Gemstone Size Conversion: Millimeters to Carat Weight

Round Diamond

Wondering how gemstone millimeter dimensions translate to carat weight? Whether you're selecting stones for custom jewelry or comparing sapphire and diamond sizes, this gemstone size conversion table makes it easy. Explore accurate mm‑to‑carat conversions for both round and oval gemstone shapes to help you choose the right size with confidence.

Round Gemstone Size to Carat Chart

Millimeters (mm)	Approx. Carat (ct)
2 mm	0.03 ct
3 mm	0.10 ct
4 mm	0.25 ct
4.5 mm	0.33 ct
5 mm	0.50 ct
5.5 mm	0.65 ct
6 mm	0.75 ct
6.5 mm	1.00 ct
7 mm	1.25 ct
8 mm	2.00 ct
9 mm	2.75 ct
10 mm	3.50 ct
11 mm	4.60 ct
12 mm	6.00 ct
13 mm	7.75 ct
14 mm	9.65 ct
15 mm	12.89 ct

Oval Gemstone Size to Carat Chart

Millimeters (mm)	Approx. Carat (ct)
5 × 3 mm	0.25 ct
6 × 4 mm	0.50 ct
7 × 5 mm	0.75 ct
7.7 × 5.7 mm	1.00 ct
8 × 6 mm	1.25 ct
8.5 × 6.5 mm	1.50 ct
9 × 6.5 mm	1.75 ct
9 × 7 mm	2.00 ct
10 × 8 mm	2.50 ct
12 × 8 mm	3.00 ct
12 × 8.5 mm	3.50 ct
12.5 × 8.5 mm	4.00 ct
12.75 × 8.5 mm	4.50 ct
12 × 10 mm	5.00 ct
14 × 10 mm	6.00 ct
14 × 12 mm	8.00 ct
16 × 12 mm	10.50 ct

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California Handbag & Jewelry Designer - Lisa Ramos

About the Author:

This guide was written by Lisa Ramos, a California jewelry designer and founder of MONOLISA. Lisa is a proud member of the International Gem Society (IGS), SNAG and Argentium Silver Guild.