Opal size chart: carat to mm for every shape

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round opal measures about 7.7 mm across, against 6.5 mm for a 1 carat diamond. Opal is lighter than diamond (2.10 against 3.52), so the same weight gives a bigger stone, 19% wider across.

7.7 mm1 carat round opal
2.10density, g/cm3
5.5 to 6.5hardness, Mohs

Carat to mm calculator

ShapeGemCarat

About 7.7 x 7.7 mm

An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.

Opal size chart: carat to mm

Face-up length and width in mm of a well-cut opal at each weight and shape. Opal is lighter than diamond (2.10 against 3.52), so the same weight gives a bigger stone, 19% wider across.

Weight	Round	Oval	Cushion	Emerald cut	Pear	Princess
0.25 ct	4.8	6.0 x 4.1	4.2 x 4.2	5.2 x 3.7	6.4 x 4.1	4.1 x 4.1
0.5 ct	6.1	7.5 x 5.2	5.3 x 5.3	6.6 x 4.7	8.0 x 5.2	5.2 x 5.2
0.75 ct	7.0	8.6 x 5.9	6.0 x 6.0	7.6 x 5.4	9.2 x 5.9	5.9 x 5.9
1 ct	7.7	9.5 x 6.5	6.7 x 6.7	8.3 x 5.9	10.1 x 6.5	6.5 x 6.5
1.5 ct	8.8	10.9 x 7.5	7.6 x 7.6	9.5 x 6.8	11.6 x 7.5	7.5 x 7.5
2 ct	9.7	12.0 x 8.2	8.4 x 8.4	10.5 x 7.5	12.7 x 8.2	8.2 x 8.2
3 ct	11.1	13.7 x 9.4	9.6 x 9.6	12.0 x 8.6	14.6 x 9.4	9.4 x 9.4
4 ct	12.2	15.1 x 10.4	10.6 x 10.6	13.2 x 9.4	16.0 x 10.4	10.4 x 10.4
5 ct	13.1	16.3 x 11.2	11.4 x 11.4	14.2 x 10.2	17.3 x 11.2	11.2 x 11.2
10 ct	16.5	20.5 x 14.1	14.3 x 14.3	17.9 x 12.8	21.8 x 14.1	14.1 x 14.1

Length x width in mm. A round stone shows one number, its diameter.

mm to carat for a round opal

If you know the size, this is the weight. Measure across the widest point of the stone, face up.

4 mm: Weight: 0.14 ct

5 mm: Weight: 0.28 ct

6 mm: Weight: 0.48 ct

7 mm: Weight: 0.76 ct

8 mm: Weight: 1.14 ct

9 mm: Weight: 1.62 ct

10 mm: Weight: 2.22 ct

12 mm: Weight: 3.84 ct

Diameter	Weight
4 mm	0.14 ct
5 mm	0.28 ct
6 mm	0.48 ct
7 mm	0.76 ct
8 mm	1.14 ct
9 mm	1.62 ct
10 mm	2.22 ct
12 mm	3.84 ct

Opal against diamond

How wide a round opal is next to a round diamond of the same weight.

0.5 ct: Opal: 6.1 mm; Diamond: 5.1 mm; Difference: +1.0 mm

1 ct: Opal: 7.7 mm; Diamond: 6.5 mm; Difference: +1.2 mm

2 ct: Opal: 9.7 mm; Diamond: 8.1 mm; Difference: +1.5 mm

3 ct: Opal: 11.1 mm; Diamond: 9.3 mm; Difference: +1.7 mm

5 ct: Opal: 13.1 mm; Diamond: 11.0 mm; Difference: +2.1 mm

Weight	Opal	Diamond	Difference
0.5 ct	6.1 mm	5.1 mm	+1.0 mm
1 ct	7.7 mm	6.5 mm	+1.2 mm
2 ct	9.7 mm	8.1 mm	+1.5 mm
3 ct	11.1 mm	9.3 mm	+1.7 mm
5 ct	13.1 mm	11.0 mm	+2.1 mm

Opal properties

Density used in the charts: Value: 2.10 g/cm3 (range 1.98 to 2.25)

Hardness (Mohs): Value: 5.5 to 6.5

Refractive index: Value: 1.37 to 1.47

Property	Value
Density used in the charts	2.10 g/cm3 (range 1.98 to 2.25)
Hardness (Mohs)	5.5 to 6.5
Refractive index	1.37 to 1.47

Very light, so a carat of opal is large. Usually cut as a cabochon, so size is given in mm.

Opal at each weight

Questions

A 1 carat round opal is about 7.7 mm across. An oval is 9.5 x 6.5 mm and a cushion cut 6.7 x 6.7 mm.

Yes. A 1 carat opal is 7.7 mm and a 1 carat diamond 6.5 mm. Opal is lighter than diamond (2.10 against 3.52), so the same weight gives a bigger stone, 19% wider across.

A 6 mm round opal weighs about 0.48 carat. An 8 mm stone weighs about 1.14 carat.

A 5 carat round opal is about 13.1 mm across (0.52 inches).

A carat is a weight, 0.2 grams. Gems differ in density, so a gram of a dense gem takes up less space than a gram of a light one. Dense stones face up smaller for their weight.