How Many Carats Is A 9 mm Diamond?
A 9 mm round brilliant weighs about 2.71 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, 9 mm
2.71 ct
About 0.543 g · 271 cents
- Formula
- 9² × 5.49 × 0.0061
- Total depth
- 5.49 mm (61.0%)
- At 59–63% depth
- 2.62–2.80 ct
- In grams
- 0.543 g
- In cents (points)
- 271
Typical sizes for well-cut rounds
| Weight | Trade charts | Formula, 61% |
|---|---|---|
| 2.00 ct | 8–8.2 mm | 8.13 mm |
| 2.50 ct | 8.6–9 mm | 8.76 mm |
| 3.00 ct | 9.3 mm | 9.31 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. 9 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.
9 mm round diamond weight at a glance
A 9 mm round brilliant cut to a typical 61% depth (5.49 mm) weighs about 2.71 ct by the standard formula: 9 × 9 × 5.49 × 0.0061. That is 0.543 g, or 271 cents. Trade size charts from Stuller and Walt's Jewelers put a 9 mm round at about 2.50–2.91 ct.
Weight by depth
| Depth | Depth in mm | Weight |
|---|---|---|
| 59% | 5.31 mm | 2.62 ct |
| 61% | 5.49 mm | 2.71 ct |
| 63% | 5.67 mm | 2.80 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 9 mm
| Diameter | At 61% depth | At 59–63% | Cents |
|---|---|---|---|
| 8.7 mm | 2.45 ct | 2.37–2.53 ct | 245 |
| 8.8 mm | 2.54 ct | 2.45–2.62 ct | 254 |
| 8.9 mm | 2.62 ct | 2.54–2.71 ct | 262 |
| 9 mm | 2.71 ct | 2.62–2.80 ct | 271 |
| 9.1 mm | 2.80 ct | 2.71–2.90 ct | 280 |
| 9.2 mm | 2.90 ct | 2.80–2.99 ct | 290 |
| 9.3 mm | 2.99 ct | 2.89–3.09 ct | 299 |
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 8.5 mm; one size up is 10 mm.
FAQ
Questions
How many carats is a 9 mm round diamond?
About 2.71 ct by the standard formula (diameter² × depth × 0.0061) with a typical 61% depth of 5.49 mm. Trade size charts from Stuller and Walt's Jewelers put a 9 mm round at about 2.50–2.91 ct.
How much does depth change the weight of a 9 mm diamond?
At 59% depth (5.31 mm) it weighs about 2.62 ct and at 63% (5.67 mm) about 2.80 ct, a spread of about 7%. That is why two 9 mm stones can be sold as different weights. Enter the real depth in the calculator for a closer figure.
What is a 9 mm diamond in grams and cents?
About 0.543 g and 271 cents (points), because 1 carat is 0.2 g and 100 cents.
How do I check that a diamond is really 9 mm?
Measure across the girdle with a millimetre gauge, or read the measurements on the grading report, written like 8.98–9.02 × 5.49 mm (smallest and largest diameter × depth). Use the average diameter and the total depth in the calculator.
Does a 9 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 9 mm lab-grown round at 61% depth also weighs about 2.71 ct. Moissanite weighs less and cubic zirconia more at the same size.
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