Carat to MM Size Chart
Find the approximate size in millimetres of a gemstone from its carat weight, for any gemstone and shape. Also shows grams and a life-size preview.
Estimates are usually within 5 to 10 percent. Change the length-to-width ratio and depth to match your stone.
Carat vs size: why carats are not millimetres
A carat is a unit of weight: one carat is 0.2 grams. Size in millimetres depends on how dense the gemstone is, how the stone is cut and how deep it is. Denser gemstones such as ruby and sapphire are smaller than diamond at the same weight, and lighter gemstones such as emerald, aquamarine and quartz are larger.
For example, a 1 carat round diamond is about 6.4 mm across, a 1 carat round ruby or sapphire is about 6.1 mm, and a 1 carat round emerald is about 7.0 mm.
Round gemstone size chart: carat weight to diameter in mm
Approximate diameter of a round stone with a depth of 60 percent of its width.
| Carats | Grams | Diamond | Ruby / Sapphire | Spinel | Emerald / Aquamarine | Topaz | Tanzanite | Quartz (amethyst / citrine) |
|---|---|---|---|---|---|---|---|---|
| 0.25 ct | 0.05 g | 4.0 mm | 3.9 mm | 4.0 mm | 4.4 mm | 4.0 mm | 4.1 mm | 4.4 mm |
| 0.5 ct | 0.10 g | 5.1 mm | 4.9 mm | 5.1 mm | 5.5 mm | 5.1 mm | 5.2 mm | 5.6 mm |
| 0.75 ct | 0.15 g | 5.8 mm | 5.6 mm | 5.8 mm | 6.3 mm | 5.8 mm | 6.0 mm | 6.4 mm |
| 1 ct | 0.20 g | 6.4 mm | 6.1 mm | 6.4 mm | 7.0 mm | 6.4 mm | 6.6 mm | 7.1 mm |
| 1.5 ct | 0.30 g | 7.3 mm | 7.0 mm | 7.3 mm | 8.0 mm | 7.3 mm | 7.5 mm | 8.1 mm |
| 2 ct | 0.40 g | 8.1 mm | 7.7 mm | 8.0 mm | 8.8 mm | 8.1 mm | 8.3 mm | 8.9 mm |
| 3 ct | 0.60 g | 9.2 mm | 8.9 mm | 9.2 mm | 10.1 mm | 9.2 mm | 9.5 mm | 10.2 mm |
| 4 ct | 0.80 g | 10.2 mm | 9.8 mm | 10.1 mm | 11.1 mm | 10.2 mm | 10.5 mm | 11.2 mm |
| 5 ct | 1.00 g | 11.0 mm | 10.5 mm | 10.9 mm | 11.9 mm | 10.9 mm | 11.3 mm | 12.1 mm |
| 7 ct | 1.40 g | 12.3 mm | 11.8 mm | 12.2 mm | 13.3 mm | 12.2 mm | 12.6 mm | 13.5 mm |
| 10 ct | 2.00 g | 13.8 mm | 13.2 mm | 13.7 mm | 15.0 mm | 13.8 mm | 14.2 mm | 15.2 mm |
Oval gemstone size chart: length × width in mm
Approximate size of an oval with a 1.35 length-to-width ratio and a depth of 60 percent of its width.
| Carats | Diamond | Ruby / Sapphire | Emerald / Aquamarine | Tanzanite |
|---|---|---|---|---|
| 0.5 ct | 6.0 × 4.4 mm | 5.8 × 4.3 mm | 6.5 × 4.8 mm | 6.2 × 4.6 mm |
| 1 ct | 7.6 × 5.6 mm | 7.2 × 5.4 mm | 8.2 × 6.1 mm | 7.8 × 5.8 mm |
| 2 ct | 9.5 × 7.1 mm | 9.1 × 6.8 mm | 10.4 × 7.7 mm | 9.8 × 7.2 mm |
| 3 ct | 10.9 × 8.1 mm | 10.5 × 7.7 mm | 11.9 × 8.8 mm | 11.2 × 8.3 mm |
| 5 ct | 12.9 × 9.6 mm | 12.4 × 9.2 mm | 14.1 × 10.4 mm | 13.3 × 9.8 mm |
| 10 ct | 16.3 × 12.1 mm | 15.6 × 11.6 mm | 17.7 × 13.1 mm | 16.7 × 12.4 mm |
Emerald cut size chart: length × width in mm
Approximate size of a step cut with a 1.40 length-to-width ratio and a depth of 62 percent of its width.
| Carats | Diamond | Ruby / Sapphire | Emerald / Aquamarine |
|---|---|---|---|
| 0.5 ct | 5.6 × 4.0 mm | 5.4 × 3.9 mm | 6.1 × 4.4 mm |
| 1 ct | 7.1 × 5.1 mm | 6.8 × 4.9 mm | 7.7 × 5.5 mm |
| 2 ct | 9.0 × 6.4 mm | 8.6 × 6.1 mm | 9.7 × 7.0 mm |
| 3 ct | 10.3 × 7.3 mm | 9.8 × 7.0 mm | 11.2 × 8.0 mm |
| 5 ct | 12.2 × 8.7 mm | 11.7 × 8.3 mm | 13.2 × 9.4 mm |
| 10 ct | 15.3 × 10.9 mm | 14.7 × 10.5 mm | 16.7 × 11.9 mm |
How the estimate works
The standard colored-stone estimate is weight in carats = length × width × depth × specific gravity × shape factor. The shape factors used are 0.0018 for round and pear, 0.0020 for oval, 0.0017 for marquise and 0.0025 for emerald cut. This calculator solves the formula for size. It assumes the length-to-width ratio and depth you enter, and uses the middle of the gemstone's published specific gravity range.
To work the other way, enter measurements in the Gemstone Weight Calculator. To convert between carats, grams and milligrams, use the Carat to Gram Converter.
Educational reference only. Typical published values vary between specimens and sources, and this page is not a gemological laboratory report. Confirm important stones with a recognised gem lab.
Frequently asked questions
How many mm is 1 carat?
Carat is a unit of weight, not size, so the size depends on the gemstone and the cut. As a guide, a 1 carat round diamond measures about 6.4 mm across, a 1 carat round ruby or sapphire about 6.1 mm, and a 1 carat round emerald about 7.0 mm.
Why is a 1 carat ruby smaller than a 1 carat diamond?
Ruby and sapphire are denser than diamond (specific gravity about 4.0 against 3.5), so each cubic millimetre weighs more. For the same weight, a ruby or sapphire is smaller. A 1 carat round ruby is about 6.1 mm across, while a 1 carat round diamond is about 6.4 mm.
How many grams is 1 carat?
One carat equals 0.2 grams, or 200 milligrams. A 5 carat stone weighs 1 gram.
How do I convert carats to mm?
There is no single conversion, because size depends on the gemstone's density, the shape and the depth of the cut. The standard estimate is weight = length × width × depth × specific gravity × a shape factor. This tool solves that formula for size, so you enter the carat weight, the gemstone and the shape, and it returns the estimated length, width and depth in millimetres.
What size is a 2 carat round diamond?
A 2 carat round diamond is about 8.1 mm across. A 2 carat round ruby or sapphire is about 7.7 mm, and a 2 carat round emerald is about 8.8 mm.
How accurate is a carat to mm chart?
Results are estimates, usually within about 5 to 10 percent. Real stones vary in depth, cut quality and outline, and a deep or shallow cut changes the weight for a given width. For an exact size, measure the stone with a caliper.
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