Moissanite vs Diamond Size
Quick answer
A "1 carat" moissanite sold by diamond equivalent weight (DEW) is the same size as a 1 ct diamond, about 6.5 mm, but really weighs about 0.91 ct because it is less dense. A moissanite that truly weighs 1.00 ct is a little larger, about 6.6 mm.
Round brilliant, typical depth; density ratio 3.22 / 3.52. Data verified .
Same size, different weight
A diamond and a moissanite cut to exactly the same shape and size hold the same volume. Weight is volume multiplied by density, and moissanite (SG 3.22) is less dense than diamond (SG 3.52). So the moissanite always weighs less, about 91.5% of the diamond's weight.
What DEW means
DEW, diamond equivalent weight, is a size label. "1 ct DEW" means "the size of a 1 carat diamond". It is not a weight you would read on a scale. The convention exists because buyers think in diamond carats, and a moissanite labelled "0.91 ct" would sound smaller than the diamond it matches.
Two real weights for the same stone
For a 6.5 mm round moissanite, you may see two different real weights:
- By density: about 0.93 ct. This takes the weight of a 6.5 mm diamond at typical depth (1.02 ct) and multiplies by 3.22 / 3.52. It is what our calculators show.
- By seller convention: often 0.88 ct. Many sellers publish this lower figure. It is consistent with moissanite being cut a bit shallower than the ideal diamond the formula assumes. We have not verified it independently, so we present it as a published convention.
Neither figure is a lie. They are estimates built on different assumptions about proportions. If the exact real weight matters, for insurance or resale, ask for the weighed figure.
Side by side by millimetre
| Diameter | Diamond (= moissanite DEW) | Moissanite real weight |
|---|---|---|
| 3 mm | 0.10 ct | 0.090 ct |
| 3.5 mm | 0.16 ct | 0.15 ct |
| 4 mm | 0.24 ct | 0.22 ct |
| 4.5 mm | 0.34 ct | 0.31 ct |
| 5 mm | 0.47 ct | 0.43 ct |
| 5.5 mm | 0.62 ct | 0.57 ct |
| 6 mm | 0.80 ct | 0.74 ct |
| 6.5 mm | 1.02 ct | 0.93 ct |
| 7 mm | 1.28 ct | 1.17 ct |
| 7.5 mm | 1.57 ct | 1.44 ct |
| 8 mm | 1.91 ct | 1.74 ct |
| 8.5 mm | 2.29 ct | 2.09 ct |
| 9 mm | 2.71 ct | 2.48 ct |
| 9.5 mm | 3.19 ct | 2.92 ct |
| 10 mm | 3.72 ct | 3.40 ct |
What this means for buyers
- Compare sizes in millimetres. It is the one number that means the same thing for both stones.
- Read the weight label. "DEW" or "diamond equivalent" means size. A plain "ct" could be either. Ask the seller.
- Do not compare price per carat across the two. A price per DEW carat and a price per real diamond carat are measuring different things.
For more sizes see the full moissanite size chart and the diamond size chart. To compare moissanite with CZ, sapphire or any other gem, use the DEW calculator.
What it means for sellers
If you list moissanite, state the millimetre size, say clearly whether a weight is DEW or real, and avoid writing a DEW figure where buyers would expect real weight. Honest, specific listings reduce returns and questions, and they build trust with buyers who have read pages like this one.
Related questions
Is a 1 carat moissanite the same size as a 1 carat diamond?
A 1 ct DEW moissanite is, by definition. A moissanite that truly weighs 1.00 ct is about 6.6 mm, about 3.0% wider than a 1 ct diamond at 6.5 mm.
Why is moissanite lighter than diamond?
Its density is lower: specific gravity 3.22 against 3.52 for diamond. The same volume of moissanite weighs about 91.5% as much.
Which weight should I compare when shopping?
Compare millimetres. If you must compare weights, compare moissanite DEW with diamond carats, because both describe the same size.
Can I tell moissanite from diamond by weight?
A gemologist can, using a precise scale and measurements, because moissanite weighs less than a diamond of the same size. At home, weight alone is not a reliable test.
Do moissanite and diamond look the same size on the hand?
At the same millimetre size and cut, yes. Differences in brilliance and fire can make one look livelier, but not bigger.
Is lab-grown diamond the same as moissanite?
No. Lab-grown diamond is real diamond, with diamond density. Moissanite is silicon carbide, a different material. See our lab-grown vs natural diamond size page.
Sources and method
Weight estimates use the colored-stone shape factors published by the International Gem Society and the Dendritics standard formula, and the diamond trade constants and length to width ratio tables published in jewelry trade references (Esslinger, Omni, Octonus). Specific gravity values are cross-checked against the International Gem Society density tables, the USGS Natural Gemstones report and GemSelect. The 0.88 ct seller figure is reported as published and not independently verified.
Every number on this page is generated by the same formula engine as the calculators. Read the full method, constants and limits and the editorial policy. Data last verified 2026-11-04.
Estimates only. This is not an appraisal, grading report or valuation. A calibrated scale or a lab report is the only real weight.