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LILY DIA

Diamond Carat Size Chart

Jared James, co-founder of LILY DIA

By Jared James · Last updated 12 July 2026

Quick answer

A well-cut 1 carat round diamond measures about 6.4mm across, a 2 carat about 8.1mm and a 3 carat about 9.3mm. This chart crosses 1, 1.5, 2, 2.5 and 3 carats with nine shapes so you can compare the face-up size in mm before you compare prices. Every value is approximate and assumes the listed length-to-width ratio.

Carat size chart in mm

Face-up measurements for well-cut stones. Symmetric shapes show one number (length equals width); elongated shapes show length by width at the assumed ratio in brackets. A stone cut deep for weight retention will face up smaller than this table.

Shape1ct1.5ct2ct2.5ct3ct
Round brilliant6.4mm7.4mm8.1mm8.7mm9.3mm
Oval (1.40)8.3 x 5.6mm9.5 x 6.4mm10.5 x 7.0mm11.3 x 7.5mm12.0 x 8.0mm
Cushion (1.00)6.0mm6.8mm7.5mm8.1mm8.6mm
Princess (1.00)5.6mm6.4mm7.0mm7.5mm8.0mm
Emerald cut (1.40)7.1 x 5.2mm8.2 x 5.9mm9.0 x 6.5mm9.7 x 7.0mm10.3 x 7.4mm
Asscher (1.00)5.6mm6.4mm7.0mm7.5mm8.0mm
Pear (1.50)8.7 x 5.6mm10.0 x 6.4mm11.0 x 7.0mm11.8 x 7.5mm12.6 x 8.0mm
Marquise (2.00)10.3 x 5.2mm11.8 x 5.9mm13.0 x 6.5mm14.0 x 7.0mm14.9 x 7.4mm
Radiant (1.10)6.3 x 5.2mm7.3 x 5.9mm8.0 x 6.5mm8.6 x 7.0mm9.2 x 7.4mm

All values are approximate. The measurements line on an individual grading report is the final word for any specific stone.

Round sizes at one scale

Round brilliant diamond outlines at 1, 1.5, 2, 2.5 and 3 carats drawn at one common scale by Lily Dia Jewellery
The five round sizes drawn at one shared scale. The jump from 1ct to 2ct is about 27% more width; from 2ct to 3ct it is another 15%. Screens scale images unpredictably, so treat the proportions as accurate and the absolute size as illustrative.

Check the size on paper

The most reliable way to preview a carat size is to draw it. Take a ruler with mm markings, draw a circle or rectangle at the measurements from the chart, cut it out and hold it against your finger. Paper does not lie the way screens do.

If you prefer to print, our ring size chart page includes a printable sizer PDF with a built-in check ruler; print at 100% scale, confirm the check ruler measures true, and anything else you print at the same setting will be true to size as well.

Why carat is not a size

Carat measures weight: one carat is 200 milligrams. The size you see on the hand is the face-up area, and the two are related but not interchangeable. Weight grows with volume, so doubling the carat adds roughly 26% to each dimension rather than doubling the width, and part of every extra carat disappears into the depth of the stone where you cannot see it.

Shape moves the equation more than most buyers expect. At every weight in the chart, the marquise is the longest stone and the princess and Asscher are the most compact, because elongated shapes spread their weight thin and square shapes stack it deep. Cut quality matters too: two stones with identical weight and ratio can differ by a few tenths of a millimetre face up depending on how well they are cut.

Size guides by weight

  1. #01

    How big is a 1 carat diamond?

    The 1ct size in mm for every shape, on the hand and next to 2 carats at one scale.

  2. #02

    How big is a 2 carat diamond?

    The 2ct size table with on-hand photos and AUD price bands.

  3. #03

    Is a 3 carat diamond big?

    The 3ct size in mm for every shape and how a stone that size wears day to day.

  4. #04

    What diamond carat weight really means

    Why carat is weight rather than size, and how cut changes what you see.

What each weight costs

  1. #01

    2 carat lab diamond ring price in Australia

    Dated AUD prices for the loose stone, the setting and the complete ring, specification held constant.

  2. #02

    3 carat lab diamond ring price in Australia

    What 3 carats costs as a lab-grown stone plus setting, and how it compares with mined.

See them in a setting

Compare carat sizes in real rings

Browse lab-grown diamond engagement rings across every shape and carat weight, made to order in Melbourne.

View lab-grown rings

Frequently asked questions

Is carat a size or a weight?
A weight. One carat is 200 milligrams. Two diamonds of the same carat weight can face up at different sizes depending on their shape, their length-to-width ratio and how deep they were cut, which is why the mm measurements on a grading report matter more than the carat number alone.
How many mm is a 1 carat diamond?
A well-cut 1 carat round brilliant measures about 6.4mm across. The same weight in an oval is about 8.3mm long, and a 1 carat marquise stretches to about 10.3mm.
How many mm is a 2 carat diamond?
A well-cut 2 carat round brilliant measures about 8.1mm across. A 2 carat oval is about 10.5mm by 7.0mm, and a 2 carat emerald cut about 9.0mm by 6.5mm.
How many mm is a 3 carat diamond?
A well-cut 3 carat round brilliant measures about 9.3mm across. A 3 carat oval is about 12.0mm by 8.0mm, and a 3 carat pear about 12.6mm long.
Why does doubling the carat not double the size?
Because weight grows with volume while the size you see is an area. Doubling the weight from 1 to 2 carats makes a round diamond about 27% wider and gives it about 60% more face-up area, not twice as much. The extra weight also sits partly in the depth of the stone.
Are the sizes in this chart exact?
No, they are typical measurements for well-cut stones at the listed length-to-width ratio. A deep-cut diamond hides weight underneath and faces up smaller, and a different ratio moves both numbers. Always check the measurements line on the individual grading report.

Want to compare stones

Not sure which carat weight suits the hand?

Send us a note with the finger size and the shapes you like, and we will show you real stones at each weight.

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