Tanzanite size chart: carat to mm for every shape

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round tanzanite measures about 6.6 mm across, against 6.5 mm for a 1 carat diamond. Tanzanite is lighter than diamond (3.35 against 3.52), so the same weight gives a bigger stone, 2% wider across.

6.6 mm1 carat round tanzanite
3.35density, g/cm3
6.5 to 7hardness, Mohs

Carat to mm calculator

ShapeGemCarat

About 6.6 x 6.6 mm

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

Tanzanite size chart: carat to mm

Face-up length and width in mm of a well-cut tanzanite at each weight and shape. Tanzanite is lighter than diamond (3.35 against 3.52), so the same weight gives a bigger stone, 2% wider across.

Weight	Round	Oval	Cushion	Emerald cut	Pear	Princess
0.25 ct	4.1	5.1 x 3.5	3.6 x 3.6	4.5 x 3.2	5.4 x 3.5	3.5 x 3.5
0.5 ct	5.2	6.5 x 4.4	4.5 x 4.5	5.6 x 4.0	6.9 x 4.4	4.4 x 4.4
0.75 ct	6.0	7.4 x 5.1	5.2 x 5.2	6.5 x 4.6	7.9 x 5.1	5.1 x 5.1
1 ct	6.6	8.1 x 5.6	5.7 x 5.7	7.1 x 5.1	8.6 x 5.6	5.6 x 5.6
1.5 ct	7.5	9.3 x 6.4	6.5 x 6.5	8.1 x 5.8	9.9 x 6.4	6.4 x 6.4
2 ct	8.3	10.2 x 7.0	7.2 x 7.2	9.0 x 6.4	10.9 x 7.0	7.0 x 7.0
3 ct	9.5	11.7 x 8.1	8.2 x 8.2	10.3 x 7.3	12.5 x 8.1	8.1 x 8.1
4 ct	10.4	12.9 x 8.9	9.0 x 9.0	11.3 x 8.1	13.7 x 8.9	8.9 x 8.9
5 ct	11.2	13.9 x 9.6	9.7 x 9.7	12.2 x 8.7	14.8 x 9.6	9.6 x 9.6
10 ct	14.1	17.5 x 12.0	12.3 x 12.3	15.3 x 11.0	18.6 x 12.0	12.0 x 12.0

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

mm to carat for a round tanzanite

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

4 mm: Weight: 0.23 ct

5 mm: Weight: 0.44 ct

6 mm: Weight: 0.77 ct

7 mm: Weight: 1.22 ct

8 mm: Weight: 1.82 ct

9 mm: Weight: 2.59 ct

10 mm: Weight: 3.55 ct

12 mm: Weight: 6.13 ct

Diameter	Weight
4 mm	0.23 ct
5 mm	0.44 ct
6 mm	0.77 ct
7 mm	1.22 ct
8 mm	1.82 ct
9 mm	2.59 ct
10 mm	3.55 ct
12 mm	6.13 ct

Tanzanite against diamond

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

0.5 ct: Tanzanite: 5.2 mm; Diamond: 5.1 mm; Difference: +0.1 mm

1 ct: Tanzanite: 6.6 mm; Diamond: 6.5 mm; Difference: +0.1 mm

2 ct: Tanzanite: 8.3 mm; Diamond: 8.1 mm; Difference: +0.1 mm

3 ct: Tanzanite: 9.5 mm; Diamond: 9.3 mm; Difference: +0.2 mm

5 ct: Tanzanite: 11.2 mm; Diamond: 11.0 mm; Difference: +0.2 mm

Weight	Tanzanite	Diamond	Difference
0.5 ct	5.2 mm	5.1 mm	+0.1 mm
1 ct	6.6 mm	6.5 mm	+0.1 mm
2 ct	8.3 mm	8.1 mm	+0.1 mm
3 ct	9.5 mm	9.3 mm	+0.2 mm
5 ct	11.2 mm	11.0 mm	+0.2 mm

Tanzanite properties

Density used in the charts: Value: 3.35 g/cm3 (range 3.30 to 3.38)

Hardness (Mohs): Value: 6.5 to 7

Refractive index: Value: 1.691 to 1.700

Property	Value
Density used in the charts	3.35 g/cm3 (range 3.30 to 3.38)
Hardness (Mohs)	6.5 to 7
Refractive index	1.691 to 1.700

A blue-violet zoisite. Softer than sapphire, so it suits a protective setting.

Tanzanite at each weight

Questions

A 1 carat round tanzanite is about 6.6 mm across. An oval is 8.1 x 5.6 mm and a cushion cut 5.7 x 5.7 mm.

Yes. A 1 carat tanzanite is 6.6 mm and a 1 carat diamond 6.5 mm. Tanzanite is lighter than diamond (3.35 against 3.52), so the same weight gives a bigger stone, 2% wider across.

A 6 mm round tanzanite weighs about 0.77 carat. An 8 mm stone weighs about 1.82 carat.

A 5 carat round tanzanite is about 11.2 mm across (0.44 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.