What is my gemstone weight?
Measure the stone in millimeters, pick its cut and the gem. The gemstone weight calculator estimates the carat weight, which helps with a stone already set in jewelry.
- Estimated weight
- 1.07
The gem weighs about 1.07 carats.
- Weight in grams
- 0.214
- Diameter used (mm)
- 6.5
- Shape factor
- 0.0018
Estimated weight: 1.07. The gem weighs about 1.07 carats.
How to calculate
Estimates a gemstone’s weight in carats from its measurements in millimeters, its cut and its specific gravity, with the International Gem Society shape formulas.
Example with the default inputs (Cut Round, Diameter (mm) 6.5, Depth (mm) 4, Gem Diamond (3.52)): The gem weighs about 1.07 carats.
Method: Rounds, ovals and squares: diameter² × depth × SG × factor. Rectangles, pears, hearts, marquises, triangles and cabochons: length × width × depth × SG × factor.
- The result is an estimate for a well-cut stone. IGS gives corrections for thick girdles, bulging pavilions and other cuts that this calculator does not apply.
- Measurements are in millimeters; the result is in metric carats (1 ct = 0.2 g).
Worked examples
Each example is checked against the calculator on every build.
- Cut Round, Diameter (mm) 6.5, Depth (mm) 4, Gem Diamond (3.52) gives Estimated weight 1.07, Weight in grams 0.214, Diameter used (mm) 6.5, Shape factor 0.0018.Source: International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight; GIA Gem Encyclopedia (specific gravity: diamond 3.52, ruby and sapphire 4.00, emerald 2.72, amethyst 2.66), https://www.gia.edu/diamond
- Cut Oval, Length (mm) 8, Width (mm) 6, Diagonal (mm) 7, Depth (mm) 4, Gem Ruby or sapphire (4.00) gives Estimated weight 1.57, Diameter used (mm) 7.Source: International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight; GIA, Ruby (SG 4.00), https://www.gia.edu/ruby
- Cut Rectangle step cut (emerald cut), Length (mm) 9, Width (mm) 7, Depth (mm) 5, Gem Emerald (2.72) gives Estimated weight 2.14.Source: International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight; GIA, Emerald (SG 2.72), https://www.gia.edu/emerald
- Cut Rectangular cushion, Length (mm) 10, Width (mm) 8, Depth (mm) 6, Gem Type my own, Specific gravity 3.1 gives Estimated weight 3.31, Diameter used (mm) 9.Source: International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight
How it works
All measurements are in millimeters; the result is in metric carats.
Diameter-based cuts: carats = D × D × depth × SG × factor, where D is
- Round: the diameter typed;
- Oval, square, square cushion, square step cut, square with cut corners: (diagonal + length + width) ÷ 3;
- Rectangular cushion: (length + width) ÷ 2.
Length × width cuts (rectangles, pears, hearts, marquises, triangles, cabochons): carats = length × width × depth × SG × factor.
Shape factors (International Gem Society):
| Cut | Formula | Factor |
|---|---|---|
| Round | diameter | 0.0018 |
| Oval | (diagonal + length + width) ÷ 3 | 0.0020 |
| Square | (diagonal + length + width) ÷ 3 | 0.0024 |
| Square cushion | (diagonal + length + width) ÷ 3 | 0.0018 |
| Square step cut | (diagonal + length + width) ÷ 3 | 0.0023 |
| Square with cut corners | (diagonal + length + width) ÷ 3 | 0.0024 |
| Rectangular cushion | (length + width) ÷ 2 | 0.0022 |
| Rectangle | length × width | 0.0026 |
| Rectangle step cut (emerald cut) | length × width | 0.0025 |
| Rectangle with cut corners | length × width | 0.0026 |
| Pear | length × width | 0.0018 |
| Heart | length × width | 0.0021 |
| Marquise | length × width | 0.0017 |
| Triangle | length × width | 0.0018 |
| Cabochon, high dome | length × width | 0.0026 |
| Cabochon, low dome | length × width | 0.0029 |
Specific gravity (SG): diamond 3.52, ruby or sapphire 4.00, emerald 2.72, amethyst 2.66 (GIA), quartz 2.65 (IGS), or a typed value from 0.5 to 25. With Type my own and no value: "Enter the specific gravity."
Grams = carats × 0.2.
Each measurement is from 0.1 to 1,000 mm. Typed values are read as exact decimals and every step is exact. Shown values are rounded half up: carats and diameter to 2 decimals, grams to 3 decimals. No girdle or pavilion correction is applied.
Worked examples by hand
Round diamond, 6.5 mm across, 4 mm deep: 6.5 × 6.5 × 4 × 3.52 × 0.0018 = 1.07 ct (1.070784), 0.214 g.
Oval ruby, 8 × 6 mm, diagonal 7 mm, 4 mm deep: D = (7 + 8 + 6) ÷ 3 = 7. 7 × 7 × 4 × 4.00 × 0.0020 = 1.57 ct (1.568).
Emerald-cut emerald, 9 × 7 × 5 mm: 9 × 7 × 5 × 2.72 × 0.0025 = 2.14 ct (2.142).
Rectangular cushion, 10 × 8 × 6 mm, SG 3.1: D = (10 + 8) ÷ 2 = 9. 9 × 9 × 6 × 3.1 × 0.0022 = 3.31 ct (3.31452).
Other questions people ask
How do I estimate a gem’s carat weight from its size?
Measure its length, width and depth in millimeters, then use the formula for its cut. For a round stone, carats = diameter × diameter × depth × specific gravity × 0.0018. A 6.5 mm round diamond 4 mm deep is about 6.5 × 6.5 × 4 × 3.52 × 0.0018 = 1.07 ct.
Why does the gem type matter?
Gems differ in density. Specific gravity is a gem’s density compared with water: a sapphire (4.00) weighs about 14% more than a diamond (3.52) of the same size, and an emerald (2.72) less.
How accurate is the estimate?
It assumes a well-proportioned stone. The International Gem Society suggests adding 1% to 10% for a thick girdle and up to 18% for a bulging pavilion, and other corrections for unusual outlines. Only a scale gives the true weight.
What is the diagonal for ovals and squares?
The IGS formula for most diameter-based cuts uses the average of three measurements across the top: length, width and the diagonal between them (corner to corner on a square). A round uses its diameter, and a rectangular cushion the average of length and width.
How many grams is a carat?
A metric carat is exactly 200 milligrams, 0.2 g. So a 1.07 ct stone weighs 0.214 g.