks like a 2 carat diamond.
How the chart was worked out
The millimetres are estimates from the International Gem Society’s formulas for well-proportioned stones: for a round, diameter × diameter × depth × specific gravity × 0.0018; for an oval, the same with 0.0020, using the average of length and width as the diameter. The chart uses diamond’s specific gravity (3.52), because DEW is a diamond weight, and a depth of about 60% (of the diameter for rounds, of the width for ovals). The round figures match the 6.5 mm = 1 carat DEW that moissanite stockists use.
How to use the chart
For studs, the per-stone size is what matters: “1 ct total” earrings are two stones of about 5.2 mm. For rings, measure an existing ring you like with a ruler across the stone, then match the millimetres, not the carats.
| Carat (DEW) | Round (mm) | Oval (mm) | Shop |
|---|---|---|---|
| 0.25 ct | 4.1 | 5 x 3.5 | Studs |
| 0.5 ct | 5.2 | 6 x 4.5 | Studs |
| 0.75 ct | 5.8 | 7 x 5 | Studs |
| 1 ct | 6.5 | 7.5 x 5.5 | Rings |
| 1.5 ct | 7.4 | 8.5 x 6.5 | Rings |
| 2 ct | 8.0 | 9.5 x 7 | Rings |
| 2.5 ct | 8.8 | 10 x 7.5 | Rings |
| 3 ct | 9.3 | 11 x 8 | Rings |
| 4 ct | 10.2 | 12 x 9 | Rings |
| 5 ct | 11.0 | 13 x 9.5 | Rings |
Frequently Asked Questions
How big is a 1 carat moissanite?
About 6.5 mm across for a round stone, the same diameter as a 1 carat diamond, because moissanite is sold by diamond equivalent weight.
Why does my moissanite weigh less than the listed carat?
Because the listing uses diamond equivalent weight. Moissanite is lighter than diamond, so a 6.5 mm round sold as 1 carat DEW weighs about 0.88 carat on a scale, according to a Charles & Colvard stockist.
What size moissanite looks best in a ring?
That is a matter of taste. As a guide, 1 to 2 carats DEW is about 6.5 to 8 mm round; hold a ruler against your finger to see the size before ordering.
Sources
- GIA 4Cs: Diamond carat weight (a metric carat is 200 milligrams; carat measures weight, not size)
- International Gem Society: How to estimate gem weight in a jewelry setting (round and oval formulas)
- GIA, Gems & Gemology: Synthetic Moissanite: A New Diamond Substitute (Winter 1997): specific gravity 3.22
- US FTC Jewelry Guides, 16 CFR 23.12: diamond specific gravity about 3.52, Cornell LII
- Debebians (Charles & Colvard stockist): Round Forever One moissanite, 6.5 mm = 0.88 ct actual, 1.00 ct DEW (2016)
Keep reading
Amazon links are affiliate links (tag thegemtray-20): buying through them costs you nothing extra and supports the site. As an Amazon Associate we earn from qualifying purchases.