Diamond Size Chart
How big a weight looks — and what a measured stone weighs.
6.44 mm
6.44 mm across · about 3.96 mm deep
1.00 ct round · 0.200 g
A carat is 0.200 g exactly, and a 1 ct round sits near 6.4–6.5 mm. Weight read from measurements is an estimate — a girdle thicker or thinner than usual will move it.
Carat weight and millimeters: how big a diamond actually looks
Carat is weight, not size. Two one-carat stones can differ by half a millimeter across the top, because weight can be hidden in a deep pavilion or a thick girdle where nobody sees it. What you see is the face-up spread, measured in millimeters, and that is what this converter works in — both directions.
A well cut one-carat round brilliant measures about 6.4 to 6.5 mm across. Elongated shapes of the same weight look larger because they cover more surface: an oval, marquise or pear spreads its weight out, while a round or a princess carries more of it downward.
Reading weight from measurements
The estimate here uses the standard trade formulas: the face-up spread multiplied by depth, a shape constant and a girdle allowance. It is how a jeweler estimates the weight of a mounted stone that cannot be put on a scale.
Treat the result as within a few percent rather than exact. A girdle thicker or thinner than usual will move it, and a stone that has been recut or chipped will not follow the formula at all.
Buying size cleverly
Three things buy visible size without buying weight. Shape: an elongated cut faces up larger than a round of the same carat. Just-under weights: a 0.90 or 1.90 carat stone looks like its rounder neighbor and costs meaningfully less per carat. And cut proportions: a shallower stone within the excellent range spreads wider, though push it too far and the brilliance goes.
One thing works against you — depth. A stone cut deep to hold weight measures small for its carat, and you pay for material you cannot see.
| Carat | Diameter | Roughly |
|---|---|---|
| 0.25 ct | 4.1 mm | A quarter carat |
| 0.50 ct | 5.2 mm | Half carat |
| 0.75 ct | 5.8 mm | Three quarters |
| 1.00 ct | 6.5 mm | One carat |
| 1.25 ct | 6.9 mm | |
| 1.50 ct | 7.4 mm | One and a half |
| 2.00 ct | 8.1 mm | Two carats |
| 2.50 ct | 8.8 mm | |
| 3.00 ct | 9.3 mm | Three carats |
| 4.00 ct | 10.2 mm | |
| 5.00 ct | 11.0 mm | Five carats |
Figures are for a well proportioned round brilliant. Fancy shapes of the same weight measure differently in each direction; use the converter above for those.
Questions we are asked
- How many mm is a 1 carat diamond?
- About 6.4 to 6.5 mm in diameter for a round brilliant of ordinary proportions. A deep-cut one-carat can measure 6.1 mm and a shallow one 6.7 mm, at the same weight.
- How do you convert mm to carat weight?
- Multiply the face-up spread by the depth and by a shape constant, then allow for girdle thickness. Switch the direction above and enter your measurements; the formulas are the ones the trade uses.
- Which diamond shape looks the biggest per carat?
- The elongated cuts — marquise, oval and pear — because they spread their weight across a larger face-up area. Round and princess cuts look smallest for their weight.
- Is a carat a size or a weight?
- A weight, exactly 0.200 grams. Size is measured in millimeters, and the two only line up when a stone is well proportioned.
Beyond the MeasurementRead the stone itself
A measurement is a fact; the grades are the other half. Photograph the stone and the instrument estimates the rest.