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

California Artist

Lisa Ramos is a California-based artist creating handcrafted jewelry and leather bags. Her work has been featured at over 200 art shows and festivals.

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