Diamond carat weight ratio
On this page, you'll find a handy conversion table to see the ratio between the diameter of a brilliant-cut diamond and its corresponding carat weight. Ideal if you want to estimate the value of a diamond in a piece of jewelry, but can't weigh the stone.
Carat weight and diameter: how does that work?
The weight of a diamond is expressed in carats. One carat is 0.2 grams. The carat weight has a significant impact on the value, but it doesn't tell the whole story: two diamonds with the same carat can still look slightly different, depending on the cut and proportions.
For a round, brilliant-cut diamond, you can often make a good estimate of the carat weight based on the diameter in millimeters. This is always an approximation: a professional jeweler's scale is needed for an exact value.
The 4 C's in Brief
With diamonds, the four C's are often discussed:
- Carat: the weight of the diamond
- Color: how colorless or tinted the stone is
- Clarity: the amount and visibility of inclusions
- Cut: the way the diamond is cut and how beautifully it sparkles
On this page, we focus primarily on the ratio between diameter and carat weight for round/brilliant-cut diamonds.
How to measure the diameter of a brilliant-cut diamond
If the diamond is already set in a piece of jewelry, you usually can't weigh it. You can then measure the diameter to estimate the carat weight.
1. Measure from above
Look straight at the diamond from above (top view). Measure the distance from one side of the stone to the other, across the widest part of the roundness (the rondist).
2. Use a caliper
For the most accurate measurement, use a caliper. Record the diameter in millimeters and find the corresponding carat weight in the table below. Note: The table is only suitable for round/brilliant cut diamonds with normal proportions.
Brilliant-cut diamond conversion chart
Use this chart to calculate the carat weight of a round/brilliant-cut diamond based on the measured diameter (in millimeters).
| Diameter (mm) | Carat (approx.) |
|---|---|
| 0.9 | 0.005 |
| 1.4 | 0.01 |
| 1.7 | 0.02 |
| 1.9 | 0.03 |
| 2.2 | 0.04 |
| 2.4 | 0.05 |
| 2.6 | 0.06 |
| 2.7 | 0.07 |
| 2.8 | 0.08 |
| 2.9 | 0.09 |
| 3.0 | 0.10 |
| 3.1 | 0.11 |
| 3.2 | 0.12 |
| 3.3 | 0.14 |
| 3.4 | 0.15 |
| 3.5 | 0.16 |
| 3.6 | 0.17 |
| 3.7 | 0.18–0.19 |
| 3.8 | 0.20–0.21 |
| 3.9 | 0.22 |
| 4.0 | 0.23–0.24 |
| 4.1 | 0.25–0.26 |
| 4.2 | 0.27–0.28 |
| 4.3 | 0.29–0.30 |
| 4.5 | 0.35 |
| 4.8 | 0.40 |
| 5.1 | 0.45 |
| 5.3 | 0.50 |
| 5.4 | 0.55 |
| 5.5 | 0.60 |
| 5.6 | 0.65 |
| 5.8 | 0.70 |
| 5.9 | 0.75 |
| 6.5 | 1.00 |
| 7.0 | 1.25 |
| 7.5 | 1.50 |
| 7.8 | 1.75 |
| 8.2 | 2.00 |
Important notes
The table provides a guideline, not an exact result. Small differences in cut depth, proportions, and setting can affect the actual carat weight.If you want to know the exact weight, a measurement on a calibrated jeweler's scale is always the best option.
Do you have a diamond or diamond jewelry and would you like to have its quality and weight professionally assessed? Please feel free to contact us. We're happy to discuss your options.