Education
MM to Carat: Conversion Charts for Every Shape
The tables you need to compare two rings honestly — including the moissanite adjustment.
Written by the Mapple Gems workshop, Surat · Updated 24 August 2026
Short answer
A 1 ct round diamond is about 6.5 mm across. Elongated shapes of the same weight look larger because more of the stone faces upward — a 1 ct oval is roughly 7.7 × 5.7 mm, a 1 ct marquise about 10 × 5 mm. For moissanite, use the millimetre column and ignore the carat one: moissanite is about 15% less dense, so a stone matching a 1 ct diamond’s size weighs nearer 0.88 ct and is sold as 1 ct DEW.
Carat is weight. Millimetres are what a person sees. Because different shapes distribute their weight differently, two stones of identical carat can differ substantially in apparent size — which is why these tables are the only fair way to compare listings.
Round brilliant
| Diameter | Diamond weight | Moissanite true weight |
|---|---|---|
| 4.0 mm | 0.25 ct | approx. 0.22 ct |
| 5.0 mm | 0.50 ct | approx. 0.44 ct |
| 5.8 mm | 0.75 ct | approx. 0.66 ct |
| 6.5 mm | 1.00 ct | approx. 0.88 ct |
| 7.4 mm | 1.50 ct | approx. 1.32 ct |
| 8.1 mm | 2.00 ct | approx. 1.76 ct |
| 8.8 mm | 2.50 ct | approx. 2.20 ct |
| 9.4 mm | 3.00 ct | approx. 2.64 ct |
Elongated shapes
Approximate dimensions at typical ratios. A longer ratio gives a larger visible face for the same weight, and a shorter one gives a smaller face.
| Shape | 0.50 ct | 1.00 ct | 2.00 ct |
|---|---|---|---|
| Oval (1.35:1) | 6.0 × 4.5 | 7.7 × 5.7 | 9.8 × 7.2 |
| Pear (1.55:1) | 6.5 × 4.3 | 8.3 × 5.4 | 10.5 × 6.8 |
| Marquise (2:1) | 8.0 × 4.0 | 10.0 × 5.0 | 12.8 × 6.4 |
| Emerald (1.4:1) | 6.0 × 4.3 | 7.3 × 5.2 | 9.2 × 6.6 |
| Radiant (1.3:1) | 5.5 × 4.2 | 6.9 × 5.3 | 8.7 × 6.7 |
| Elongated cushion (1.3:1) | 5.6 × 4.3 | 7.0 × 5.4 | 8.8 × 6.8 |
Square and symmetrical shapes
| Shape | 0.50 ct | 1.00 ct | 2.00 ct |
|---|---|---|---|
| Princess | 4.4 mm | 5.5 mm | 7.0 mm |
| Asscher | 4.4 mm | 5.5 mm | 7.0 mm |
| Cushion (square) | 4.6 mm | 5.7 mm | 7.2 mm |
| Heart | 5.0 mm | 6.5 mm | 8.2 mm |
Note how much smaller a 1 ct princess reads than a 1 ct round — 5.5 mm against 6.5 mm. Square shapes carry far more of their weight in depth.
Why these are approximations, and what makes them wrong. Every figure here assumes standard proportions. A stone cut deep hides weight below the girdle where nobody sees it, so it measures smaller than the table suggests for its carat. A stone cut shallow spreads wide and measures larger — but usually leaks light through the bottom and looks glassy. So a stone measuring noticeably above its expected size is not a bargain; it is a warning. This is precisely why we quote millimetres on every listing rather than leaving you to work backwards from a weight.
The moissanite adjustment, in one line
Moissanite has a specific gravity of about 3.21 against diamond’s 3.52 — roughly 15% less dense. So a moissanite cut to any dimension in these tables weighs about 15% less than the diamond figure beside it.
The trade handles this by quoting DEW, diamond equivalent weight: a “1 ct moissanite” means the size of a 1 ct diamond, which is 6.5 mm. Its actual weight is about 0.88 ct. Nothing is being hidden — it is a size statement expressed in weight language, because that is the language buyers know.
Practical upshot: when comparing moissanite listings, use millimetres only. Carat figures are DEW and may be inconsistently applied between sellers.
How to use these tables when shopping
- Find the millimetre size of each stone you are comparing. Ask if it is not stated.
- Check it against the table for that shape. A large discrepancy means unusual proportions — ask why.
- Compare across shapes on face dimensions, not carat. A 1 ct oval and a 1 ct princess are not comparable purchases.
- Check the size against your finger. A 6.5 mm stone reads very differently on a size 5 hand and a size 9.
- Hold a ruler to the screen. Almost everyone imagines these numbers larger than they are.
Keep reading
Frequently asked questions
How many mm is a 1 carat diamond?
A 1 ct round brilliant is about 6.5 mm in diameter at standard proportions. Other shapes differ: a 1 ct oval is roughly 7.7 × 5.7 mm, a 1 ct princess about 5.5 mm square, and a 1 ct marquise around 10 × 5 mm.
Why does a 1 carat princess look smaller than a 1 carat round?
Because square shapes carry more of their weight below the girdle, in depth you never see. A 1 ct princess measures about 5.5 mm across against a round’s 6.5 mm. Elongated shapes go the other way, spreading weight across a larger visible face.
Is a stone that measures larger than its carat weight a good deal?
Usually not. A stone measuring noticeably wider than the table predicts has been cut shallow, spreading its weight across a larger face — which typically leaks light through the bottom and gives a glassy, lifeless look. Slightly under the expected size is more often a sign of good proportions.
How do I convert moissanite carat to mm?
Use the diamond millimetre figure directly, because moissanite is sold by diamond equivalent weight. A stone described as 1 ct moissanite is 6.5 mm, the same as a 1 ct diamond, though its true weight is nearer 0.88 ct since silicon carbide is about 15% less dense. When comparing moissanite listings, compare millimetres rather than carats.
Every listing of ours quotes millimetres.
So you never have to work backwards from a weight to find out what you are actually buying.
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