Gem size chart: carat to mm

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round diamond measures about 6.5 mm across. Other shapes of the same weight face up from 5.5 mm to 10.4 mm long. Find your stone below, or use the calculator.

Carat to mm calculator

ShapeGemCarat

About 6.5 x 6.5 mm

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

1 carat diamond size by shape

Weight is not size. These are the face-up measurements of a well-cut 1 carat diamond. Pick a shape for its full chart from 0.25 to 5 carats.

Round diamond: 1 ct: 6.5 mm; Length to width: 1.00:1; Against a round: -

Oval diamond: 1 ct: 8.0 x 5.5 mm; Length to width: 1.45:1; Against a round: +6%

Cushion cut diamond: 1 ct: 5.6 x 5.6 mm; Length to width: 1.00:1; Against a round: -14%

Princess cut diamond: 1 ct: 5.5 x 5.5 mm; Length to width: 1.00:1; Against a round: -8%

Emerald cut diamond: 1 ct: 7.0 x 5.0 mm; Length to width: 1.40:1; Against a round: +4%

Asscher cut diamond: 1 ct: 5.5 x 5.5 mm; Length to width: 1.00:1; Against a round: -16%

Radiant cut diamond: 1 ct: 6.5 x 5.0 mm; Length to width: 1.30:1; Against a round: -2%

Pear shaped diamond: 1 ct: 8.5 x 5.5 mm; Length to width: 1.55:1; Against a round: -3%

Marquise diamond: 1 ct: 10.4 x 5.2 mm; Length to width: 2.00:1; Against a round: +16%

Heart shaped diamond: 1 ct: 6.5 x 6.5 mm; Length to width: 1.00:1; Against a round: -17%

Trillion cut diamond: 1 ct: 7.0 x 7.0 mm; Length to width: 1.00:1; Against a round: -14%

Baguette diamond: 1 ct: 8.0 x 4.0 mm; Length to width: 2.00:1; Against a round: -2%

Shape	1 ct	Length to width	Against a round
Round diamond	6.5 mm	1.00:1	-
Oval diamond	8.0 x 5.5 mm	1.45:1	+6%
Cushion cut diamond	5.6 x 5.6 mm	1.00:1	-14%
Princess cut diamond	5.5 x 5.5 mm	1.00:1	-8%
Emerald cut diamond	7.0 x 5.0 mm	1.40:1	+4%
Asscher cut diamond	5.5 x 5.5 mm	1.00:1	-16%
Radiant cut diamond	6.5 x 5.0 mm	1.30:1	-2%
Pear shaped diamond	8.5 x 5.5 mm	1.55:1	-3%
Marquise diamond	10.4 x 5.2 mm	2.00:1	+16%
Heart shaped diamond	6.5 x 6.5 mm	1.00:1	-17%
Trillion cut diamond	7.0 x 7.0 mm	1.00:1	-14%
Baguette diamond	8.0 x 4.0 mm	2.00:1	-2%

Round diamond size chart: carat to mm

Every side grows with the cube root of the weight. Double the carat weight and the stone is about 26% wider, not twice as wide.

0.25: Size: 4.1 mm

0.33: Size: 4.5 mm

0.5: Size: 5.1 mm

0.75: Size: 5.9 mm

1: Size: 6.5 mm

1.25: Size: 6.9 mm

1.5: Size: 7.4 mm

2: Size: 8.1 mm

2.5: Size: 8.8 mm

3: Size: 9.3 mm

4: Size: 10.2 mm

5: Size: 11.0 mm

Carat	Size
0.25	4.1 mm
0.33	4.5 mm
0.5	5.1 mm
0.75	5.9 mm
1	6.5 mm
1.25	6.9 mm
1.5	7.4 mm
2	8.1 mm
2.5	8.8 mm
3	9.3 mm
4	10.2 mm
5	11.0 mm

Round diamond mm to carat chart

From the face-up diameter to the estimated weight of a round diamond, with the same proportions as the chart above.

3.0 mm: Carat (about): 0.1

4.0 mm: Carat (about): 0.24

5.0 mm: Carat (about): 0.47

5.5 mm: Carat (about): 0.62

6.0 mm: Carat (about): 0.8

6.5 mm: Carat (about): 1.02

7.0 mm: Carat (about): 1.28

7.5 mm: Carat (about): 1.57

8.0 mm: Carat (about): 1.91

9.0 mm: Carat (about): 2.72

10.0 mm: Carat (about): 3.73

11.0 mm: Carat (about): 4.96

12.0 mm: Carat (about): 6.44

Diameter	Carat (about)
3.0 mm	0.1
4.0 mm	0.24
5.0 mm	0.47
5.5 mm	0.62
6.0 mm	0.8
6.5 mm	1.02
7.0 mm	1.28
7.5 mm	1.57
8.0 mm	1.91
9.0 mm	2.72
10.0 mm	3.73
11.0 mm	4.96
12.0 mm	6.44

How big is a 10, 20 or 40 carat diamond?

Round diamond sizes for big stones, in mm and inches.

6: Size: 11.7 mm; Inches: 0.46

8: Size: 12.9 mm; Inches: 0.51

10: Size: 13.9 mm; Inches: 0.55

15: Size: 15.9 mm; Inches: 0.63

20: Size: 17.5 mm; Inches: 0.69

25: Size: 18.9 mm; Inches: 0.74

30: Size: 20.0 mm; Inches: 0.79

40: Size: 22.1 mm; Inches: 0.87

50: Size: 23.8 mm; Inches: 0.94

100: Size: 29.9 mm; Inches: 1.18

Carat	Size	Inches
6	11.7 mm	0.46
8	12.9 mm	0.51
10	13.9 mm	0.55
15	15.9 mm	0.63
20	17.5 mm	0.69
25	18.9 mm	0.74
30	20.0 mm	0.79
40	22.1 mm	0.87
50	23.8 mm	0.94
100	29.9 mm	1.18

Other gems: sapphire, ruby, emerald and more

Gems are not all as dense as diamond. A 1 carat sapphire faces up smaller than a 1 carat diamond, and a 1 carat emerald faces up bigger. See the Gemstone size chart: carat to mm.

How these sizes are worked out

We start from the face-up size of a well-cut 1 carat diamond of each shape and scale it by weight and by the gem's density. These are estimates for a typical cut. A deep stone of the same weight faces up smaller, a shallow one bigger. For an exact stone, use the measurements on its certificate.

Calculators and converters

Carat to mm calculator: the size of any stone from its weight.
mm to carat calculator: the weight of a stone from its size.
Diamond weight calculator: carat weight from length, width and depth.
Diamond size comparison tool: two stones side by side, to scale.
Carat converter: carats to grams, ounces and points.

At actual size, and on a finger

Diamond actual size on your screen: calibrate once with a bank card, then see any stone life-size.
Printable actual size chart: print at 100% and lay a ring on it.
Diamond size on a finger: see any carat weight and shape on your ring size.
What size diamond should I get?: pick a look and a ring size, get a carat weight.
Ring size chart in mm and stud earring size chart.

Charts by carat weight, shape and size

Every carat weight from 0.1 to 100 has its own page, for example 1 carat diamond size, 2 carat and 3 carat. Each shape has a page for 18 weights, such as the 1 carat oval. Going the other way, a 6.5 mm diamond in carats and every other size from 1 to 20 mm.

Diamond carat size chart: every weight on one page.
Diamond size chart for all shapes: 12 shapes by weight.
Which diamond shape looks biggest: shapes ranked by face-up size.

Gemstones, moissanite and lab grown

Moissanite size chart: mm to carat, in diamond equivalent weight.
Lab grown diamond size chart.
Sapphire size chart, ruby, emerald, aquamarine, tanzanite and every other gem on the gemstone size chart.

Gemstone color charts

Gemstone color chart: shades, the color considered finest and treatments, for 16 gems.
Tsavorite color chart, sapphire, ruby, emerald and tanzanite.

For jewelers and designers

Melee diamond size chart, calibrated gemstone sizes and the round diamond CAD size table.
How to measure a gem and the glossary.
Methodology, the data as CSV and JSON and a chart to embed on your site.

Questions

A 1 carat round diamond is about 6.5 mm across. A 1 carat oval is about 8.0 x 5.5 mm, a 1 carat princess cut about 5.5 x 5.5 mm and a 1 carat emerald cut about 7.0 x 5.0 mm.

A 40 carat round diamond is about 22.1 mm across, or 0.87 inches.

Carat is weight and mm is size, so it depends on the shape and the gem. For a round diamond, the diameter in mm is about 6.45 times the cube root of the carat weight. The chart and calculator above do this for every shape.

No. A sapphire is denser than a diamond, so a 1 carat sapphire is about 6.2 mm across against 6.5 mm for the diamond.

No, they are estimates for a typical well-cut stone. The cut depth changes the face-up size. For a stone you are buying, ask for the length and width on its certificate.

For a round diamond, the weight is about (diameter in mm divided by 6.45) cubed, in carats. A 6.5 mm round diamond is about 1 carat, and a 5 mm one about 0.5 carat. Use the mm to carat chart or the calculator.

A 1 carat round diamond is about 6.5 mm wide. On a US size 6 finger it covers about 39% of the finger's width. The shape pages show the same for every shape and ring size.