How Many Carats Is A 3 mm Diamond?
A 3 mm round brilliant weighs about 0.10 ct. See how depth changes it, the weight in grams and cents, its sieve size and the sizes around it, then adjust the stone below.
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Round brilliant estimate. Size charts from Stuller (May 2025) and Walt's Jewelers, checked 9 November 2026.
Your stone
Shape
Measured across the girdle. On a certificate, average the smallest and largest diameter.
Table to culet, in mm or as a % of the diameter. Leave it blank to assume 61%, typical for a well-cut round.
Estimated weight
Round, 3 mm
0.10 ct
About 0.0201 g · 10 cents
- Formula
- 3² × 1.83 × 0.0061
- Total depth
- 1.83 mm (61.0%)
- At 59–63% depth
- 0.10–0.10 ct
- In grams
- 0.0201 g
- In cents (points)
- 10
- Stones per carat
- about 10
- Sieve size
- +12.5 -13
Typical sizes for well-cut rounds
| Weight | Trade charts | Formula, 61% |
|---|---|---|
| 0.07 ct | 2.7 mm | 2.66 mm |
| 0.10 ct | 3 mm | 3 mm |
| 0.15 ct | 3.4 mm | 3.43 mm |
Diameters from Stuller's size chart (2025) and Walt's Jewelers' chart. The closest is highlighted.
An estimate from the stone's measurements. A thick girdle or a bulging pavilion adds weight; weigh a loose stone to be sure.
How to use it
Measure the stone across the girdle. 3 mm is filled in for you; change it to your stone's size.
Add the total depth from the certificate if you have it, or leave it blank to assume 61%.
Read the weight in carats, grams and cents, with the sieve size, and compare it with the trade charts.
3 mm round diamond weight at a glance
A 3 mm round brilliant cut to a typical 61% depth (1.83 mm) weighs about 0.10 ct by the standard formula: 3 × 3 × 1.83 × 0.0061. That is 0.0201 g, or 10 cents. Trade size charts from Stuller and Walt's Jewelers put a 3 mm round at 0.10 ct.
Weight by depth
| Depth | Depth in mm | Weight |
|---|---|---|
| 59% | 1.77 mm | 0.10 ct |
| 61% | 1.83 mm | 0.10 ct |
| 63% | 1.89 mm | 0.10 ct |
A deeper stone hides more of its weight below the girdle, so it faces up smaller for its weight. Well-cut rounds are usually between about 59% and 63%.
Sizes around 3 mm
| Diameter | At 61% depth | At 59–63% | Cents |
|---|---|---|---|
| 2.7 mm | 0.073 ct | 0.071–0.076 ct | 7.3 |
| 2.8 mm | 0.082 ct | 0.079–0.084 ct | 8.2 |
| 2.9 mm | 0.091 ct | 0.088–0.094 ct | 9.1 |
| 3 mm | 0.10 ct | 0.10–0.10 ct | 10 |
| 3.1 mm | 0.11 ct | 0.11–0.11 ct | 11 |
| 3.2 mm | 0.12 ct | 0.12–0.13 ct | 12 |
| 3.3 mm | 0.13 ct | 0.13–0.14 ct | 13 |
Sieve size
A 3 mm round falls in the +12.5 -13 sieve (3.00–3.10 mm). Published sieve charts put stones in that range at about 0.108 ct each, roughly 9.3 stones per carat. See the full diamond sieve size chart to compare neighbouring sieves.
Other shapes and sizes
For an oval, pear, princess or other fancy shape, or to find the size for a carat weight, use the diamond mm to carat calculator. One size down is 2.5 mm; one size up is 3.5 mm.
FAQ
Questions
How many carats is a 3 mm round diamond?
About 0.10 ct by the standard formula (diameter² × depth × 0.0061) with a typical 61% depth of 1.83 mm. Trade size charts from Stuller and Walt's Jewelers put a 3 mm round at 0.10 ct.
How much does depth change the weight of a 3 mm diamond?
At 59% depth (1.77 mm) it weighs about 0.10 ct and at 63% (1.89 mm) about 0.10 ct, a spread of about 7%. That is why two 3 mm stones can be sold as different weights. Enter the real depth in the calculator for a closer figure.
What is a 3 mm diamond in grams and cents?
About 0.0201 g and 10 cents (points), because 1 carat is 0.2 g and 100 cents.
What sieve size is a 3 mm diamond?
It falls in the +12.5 -13 sieve (3.00–3.10 mm). Published sieve charts put stones in that range at about 0.108 ct each, roughly 9.3 stones per carat. Charts vary a little between suppliers, so agree the range with yours.
Does a 3 mm lab-grown diamond weigh the same?
Yes, if it is cut the same. Lab-grown and natural diamonds are the same material with the same density, so a 3 mm lab-grown round at 61% depth also weighs about 0.10 ct. Moissanite weighs less and cubic zirconia more at the same size.
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