A mounted diamond cannot be put on a scale, so jewellers and appraisers estimate its weight from measurements taken with a gauge. The method multiplies the stone’s dimensions in millimetres by a shape factor: an empirical constant that combines how much of the bounding box a typical stone of that cut fills with diamond’s density of 3.52 g/cm³. A well-cut round brilliant uses 0.0061 × diameter² × depth; fancy shapes use length × width × depth with their own factor, which for emeralds, pears, marquises and radiants depends on how elongated the stone is.
Real stones differ from the ideal proportions the factors assume. A thick girdle hides extra weight around the rim, and bulging pavilions or wide “wings” on pears and marquises add more. The calculator lets you add a girdle correction (0% for thin to medium girdles up to about 10% for extremely thick ones) and a shape correction from −5% to +18%.
Expect the estimate to land within a few percent of the true weight for well-proportioned stones; poorly cut or unusual stones can differ more. For a sale or insurance, a laboratory report with the weighed carat value is the reference.
How to use the Diamond Weight Calculator
- Diamond shape
- Diameter (average girdle diameter)
- Length
- Width
- Depth (table to culet)
- Girdle thickness
- Custom girdle correction
- Shape correction (wings, bulge, culet) — Optional: up to +10% for wide wings or high shoulders on pears, ovals, marquises and hearts; up to +18% for a bulging step-cut pavilion.
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the shape, measure the stone in millimetres (or switch the unit), pick the girdle thickness and any shape correction. The calculator selects or interpolates the shape factor from the length-to-width ratio and multiplies everything out.
Worked example
A round brilliant measures 6.4 mm across and 3.9 mm deep (61% depth), with a medium girdle. Weight = 6.4² × 3.9 × 0.0061 = 40.96 × 3.9 × 0.0061 = 0.974 ct, or 0.195 g. If the girdle were thick (+5%), the estimate would rise to 1.023 ct – the difference between just under and just over the 1-carat mark.
Shape factors
| Shape | Formula (mm) | Factor |
|---|---|---|
| Round | D² × depth | 0.0061 |
| Oval | ((L + W)/2)² × depth | 0.0062 |
| Princess | L × W × depth | 0.0083 |
| Cushion | L × W × depth | 0.00815 |
| Emerald | L × W × depth | 0.0080 (1:1) – 0.0092 (1.5:1) – 0.0100 (2:1) – 0.0106 (2.5:1) |
| Asscher | L × W × depth | 0.0080 |
| Radiant | L × W × depth | 0.0081 (1:1) – 0.0084 (1.5:1) |
| Pear | L × W × depth | 0.00615 (1.25:1) – 0.0060 (1.5:1) – 0.0059 (1.66:1) – 0.00575 (2:1) |
| Marquise | L × W × depth | 0.00565 (1.5:1) – 0.0058 (2:1) – 0.00585 (2.5:1) – 0.00595 (3:1) |
| Heart | L × W × depth | 0.0059 |
| Trillion | L × W × depth | 0.0057 |
| Baguette | L × W × depth | 0.00915 |
Inputs explained
| Field | What to enter |
|---|---|
| Diamond shape | Choose one: Round brilliant, Oval, Princess, Cushion … |
| Diameter (average girdle diameter) | Units: mm, cm, in |
| Length | Units: mm, cm, in |
| Width | Units: mm, cm, in |
| Depth (table to culet) | Units: mm, cm, in |
| Girdle thickness | Choose one: Very thin to medium (+0%), Slightly thick (+3%), Thick (+5%), Very thick (+8%) … |
| Custom girdle correction | In % |
| Shape correction (wings, bulge, culet) | In %. Optional: up to +10% for wide wings or high shoulders on pears, ovals, marquises and hearts; up to +18% for a bulging step-cut pavilion. |
What each result means
- Weight in grams = [Estimated weight]×0.2
- Weight in points = [Estimated weight]×100
Formula
Round: ct = D² × depth × 0.0061 · Oval: ct = ((L + W)/2)² × depth × 0.0062
Other cuts: ct = L × W × depth × k (k from the shape and length-to-width ratio)
Then × (1 + girdle %) × (1 + shape correction %) · dimensions in mm
Worked example
Inputs — Diamond shape: Round brilliant; Diameter (average girdle diameter): 6.4 mm; Depth (table to culet): 3.9 mm; Girdle thickness: Very thin to medium (+0%).
Results — Shape factor used = 0.0061; Correction multiplier = 1; Estimated weight = 0.974 ct; Depth percentage = 60.9 %; Weight in grams = 0.1949 g; Weight in points = 97 pt.
Key terms
- Carat (ct)
- Unit of mass for gems, exactly 200 milligrams; 1 ct = 100 points.
- Shape factor
- Empirical constant that turns length × width × depth (in mm) into carats for one cut; it reflects the stone's volume fraction and diamond's density (3.52 g/cm³).
- Girdle
- The thin rim around the widest part of the stone; a thick girdle adds weight that the dimensions alone do not show.
- Length-to-width ratio
- Length ÷ width of a fancy shape; elongated emeralds, pears and marquises use a ratio-dependent factor.
- Depth percentage
- Total depth ÷ width (or diameter) × 100; about 59–63% for a well-cut round brilliant.
Frequently asked questions
How do you estimate a diamond's carat weight?
Measure it in millimetres and use the shape formula: for a round brilliant, diameter² × depth × 0.0061. A 6.4 mm stone 3.9 mm deep weighs about 0.97 ct.
What size is a 1-carat round diamond?
About 6.4–6.5 mm across with a depth of 3.9–4.0 mm (around 61% depth).
Why do fancy shapes need a ratio?
A long emerald fills its bounding box more fully than a square one, so its factor rises from 0.0080 at 1:1 to 0.0106 at 2.5:1; pears and marquises change in the opposite or a smaller way.
How much does a thick girdle add?
Typically 3–8% depending on thickness, and up to 10% for an extremely thick girdle.
How accurate are these estimates?
Usually within about 5% for well-proportioned stones; heavily bulging pavilions or very uneven girdles increase the error.
Is a carat a measure of size?
No, it is a mass: 1 carat = 0.2 g. Two 1-carat diamonds of different cuts can look quite different in size.
Can I use this for other gemstones?
Not directly – coloured stones have different densities (e.g. ruby and sapphire 4.0 g/cm³), so the factor must be multiplied by their specific gravity ÷ 3.52.