Ruby size chart: carat to mm for every shape

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round ruby measures about 6.2 mm across, against 6.5 mm for a 1 carat diamond. Ruby is denser than diamond (4.00 against 3.52), so it packs more weight into less space and faces up 4% narrower for the same weight.

6.2 mm1 carat round ruby
4.00density, g/cm3
9hardness, Mohs

Carat to mm calculator

ShapeGemCarat

About 6.2 x 6.2 mm

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

Ruby size chart: carat to mm

Face-up length and width in mm of a well-cut ruby at each weight and shape. Ruby is denser than diamond (4.00 against 3.52), so it packs more weight into less space and faces up 4% narrower for the same weight.

Weight	Round	Oval	Cushion	Emerald cut	Pear	Princess
0.25 ct	3.9	4.8 x 3.3	3.4 x 3.4	4.2 x 3.0	5.1 x 3.3	3.3 x 3.3
0.5 ct	4.9	6.1 x 4.2	4.3 x 4.3	5.3 x 3.8	6.5 x 4.2	4.2 x 4.2
0.75 ct	5.6	7.0 x 4.8	4.9 x 4.9	6.1 x 4.4	7.4 x 4.8	4.8 x 4.8
1 ct	6.2	7.7 x 5.3	5.4 x 5.4	6.7 x 4.8	8.1 x 5.3	5.3 x 5.3
1.5 ct	7.1	8.8 x 6.0	6.1 x 6.1	7.7 x 5.5	9.3 x 6.0	6.0 x 6.0
2 ct	7.8	9.7 x 6.6	6.8 x 6.8	8.5 x 6.0	10.3 x 6.6	6.6 x 6.6
3 ct	8.9	11.1 x 7.6	7.7 x 7.7	9.7 x 6.9	11.7 x 7.6	7.6 x 7.6
4 ct	9.8	12.2 x 8.4	8.5 x 8.5	10.6 x 7.6	12.9 x 8.4	8.4 x 8.4
5 ct	10.6	13.1 x 9.0	9.2 x 9.2	11.5 x 8.2	13.9 x 9.0	9.0 x 9.0
10 ct	13.3	16.5 x 11.4	11.6 x 11.6	14.5 x 10.3	17.5 x 11.4	11.4 x 11.4

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

mm to carat for a round ruby

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

4 mm: Weight: 0.27 ct

5 mm: Weight: 0.53 ct

6 mm: Weight: 0.91 ct

7 mm: Weight: 1.45 ct

8 mm: Weight: 2.17 ct

9 mm: Weight: 3.09 ct

10 mm: Weight: 4.23 ct

12 mm: Weight: 7.32 ct

Diameter	Weight
4 mm	0.27 ct
5 mm	0.53 ct
6 mm	0.91 ct
7 mm	1.45 ct
8 mm	2.17 ct
9 mm	3.09 ct
10 mm	4.23 ct
12 mm	7.32 ct

Ruby against diamond

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

0.5 ct: Ruby: 4.9 mm; Diamond: 5.1 mm; Difference: -0.2 mm

1 ct: Ruby: 6.2 mm; Diamond: 6.5 mm; Difference: -0.3 mm

2 ct: Ruby: 7.8 mm; Diamond: 8.1 mm; Difference: -0.3 mm

3 ct: Ruby: 8.9 mm; Diamond: 9.3 mm; Difference: -0.4 mm

5 ct: Ruby: 10.6 mm; Diamond: 11.0 mm; Difference: -0.5 mm

Weight	Ruby	Diamond	Difference
0.5 ct	4.9 mm	5.1 mm	-0.2 mm
1 ct	6.2 mm	6.5 mm	-0.3 mm
2 ct	7.8 mm	8.1 mm	-0.3 mm
3 ct	8.9 mm	9.3 mm	-0.4 mm
5 ct	10.6 mm	11.0 mm	-0.5 mm

Ruby properties

Density used in the charts: Value: 4.00 g/cm3 (range 3.95 to 4.03)

Hardness (Mohs): Value: 9

Refractive index: Value: 1.762 to 1.770

Property	Value
Density used in the charts	4.00 g/cm3 (range 3.95 to 4.03)
Hardness (Mohs)	9
Refractive index	1.762 to 1.770

Red corundum, the same mineral as sapphire. Denser than diamond, so it faces up smaller for its weight.

Ruby at each weight

Questions

A 1 carat round ruby is about 6.2 mm across. An oval is 7.7 x 5.3 mm and a cushion cut 5.4 x 5.4 mm.

No. A 1 carat ruby is 6.2 mm and a 1 carat diamond 6.5 mm. Ruby is denser than diamond (4.00 against 3.52), so it packs more weight into less space and faces up 4% narrower for the same weight.

A 6 mm round ruby weighs about 0.91 carat. An 8 mm stone weighs about 2.17 carat.

A 5 carat round ruby is about 10.6 mm across (0.42 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.