Diamond size chart: carat to millimetres
Enter a carat weight to see the approximate diameter of a round brilliant diamond, or enter a diameter to estimate the weight. The chart shows typical sizes from 0.05 to 5 carats, with the matching weights of moissanite and cubic zirconia at the same size.
Round brilliant estimate
Size chart
| Carat | Approx. diameter | Points |
|---|---|---|
| 0.05 ct | 2.4 mm | 5 |
| 0.10 ct | 3.0 mm | 10 |
| 0.15 ct | 3.5 mm | 15 |
| 0.20 ct | 3.8 mm | 20 |
| 0.25 ct | 4.1 mm | 25 |
| 0.30 ct | 4.4 mm | 30 |
| 0.40 ct | 4.8 mm | 40 |
| 0.50 ct | 5.2 mm | 50 |
| 0.60 ct | 5.5 mm | 60 |
| 0.70 ct | 5.8 mm | 70 |
| 0.75 ct | 5.9 mm | 75 |
| 0.90 ct | 6.3 mm | 90 |
| 1.00 ct | 6.5 mm | 100 |
| 1.25 ct | 7.0 mm | 125 |
| 1.50 ct | 7.4 mm | 150 |
| 1.75 ct | 7.8 mm | 175 |
| 2.00 ct | 8.2 mm | 200 |
| 2.50 ct | 8.8 mm | 250 |
| 3.00 ct | 9.4 mm | 300 |
| 4.00 ct | 10.3 mm | 400 |
| 5.00 ct | 11.1 mm | 500 |
Approximate values for round brilliant diamonds with typical proportions. Deep or shallow cuts measure differently for the same weight, and fancy shapes such as ovals or pears are measured by length and width. For a specific stone, use the measurements on its grading report.
How to use the chart
Type a carat weight and the diameter updates, or type a diameter in millimetres and the carat weight updates. The results also show what a moissanite or cubic zirconia stone of the same diameter would weigh. Below the calculator, a visual chart draws stones from a quarter carat upwards to scale with each other, and a table lists approximate diameters from 0.05 to 5 carats, with their weight in points.
Carat is weight, not size
A carat is a unit of weight equal to 0.2 grams, divided into 100 points. A 0.25 carat stone is often called a 25 pointer. Because carat measures weight, two diamonds of the same weight can look different in size if one is cut deeper than the other. A deep cut hides weight below the girdle, so the stone looks smaller from above.
For round brilliants, weight grows with the cube of the diameter. That is why doubling the carat weight does not double the visible size. A well proportioned 1 carat round is about 6.5 mm across, while a 2 carat round is about 8.2 mm, only about a quarter wider. Seen from above, the face of the stone grows less than its weight.
How the chart is calculated
The chart uses a simple rule for well cut round brilliants. A 1 carat stone measures about 6.5 mm, and the diameter changes with the cube root of the weight. This tracks common industry charts closely for typical proportions. Real stones vary, so the measurements on a grading report always beat a chart.
Fancy shapes such as ovals, pears, marquises, emerald cuts and princess cuts are measured by length and width, and their proportions vary a lot between cutters, so this chart does not cover them.
Moissanite, cubic zirconia and lab-grown diamond
- Lab-grown diamond is diamond, with the same density, so the same weight gives the same size.
- Moissanite is lighter than diamond. A moissanite of the same diameter weighs roughly 9 percent less, which is why it is often sold by its size in millimetres or its diamond equivalent.
- Cubic zirconia is much denser. A stone of the same diameter weighs roughly 65 percent more than a diamond.
The glossary covers moissanite, cubic zirconia and lab-grown diamond in more detail.
Choosing a stone size for a design
Stone size changes how a design reads. A 1 carat centre stone looks larger on a slim band than on a wide one, and a halo of small stones makes the centre look bigger than its weight. Finger size matters too, since the same stone covers more of a slender finger.
When you design in CAD, work from the stone diameter rather than the weight, and build the setting around the actual stone dimensions. In RenderMyJewels, stones are recognised by their layer or object names, such as Diamond or Gem, and can be shown in any of 58 gem presets, so you can compare a diamond with a coloured stone in seconds before anything is cast. Pair this chart with the ring size converter to plan a complete engagement ring, and see how to render diamonds realistically for presentation tips.
Small stones and melee
Accent stones are usually described by diameter rather than weight, because they are bought and set by size. A pavé band might use stones from about 1 mm to 2 mm, and a halo is often built from one consistent size. The chart starts at 0.05 carat, about 2.4 mm. For smaller stones, a supplier's sieve size or millimetre chart is the better reference.