# What is my gemstone weight?

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.

- Page: https://www.acalculator.org/everyday-life/gemstone-weight-calculator
- JSON spec: https://www.acalculator.org/everyday-life/gemstone-weight-calculator.json
- Version: ecf89c67ed9c

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| shape | Cut | The shape and cut of the gem. |
| dia | Diameter (mm) | The width across a round stone, in millimeters. |
| l | Length (mm) | The longest measurement across the top, in millimeters. |
| w | Width (mm) | The measurement across the top at right angles to the length, in millimeters. |
| diag | Diagonal (mm) | The measurement across the top halfway between the length and the width (corner to corner on a square), in millimeters. |
| d | Depth (mm) | From the top (table) to the bottom point (culet), in millimeters. |
| gem | Gem | The gem, for its specific gravity (density compared with water). |
| sg | Specific gravity | The gem’s specific gravity, from a gem reference. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| carats | Estimated weight | The IGS estimate in carats. |
| grams | Weight in grams | Carats × 0.2 (a metric carat is 200 mg). |
| size | Diameter used (mm) | The diameter in the formula, for rounds, ovals and squares. |
| factor | Shape factor | The IGS factor for the cut. |

## Method

Rounds, ovals and squares: diameter² × depth × SG × factor. Rectangles, pears, hearts, marquises, triangles and cabochons: length × width × depth × SG × factor.

## Assumptions

- 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

1. shape = round, dia = 6.5, d = 4, gem = diamond gives carats = 1.07, grams = 0.214, size = 6.5, 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.
2. shape = oval, l = 8, w = 6, diag = 7, d = 4, gem = corundum gives carats = 1.57, size = 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.
3. shape = rect-step, l = 9, w = 7, d = 5, gem = emerald gives carats = 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.
4. shape = rect-cushion, l = 10, w = 8, d = 6, gem = custom, sg = 3.1 gives carats = 3.31, size = 9. Source: International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight.

## FAQ

### 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.

## Sources

- International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (formulas and shape factors for faceted gems and cabochons; girdle and pavilion corrections; updated 2023-04-26). https://www.gemsociety.org/article/estimating-gem-weight (retrieved 2026-10-19)
- GIA Gem Encyclopedia: diamond (specific gravity 3.52), https://www.gia.edu/diamond; ruby (4.00), https://www.gia.edu/ruby; sapphire (4.00), https://www.gia.edu/sapphire; emerald (2.72), https://www.gia.edu/emerald; amethyst (2.66), https://www.gia.edu/amethyst (retrieved 2026-10-19)
- International Gem Society, Specific Gravity Values of Selected Gems (quartz 2.65). https://www.gemsociety.org/article/select-gems-ordered-density/ (retrieved 2026-10-19)
- NIST Special Publication 811 (2008), Appendix B.8: carat, metric = 2 × 10⁻⁴ kg exactly. https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-19)