Moissanite carat size chart: carat to mm for every shape

Moissanite is sold by diamond-equivalent weight (DEW): a "1 ct" moissanite is cut to the face size of a 1 ct diamond, 6.5 mm across for a round, even though it weighs less. So read sizes in millimeters. Round: 0.5 ct is 5.1 mm, 1 ct is 6.5 mm, 2 ct is 8.0 mm and 3 ct is 9.3 mm.

• 1 ct DEW round moissanite = 6.5 mm, same as diamond.
• Moissanite is about 10% lighter than diamond at the same size.
• Density: moissanite 3.21 to 3.22, diamond 3.52.
• 2 ct shows only about 1.5 times the face of 1 ct.
• These tables match the sizes in our ring pickers.

Why moissanite is sold by diamond-equivalent weight

A carat is a weight, 0.2 grams. Diamond and moissanite have different densities (how heavy a material is for its size): diamond is 3.52 grams per cubic centimeter, moissanite 3.21 to 3.22. So a moissanite cut to exactly the same size as a diamond weighs less: about 91% as much by density alone. Put simply, moissanite weighs about 10% less than a diamond of the same size.

If sellers listed moissanite by its true weight, a stone that looks exactly like a 1 ct diamond would be sold as a 0.9 ct, which confuses everyone. So the trade uses DEW: the carat of the diamond the moissanite matches in size. When you see "1 ct DEW," think "looks like a 1 ct diamond, 6.5 mm."

Some size charts online say moissanite is 15 to 18% lighter. Density alone does not explain that: 3.21 divided by 3.52 is 0.91, a gap of about 9%. Moissanite is often cut a little shallower than diamond, so a real stone can weigh somewhat less than the density math says. The face size is the same either way.

Round moissanite: carat to mm

Round is the most common shape and the easiest to compare. The last column shows how much face you see compared with a 1 ct, which is what your eye notices.

Look at the last column. Doubling the carat from 1 to 2 gives you about 1.5 times the face, not twice. That is because carat is volume (three dimensions) and you see only the top (two). Going from 1 ct to 1.5 ct adds 0.9 mm across and about 30% more face.

Charts from different sellers differ by a few tenths of a millimeter, because cutters vary proportions. A 1 ct round diamond, for example, is listed anywhere from about 6.3 to 6.5 mm. Always check the mm of the exact stone.

Oval, pear, marquise, emerald, radiant and square shapes

Fancy shapes are listed as length x width. Long shapes such as marquise and pear spread the same weight over a longer outline, so they look bigger per carat than a round. A 1 ct marquise is 10.0 mm long; a 1 ct round is 6.5 mm across.

Square shapes are listed by side length. Cushion, princess and asscher use the same sizes in our pickers:

Heart shape is not in these charts.

How big each size looks on your finger

The size of your finger changes how big a stone looks. A ring's inner diameter in mm is 11.63 + 0.8128 x your US ring size, and that is roughly how wide your finger is where the ring sits. Divide the stone's width by that number and you get the share of your finger the stone covers. This is a geometric estimate: fingers are not perfect circles and knuckles vary.

Some plain rules from the table:

• Under about a third of finger width (0.5 ct on most fingers) reads as dainty and minimal.
• About 35 to 47% (1 to 1.5 ct on sizes 5 to 8) reads as a classic engagement ring.
• Half the finger or more (3 ct on sizes 4 to 8, or 2 ct on a size 4 or 5) reads as a statement stone.

The same carat looks bigger on a small finger: a 1 ct covers 44% of a size 4 finger but 33% of a size 10. For long shapes, the width runs across the finger and the length runs along it, so a 1 ct oval (5.7 mm wide) covers less width than a 1 ct round but looks longer. Get your size right first with the ring size guide: rings are made to order and cannot be resized later.

Stud earring and pendant sizes

For studs, check whether the listing gives the carat per earring or for the pair. A pair listed as "1 ct tw" (total weight) is two 0.5 ct stones, 5.1 mm each. Our studs run from 0.25 ct to 2 ct and our pendants from 0.5 ct to 3 ct.

A 0.25 ct round is about 4 mm across on standard diamond charts (it is below our ring picker range, so it is not in the table above). Small studs of 0.25 to 0.5 ct work for everyday and second piercings. From 1 ct up, studs are easy to see across a room. Pendants hang against the chest and are seen from further away, so they can go a size bigger than you would pick for studs. Shop moissanite studs or moissanite pendants.

Picking your size

1. Know your ring size. It sets the finger width in the table above.
2. Pick a shape. Long shapes look bigger per carat than round or square.
3. Pick by mm, not by carat. Cut a circle of that diameter out of paper and hold it on your finger. It takes two minutes and beats any photo.
4. Remember the setting adds size. A halo of small stones around the center makes the whole top look wider than the center stone alone.

If you are still choosing between stones, read moissanite vs lab diamond. Lab-grown diamonds follow the same carat to mm sizes, so this chart works for both. Browse moissanite engagement rings and each ring's carat picker will show the mm for your chosen shape.

Questions people ask

How many mm is a 1 carat moissanite?

A 1 ct DEW round moissanite is 6.5 mm across, the same as a 1 ct round diamond. A 1 ct oval is about 7.7 x 5.7 mm and a 1 ct cushion or princess about 5.5 mm per side.

Is a 1 carat moissanite the same size as a 1 carat diamond?

Yes, when it is sold by DEW, which is how we sell it. It is cut to the diamond's face size and weighs about 10% less because moissanite is less dense.

Why is the actual weight on a moissanite lower than its carat?

The carat on most moissanite listings is the diamond-equivalent weight. The real weight is lower because moissanite (3.21 to 3.22 grams per cubic centimeter) is lighter than diamond (3.52). The size you see is the same.

What carat moissanite looks good on a size 6 finger?

A size 6 finger is about 16.5 mm wide. A 1 ct round (6.5 mm) covers about 39% of it, a classic look; 1.5 ct (7.4 mm) covers about 45%; 2 ct (8.0 mm) covers about 48%. It comes down to taste.

How big is a 2 carat moissanite?

A 2 ct DEW round is 8.0 mm across, with about 1.5 times the face of a 1 ct. A 2 ct oval is about 9.7 x 7.2 mm and a 2 ct emerald cut about 8.8 x 6.3 mm.

Why do moissanite size charts disagree?

Cutters use slightly different proportions, so published charts differ by a few tenths of a millimeter at the same carat. Some also quote the actual weight instead of DEW. Compare stones by mm to be sure.

Sources

1. FTC Jewelry Guides, 16 CFR 23.12 (diamond specific gravity about 3.52), checked 2026-09-28.
2. GIA, Gems and Gemology (Winter 1997): Synthetic Moissanite: A New Diamond Substitute (specific gravity 3.22), checked 2026-09-28.
3. Webmineral: Moissanite mineral data (density 3.218 to 3.22), checked 2026-09-28.
4. GIA: Diamond Quality Factors (1 carat = 0.2 g), checked 2026-09-28.