Spinel size chart: carat to mm for every shape

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round spinel measures about 6.4 mm across, against 6.5 mm for a 1 carat diamond. Spinel is denser than diamond (3.60 against 3.52), so it packs more weight into less space and faces up 1% narrower for the same weight.

6.4 mm1 carat round spinel
3.60density, g/cm3
8hardness, Mohs

Carat to mm calculator

ShapeGemCarat

About 6.4 x 6.4 mm

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

Spinel size chart: carat to mm

Face-up length and width in mm of a well-cut spinel at each weight and shape. Spinel is denser than diamond (3.60 against 3.52), so it packs more weight into less space and faces up 1% narrower for the same weight.

Weight	Round	Oval	Cushion	Emerald cut	Pear	Princess
0.25 ct	4.0	5.0 x 3.4	3.5 x 3.5	4.4 x 3.1	5.3 x 3.4	3.4 x 3.4
0.5 ct	5.1	6.3 x 4.3	4.4 x 4.4	5.5 x 3.9	6.7 x 4.3	4.3 x 4.3
0.75 ct	5.8	7.2 x 5.0	5.0 x 5.0	6.3 x 4.5	7.7 x 5.0	5.0 x 5.0
1 ct	6.4	7.9 x 5.5	5.6 x 5.6	6.9 x 5.0	8.4 x 5.5	5.5 x 5.5
1.5 ct	7.3	9.1 x 6.2	6.4 x 6.4	8.0 x 5.7	9.7 x 6.2	6.2 x 6.2
2 ct	8.1	10.0 x 6.9	7.0 x 7.0	8.8 x 6.3	10.6 x 6.9	6.9 x 6.9
3 ct	9.2	11.5 x 7.9	8.0 x 8.0	10.0 x 7.2	12.2 x 7.9	7.9 x 7.9
4 ct	10.2	12.6 x 8.7	8.8 x 8.8	11.0 x 7.9	13.4 x 8.7	8.7 x 8.7
5 ct	10.9	13.6 x 9.3	9.5 x 9.5	11.9 x 8.5	14.4 x 9.3	9.3 x 9.3
10 ct	13.8	17.1 x 11.8	12.0 x 12.0	15.0 x 10.7	18.2 x 11.8	11.8 x 11.8

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

mm to carat for a round spinel

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

4 mm: Weight: 0.24 ct

5 mm: Weight: 0.48 ct

6 mm: Weight: 0.82 ct

7 mm: Weight: 1.31 ct

8 mm: Weight: 1.95 ct

9 mm: Weight: 2.78 ct

10 mm: Weight: 3.81 ct

12 mm: Weight: 6.59 ct

Diameter	Weight
4 mm	0.24 ct
5 mm	0.48 ct
6 mm	0.82 ct
7 mm	1.31 ct
8 mm	1.95 ct
9 mm	2.78 ct
10 mm	3.81 ct
12 mm	6.59 ct

Spinel against diamond

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

0.5 ct: Spinel: 5.1 mm; Diamond: 5.1 mm; Difference: about the same

1 ct: Spinel: 6.4 mm; Diamond: 6.5 mm; Difference: about the same

2 ct: Spinel: 8.1 mm; Diamond: 8.1 mm; Difference: -0.1 mm

3 ct: Spinel: 9.2 mm; Diamond: 9.3 mm; Difference: -0.1 mm

5 ct: Spinel: 10.9 mm; Diamond: 11.0 mm; Difference: -0.1 mm

Weight	Spinel	Diamond	Difference
0.5 ct	5.1 mm	5.1 mm	about the same
1 ct	6.4 mm	6.5 mm	about the same
2 ct	8.1 mm	8.1 mm	-0.1 mm
3 ct	9.2 mm	9.3 mm	-0.1 mm
5 ct	10.9 mm	11.0 mm	-0.1 mm

Spinel properties

Density used in the charts: Value: 3.60 g/cm3 (range 3.58 to 3.61)

Hardness (Mohs): Value: 8

Refractive index: Value: 1.712 to 1.762

Property	Value
Density used in the charts	3.60 g/cm3 (range 3.58 to 3.61)
Hardness (Mohs)	8
Refractive index	1.712 to 1.762

Close to diamond in density, so the same weight gives almost the same size.

Spinel at each weight

Questions

A 1 carat round spinel is about 6.4 mm across. An oval is 7.9 x 5.5 mm and a cushion cut 5.6 x 5.6 mm.

No. A 1 carat spinel is 6.4 mm and a 1 carat diamond 6.5 mm. Spinel is denser than diamond (3.60 against 3.52), so it packs more weight into less space and faces up 1% narrower for the same weight.

A 6 mm round spinel weighs about 0.82 carat. An 8 mm stone weighs about 1.95 carat.

A 5 carat round spinel is about 10.9 mm across (0.43 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.