How many mm is a 1 carat diamond?

A 1 ct round brilliant is about 6.5 mm across at typical proportions, likely 6.3 to 6.6 mm.

What size is a half carat diamond in mm?

About 5.1 mm for a round at 61% depth.

How big is a 3 carat diamond?

About 9.3 mm round. A 3 ct oval is roughly 11.5 mm long.

Which diamond shape looks biggest for its weight?

Elongated shapes. A 1 ct marquise is about 10.5 mm long, and a 1 ct oval about 8.0 mm, against 6.5 mm for a round. Square shapes such as princess and Asscher look smallest face up.

Do lab-grown diamonds follow the same size chart?

Yes. They have the same density and the same cutting styles, so the chart applies unchanged.

Why does a retailer chart show slightly different sizes?

Every chart assumes some proportions. Ours shows its assumptions and a range. Small differences of a tenth of a millimetre are normal between charts.

Diamond Size Chart in mm, All Shapes

Round diamond sizes

This chart is generated by our formula engine using the standard trade formula for round brilliants, which already includes diamond's density. It assumes a total depth of 61% of the diameter, which is typical of a well-cut stone. The range column shows where most real stones of that weight will fall.

Round brilliant diamond: carat weight to diameter
Carats	Diameter	Likely range	Depth
0.050 ct	2.4 mm	2.3 to 2.4 mm	1.5 mm
0.10 ct	3.0 mm	2.9 to 3.0 mm	1.8 mm
0.15 ct	3.4 mm	3.3 to 3.5 mm	2.1 mm
0.20 ct	3.8 mm	3.7 to 3.8 mm	2.3 mm
0.25 ct	4.1 mm	4.0 to 4.1 mm	2.5 mm
0.30 ct	4.3 mm	4.2 to 4.4 mm	2.6 mm
0.40 ct	4.8 mm	4.6 to 4.8 mm	2.9 mm
0.50 ct	5.1 mm	5.0 to 5.2 mm	3.1 mm
0.60 ct	5.4 mm	5.3 to 5.5 mm	3.3 mm
0.70 ct	5.7 mm	5.6 to 5.8 mm	3.5 mm
0.75 ct	5.9 mm	5.7 to 6.0 mm	3.6 mm
0.80 ct	6.0 mm	5.8 to 6.1 mm	3.7 mm
0.90 ct	6.2 mm	6.1 to 6.3 mm	3.8 mm
1.00 ct	6.5 mm	6.3 to 6.6 mm	3.9 mm
1.25 ct	7.0 mm	6.8 to 7.1 mm	4.2 mm
1.50 ct	7.4 mm	7.2 to 7.5 mm	4.5 mm
1.75 ct	7.8 mm	7.6 to 7.9 mm	4.7 mm
2.00 ct	8.1 mm	7.9 to 8.3 mm	5.0 mm
2.50 ct	8.8 mm	8.5 to 8.9 mm	5.3 mm
3.00 ct	9.3 mm	9.1 to 9.5 mm	5.7 mm
3.50 ct	9.8 mm	9.5 to 10.0 mm	6.0 mm
4.00 ct	10.2 mm	10.0 to 10.4 mm	6.3 mm
5.00 ct	11.0 mm	10.7 to 11.2 mm	6.7 mm

A deeper cut hides weight below the girdle. The same 1 ct stone at 65% depth is only 6.3 mm across. That is why two 1 ct diamonds side by side can look different, and why a slightly lighter, well-cut stone can face up as large as a heavier, deep one.

Fancy shape diamond sizes

Fancy shapes use their own trade formulas. Emerald, radiant, pear and marquise also depend on the length to width ratio. Each table below uses a typical ratio and depth for the shape. Change them in the calculator further down if your stone is longer or shorter.

Oval: typical ratio 1.4 to 1, depth 61%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	5.0 x 3.6	6.3 x 4.5	7.2 x 5.2	8.0 x 5.7	9.1 x 6.5	10.0 x 7.2	11.5 x 8.2

Princess (square brilliant): typical ratio 1 to 1, depth 70%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	3.5	4.4	5.1	5.6	6.4	7.0	8.0

Cushion (square): typical ratio 1 to 1, depth 70%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	3.8	4.8	5.5	6.1	7.0	7.7	8.8

Emerald cut (rectangular step): typical ratio 1.4 to 1, depth 62%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	4.5 x 3.2	5.6 x 4.0	6.4 x 4.6	7.1 x 5.1	8.1 x 5.8	8.9 x 6.4	10.2 x 7.3

Square step cut (Asscher style): typical ratio 1 to 1, depth 64%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	3.7	4.6	5.3	5.8	6.6	7.3	8.4

Rectangle with cut corners (radiant style): typical ratio 1.25 to 1, depth 64%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	4.1 x 3.3	5.2 x 4.2	6.0 x 4.8	6.6 x 5.3	7.5 x 6.0	8.3 x 6.6	9.5 x 7.6

Pear: typical ratio 1.55 to 1, depth 61%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	5.5 x 3.5	6.9 x 4.5	7.9 x 5.1	8.7 x 5.6	10.0 x 6.4	11.0 x 7.1	12.6 x 8.1

Marquise: typical ratio 2 to 1, depth 60%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	6.6 x 3.3	8.3 x 4.2	9.5 x 4.8	10.5 x 5.2	12.0 x 6.0	13.2 x 6.6	15.1 x 7.6

Heart: typical ratio 0.95 to 1, depth 60%
Carats	0.25	0.50	0.75	1.00	1.50	2.00	3.00
Size (mm)	4.3 x 4.1	5.4 x 5.1	6.2 x 5.9	6.8 x 6.5	7.8 x 7.4	8.6 x 8.1	9.8 x 9.3

One carat in six shapes, to scale

Each outline is a 1 ct diamond at the typical proportions above, drawn to the millimetre ruler.

Same weight, very different looks. The 1 ct marquise stretches to about 10.5 mm, the emerald cut is 7.1 x 5.1 mm, and the princess is only 5.6 mm square because it carries more weight in its corners and depth.

Find any size

Use the size finder for any weight, shape, depth or ratio not in the tables.

How to read a diamond size chart

Charts describe typical stones. A real diamond's size depends on its exact cut. Grading reports list the measurements, and those beat any chart.
Small differences are invisible. A tenth of a millimetre is hard to see on the hand. Two tenths starts to show on larger stones.
Settings change the look. Thin prongs and a halo make a stone look bigger. A thick bezel hides some of the edge.

Comparing a diamond with moissanite? See the moissanite vs diamond size guide. Comparing natural and lab-grown? See lab-grown vs natural diamond size. For a stone you can measure, the mm to carat calculator works the other way.

Sources and method

Weight estimates use the colored-stone shape factors published by the International Gem Society and the Dendritics standard formula, and the diamond trade constants and length to width ratio tables published in jewelry trade references (Esslinger, Omni, Octonus). Specific gravity values are cross-checked against the International Gem Society density tables, the USGS Natural Gemstones report and GemSelect.

Every number on this page is generated by the same formula engine as the calculators. Read the full method, constants and limits and the editorial policy. Data last verified 2026-11-04.

Estimates only. This is not an appraisal, grading report or valuation. A calibrated scale or a lab report is the only real weight.

Diamond Size Chart in mm

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