How Many Carats Is A 12 mm Diamond?
A 12 mm round brilliant weighs about 6.43 ct. See how depth changes it, the weight in grams and cents 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, 12 mm
6.43 ct
About 1.29 g · 643 cents
- Formula
- 12² × 7.32 × 0.0061
- Total depth
- 7.32 mm (61.0%)
- At 59–63% depth
- 6.22–6.64 ct
- In grams
- 1.29 g
- In cents (points)
- 643
Typical sizes for well-cut rounds
| Weight | Trade charts | Formula, 61% |
|---|---|---|
| 5.00 ct | 11 mm | 11.03 mm |
| 6.00 ct | 11.7 mm | 11.73 mm |
| 7.00 ct | 12.4 mm | 12.34 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. 12 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, and compare it with the trade charts.
12 mm round diamond weight at a glance
A 12 mm round brilliant cut to a typical 61% depth (7.32 mm) weighs about 6.43 ct by the standard formula: 12 × 12 × 7.32 × 0.0061. That is 1.29 g, or 643 cents. The trade size chart from Walt's Jewelers puts a 12 mm round at about 6.43 ct.
Weight by depth
| Depth | Depth in mm | Weight |
|---|---|---|
| 59% | 7.08 mm | 6.22 ct |
| 61% | 7.32 mm | 6.43 ct |
| 63% | 7.56 mm | 6.64 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 12 mm
| Diameter | At 61% depth | At 59–63% | Cents |
|---|---|---|---|
| 11.7 mm | 5.96 ct | 5.76–6.15 ct | 596 |
| 11.8 mm | 6.11 ct | 5.91–6.31 ct | 611 |
| 11.9 mm | 6.27 ct | 6.06–6.48 ct | 627 |
| 12 mm | 6.43 ct | 6.22–6.64 ct | 643 |
| 12.1 mm | 6.59 ct | 6.38–6.81 ct | 659 |
| 12.2 mm | 6.76 ct | 6.54–6.98 ct | 676 |
| 12.3 mm | 6.92 ct | 6.70–7.15 ct | 692 |
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 11 mm.
FAQ
Questions
How many carats is a 12 mm round diamond?
About 6.43 ct by the standard formula (diameter² × depth × 0.0061) with a typical 61% depth of 7.32 mm. The trade size chart from Walt's Jewelers puts a 12 mm round at about 6.43 ct.
How much does depth change the weight of a 12 mm diamond?
At 59% depth (7.08 mm) it weighs about 6.22 ct and at 63% (7.56 mm) about 6.64 ct, a spread of about 7%. That is why two 12 mm stones can be sold as different weights. Enter the real depth in the calculator for a closer figure.
What is a 12 mm diamond in grams and cents?
About 1.29 g and 643 cents (points), because 1 carat is 0.2 g and 100 cents.
How do I check that a diamond is really 12 mm?
Measure across the girdle with a millimetre gauge, or read the measurements on the grading report, written like 11.98–12.02 × 7.32 mm (smallest and largest diameter × depth). Use the average diameter and the total depth in the calculator.
Does a 12 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 12 mm lab-grown round at 61% depth also weighs about 6.43 ct. Moissanite weighs less and cubic zirconia more at the same size.
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