Diamond Carat Weight Calculator
This calculator converts between diamond carat weight and grams, and estimates carat weight from physical dimensions using GIA dimension-to-weight approximation formulas. The metric carat (0.2 g exactly) is the international standard unit for gemstone weight, defined by the International Bureau of Weights and Measures. Note that dimension-based estimates are approximations only; accurate carat weight requires weighing on a calibrated precision balance.
Carat conversion formula
1 carat (ct) = 0.2 grams (g) = 200 milligrams (mg)
GIA round brilliant weight estimate: ct = d2 x depth x 0.0061, where d = average diameter in mm and depth = total depth in mm. This is an approximation for well-proportioned round brilliants. Source: Gemological Institute of America (GIA) Gem Diamond course; BIPM SI Brochure (metric carat definition).
Typical round brilliant dimensions by carat
| Carat Weight | Approx. Diameter (mm) | Approx. Depth (mm) |
|---|---|---|
| 0.25 ct | 4.1 | 2.5 |
| 0.50 ct | 5.2 | 3.1 |
| 0.75 ct | 5.9 | 3.6 |
| 1.00 ct | 6.5 | 4.0 |
| 1.50 ct | 7.4 | 4.5 |
| 2.00 ct | 8.2 | 5.0 |
Diamond carat weight calculator: frequently asked questions
What is a carat in gemology?
A metric carat (ct) is the standard unit of mass for gemstones. One metric carat equals exactly 0.2 grams (200 milligrams). The metric carat has been the international standard since 1907 and is defined by the International Bureau of Weights and Measures (BIPM). It is not related to karat (purity of gold alloys).
How is diamond weight estimated from dimensions?
The GIA (Gemological Institute of America) provides dimension-based formulas. For a round brilliant cut: estimated carat weight = diameter2 x depth x 0.0061. For an oval cut: length x width x depth x 0.0062. These are approximations; actual carat weight requires weighing on a precision balance.
What is the point system for small diamonds?
One carat is divided into 100 points. A 0.50 ct diamond is called a 50-point or half-carat diamond. A 0.25 ct diamond is a 25-point or quarter-carat diamond. The point system is widely used in the trade for diamonds below one carat.
Does carat weight equal size?
Carat is a unit of mass, not size. Two diamonds of equal carat weight can appear different sizes depending on their cut proportions. A well-cut round brilliant at 1.00 ct has an average diameter of about 6.5 mm, but poorly cut diamonds of the same weight may appear smaller because their mass is hidden in excess depth.
What carat weights are standard commercial sizes?
Common commercial sizes: 0.25 ct (quarter carat), 0.33 ct (one-third), 0.50 ct (half carat), 0.75 ct (three-quarter), 1.00 ct, 1.50 ct, 2.00 ct, and so on. Stones just below these milestones (such as 0.97 ct) are significantly less expensive than stones at or above the milestone weight.
Official sources
- Gemological Institute of America (GIA): GIA Diamond Quality Factors.
- International Bureau of Weights and Measures (BIPM): BIPM Units of Measurement (metric carat).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.