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Diamond Size Converter
Convert carat weight to millimeter size for round diamonds — and back — then see how big the same weight looks in other shapes.
Enter a carat weight above to see the diameter in millimeters.
Carat is a weight; millimeters are the face-up size. Cut and shape change how large a stone looks.
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Round diamond size chart
Carat weight and the face-up diameter it maps to for a round brilliant.
| Carat | Diameter (mm) | Inches |
|---|---|---|
| 0.25 | 4.1 | 0.16 |
| 0.33 | 4.5 | 0.18 |
| 0.50 | 5.2 | 0.20 |
| 0.75 | 5.9 | 0.23 |
| 1.00 | 6.5 | 0.26 |
| 1.25 | 7.0 | 0.28 |
| 1.50 | 7.4 | 0.29 |
| 1.75 | 7.8 | 0.31 |
| 2.00 | 8.2 | 0.32 |
| 2.50 | 8.8 | 0.35 |
| 3.00 | 9.4 | 0.37 |
Diamond size by shape (face-up mm)
Typical face-up dimensions at each carat weight. Elongated shapes look larger than a round of the same weight.
| Shape | 0.5 ct | 1.0 ct | 1.5 ct | 2.0 ct |
|---|---|---|---|---|
| Round | 5.2 | 6.5 | 7.4 | 8.2 |
| Princess | 4.4 | 5.5 | 6.3 | 6.9 |
| Cushion | 4.4 | 5.6 | 6.4 | 7.1 |
| Oval | 6.0×4.4 | 7.5×5.5 | 8.6×6.3 | 9.4×6.9 |
| Emerald | 5.4×4.0 | 6.8×5.0 | 7.8×5.7 | 8.6×6.3 |
| Pear | 6.5×4.4 | 8.2×5.5 | 9.4×6.3 | 10.3×6.9 |
| Marquise | 7.9×4.0 | 10.0×5.0 | 11.4×5.7 | 12.6×6.3 |
Round is a diameter; other shapes are length×width in mm. Fancy-shape dimensions vary with cut proportions — always check the grading report.
Frequently asked questions
How big is a 1 carat diamond?
A well-cut 1.00 carat round diamond measures about 6.5 mm across (roughly a quarter inch). Carat is a weight, not a size, so the face-up diameter depends on the shape and cut — an elongated 1 ct oval looks larger than a 1 ct round because its weight is spread over a longer outline.
How do I convert carat to millimeters?
For a round brilliant, the diameter grows with the cube root of the weight: mm ≈ 6.5 × ∛carat. So a 0.50 ct round is about 5.2 mm and a 2.00 ct round about 8.2 mm. The converter above does this instantly in both directions.
Does a higher carat always look bigger?
Not necessarily. Two diamonds of equal carat can look different sizes depending on cut. A deep-cut stone hides weight below the girdle, so it looks smaller face-up, while a shallow or well-proportioned stone spreads wider. Shape matters too — ovals, marquises, and pears look larger than rounds of the same weight.
Is carat the same as diamond size?
No. Carat measures weight (1 carat = 0.2 grams), while size is the physical face-up dimension in millimeters. This tool converts between them for round diamonds and shows typical millimeter sizes for other shapes, but always confirm the exact mm dimensions on a diamond's grading report.
What ring size is unrelated to diamond size?
Correct — diamond size (the stone) and ring size (the band that fits your finger) are separate measurements. To find your finger size, use our ring size converter; to picture the stone, use this diamond size tool.