Diamond Carat to MM Calculator: Round, Oval, Princess, Pear and More
Free tool · Updated October 2, 2026 · By CouponTalk
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Quick answer
Carat is weight, not size, so a diamond's mm size depends on its shape and cut. A well-cut 1-carat round diamond is about 6.5 mm across; 0.5 carat is about 5.1 mm. Elongated shapes look bigger for the same weight: a 1-carat oval is about 7.8 × 5.8 mm. Use the mm to compare stones and check how big a diamond will look on the hand.
How to measure
- Pick the diamond shape.
- Choose carat to mm, or mm to carat.
- Enter the weight or the width in mm.
Diamond Carat to MM Size Chart
| Carat | Round (diameter) | Princess (side) | Oval (L × W) |
|---|---|---|---|
| 0.25 ct | 4.1 mm | 3.5 mm | 4.9 × 3.7 mm |
| 0.5 ct | 5.1 mm | 4.4 mm | 6.2 × 4.6 mm |
| 0.75 ct | 5.9 mm | 5 mm | 7.1 × 5.3 mm |
| 1 ct | 6.5 mm | 5.5 mm | 7.8 × 5.8 mm |
| 1.5 ct | 7.4 mm | 6.3 mm | 9 × 6.6 mm |
| 2 ct | 8.1 mm | 6.9 mm | 9.9 × 7.3 mm |
| 3 ct | 9.3 mm | 7.9 mm | 11.3 × 8.4 mm |
Sizes are estimates for well-proportioned stones. The certificate gives exact measurements.
Frequently asked questions
How big is a 1 carat diamond in mm?
A well-cut round 1-carat diamond is about 6.5 mm across. Deep cuts look smaller; shallow cuts look bigger but sparkle less.
Do lab-grown diamonds have the same size per carat?
Yes. Lab-grown diamonds have the same density as natural ones, so size per carat is the same.
What size diamond studs should I buy?
For everyday studs, 0.25–0.5 carat each (about 4–5 mm) is popular. "1 carat total weight" means 0.5 carat per earring.
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