Lab Diamond Carat Size Chart: mm by Shape

Carat is weight, not size. This chart shows the typical face-up measurements of well-cut lab-grown diamonds in round, oval and emerald shapes, so you can picture each carat weight before you choose.

Lab diamond carat size chart: round, oval and emerald cut outlines from 0.5 to 4 carats with mm measurements

See it on a hand at true scale

Our ring builder has an On hand view that draws your chosen shape and carat to scale on a US size 6 finger. Open it, pick a diamond, and switch between "Scale drawing" and "On hand".

Open the on-hand view

Carat to mm by shape

Typical ranges for ideal and excellent proportions (oval length-to-width about 1.35 to 1.45, emerald about 1.4). Individual stones vary by a few tenths of a millimetre; the IGI report of each diamond states its exact measurements.

Round

Carat	Face-up size (mm)
0.5 ct	5.1
0.75 ct	5.8
1 ct	6.5
1.5 ct	7.4
2 ct	8.1
2.5 ct	8.7
3 ct	9.3
4 ct	10.2
5 ct	11.0

Oval

Carat	Face-up size (mm)
0.5 ct	6.0 × 4.1
0.75 ct	6.9 × 4.8
1 ct	7.7 × 5.6
1.5 ct	8.8 × 6.3
2 ct	9.6 × 6.9
2.5 ct	10.3 × 7.4
3 ct	11.0 × 7.8
4 ct	12.0 × 8.6
5 ct	13.0 × 9.2

Emerald

Carat	Face-up size (mm)
0.5 ct	6.0 × 4.0
0.75 ct	6.6 × 4.6
1 ct	7.0 × 5.0
1.5 ct	8.0 × 5.6
2 ct	8.7 × 6.1
2.5 ct	9.3 × 6.6
3 ct	9.8 × 7.0
4 ct	10.8 × 7.6
5 ct	11.6 × 8.2

Why size grows slower than carat

Bar chart of round lab diamond diameter in mm from 0.5 to 5 carats

A diamond's weight follows its volume, so its face-up spread grows roughly with the cube root of carat. Going from 1 ct to 2 ct adds about 1.6 mm to a round's diameter, not double the width. Cut quality matters too: a shallow stone can look larger but loses brightness, and a deep stone hides weight in its base.

Choosing a size that suits the hand

On slimmer fingers (US 4 to 6), 1 to 2 ct rounds and ovals already cover much of the finger width.
A hidden halo adds sparkle under the stone without changing the face-up size; a visible halo adds about 1 to 1.5 mm each side.
Toi et moi rings split the weight across two stones, so check each stone's mm, not only total carat.

Read more in our 4Cs guide and how to read an IGI report, or browse the diamond library.

FAQ

How big is a 1 carat lab diamond in mm?

A well-cut 1 ct round lab diamond is typically about 6.4 to 6.5 mm across. A 1 ct oval is about 7.7 x 5.6 mm and a 1 ct emerald cut about 7.0 x 5.0 mm.

Is a lab diamond the same size as a mined diamond of the same carat?

Yes. Carat is weight, and lab and mined diamonds have the same density, so the same cut and proportions give the same size.

Why does a 2 carat diamond not look twice as big as a 1 carat?

Weight grows with volume. Doubling the carat increases the face-up diameter by only about 25 to 26 percent.

Which shape looks biggest for its carat weight?

Elongated shapes such as oval, pear and marquise usually show a larger face-up length than a round of the same weight. Emerald cuts often look slightly smaller because of their deeper, step-cut proportions.

Where can I see the exact size of my diamond?

Every STELLIVON centre diamond comes with an IGI report that lists its measured length, width and depth in mm.