Coloured gemstone carat-weight calculator
Estimate a coloured gemstone’s carat weight from its measurements.
Made clearer, step by step
Your estimate starts here
Choose your gemstone and its shape, then enter its measurements. We’ll show the estimated weight here.
How it works & reference guideMeasuring tips, examples and the details behind this tool
Methodology
How the formula works
estimated carats = length × width × depth × specific gravity × shape factor
Dimensions are millimetres and specific gravity (SG) is dimensionless. The cited shape factor incorporates the millimetre-to-carat scaling; no additional gram-to-carat conversion is applied.
The lower and upper typical SG values give the two ends of the range. Each shape factor assumes the typical (average) proportions shown in the measuring guide, so a stone with unusual proportions can fall outside the range. No extra margin or professional weight-correction factor is added.
Round outlines use the product of two perpendicular face-up diameters. Square outlines use two maximum side measurements. Elongated outlines use maximum length and width. Reversed face-up measurements are normalised visibly in the result.
Worked example: natural solid corundum
A natural ruby or sapphire, oval faceted brilliant, measured loose at 10 × 8 × 5 mm, with typical proportions and no filling, coating or assembly:
10 × 8 × 5 × (3.80–4.05) × 0.00210 = 3.192–3.402 ct
3.19–3.41 ct
The estimated weight range, rounded outwards. The two ends come from the lowest and highest typical specific gravity; the actual weight can fall outside it.
1 ct = 100 points. When the entire interval is below 0.10 ct, results use points, rounded outward to 0.1 point, with equivalent carats to 0.001 ct. Otherwise genuine intervals round outward to 0.01 ct. An equal-bound result is shown as one approximate value.
Measuring guide
How to measure your stone
These diagrams show where to measure and the typical proportions each estimate assumes. They can't identify a stone or check its proportions. Measure the depth on a loose stone; a depth guessed through a setting won't give a reliable estimate.
Length, width and full depth
Measure round stones across the top twice, at right angles to each other, and square stones from side to side in both directions (not corner to corner). Full depth runs from the highest point of the top or dome to the lowest point underneath.
Average faceted brilliant
Approximately 30% crown and 70% pavilion, a thin-to-medium girdle and no culet. A stone that is unusually shallow, deep, flat, bulging or lopsided can weigh noticeably more or less than the estimate.
Average rectangular emerald cut
The estimate assumes a keel (the ridge where the base facets meet) about one-third of the stone's length. It covers rectangular emerald cuts only, not other step cuts.
Average cabochon
The estimate assumes an average dome and shoulders like this. Choose a cabochon option only for a smooth stone with no facets; an unusually flat, high or bulging dome can weigh noticeably more or less.
Supported specific-gravity ranges
These nine exact material records use Gem-A's 2017 Foundation constants sheet. They are typical reference ranges; an individual specimen can fall outside them. The construction and treatment limits listed below apply to every material.
| Known material | Typical SG | Reference identity | Applicability and limitations |
|---|---|---|---|
| Emerald or aquamarine — natural solid beryl | 2.65–2.80 | gem-beryl-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Natural solid only; no synthetic beryl, filling, coating, impregnation or assembly. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Ruby or sapphire — natural solid corundum | 3.80–4.05 | gem-corundum-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Natural solid only; no synthetic, filled, coated or composite material. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Natural solid opal | 2.00–2.20 | gem-opal-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Cabochon factors only; no synthetic opal, doublet, triplet, backing, impregnation, filling or matrix-bearing composite. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Natural peridot | 3.32–3.37 | gem-peridot-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Natural solid only; no imitation, assembly, coating or filling. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Natural spinel | 3.58–3.61 | gem-spinel-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Natural solid only; no synthetic spinel or composite. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Verneuil synthetic spinel | 3.61–3.67 | gem-spinel-verneuil-synthetic-solid gem-a-foundation-2017.v1 Gem-A source | Verneuil synthetic solid Known Verneuil material only; not another or unknown synthetic process. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Natural tanzanite | 3.15–3.38 | gem-tanzanite-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Natural solid only; no coating, filling, assembly or imitation. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Natural topaz | 3.50–3.60 | gem-topaz-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Natural solid only; no coating, filling, assembly or imitation. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
| Natural tourmaline | 3.00–3.10 | gem-tourmaline-natural-solid gem-a-foundation-2017.v1 Gem-A source | Natural solid Natural solid only; no coating, filling, assembly or imitation. Gem-A typical reference range; an individual specimen can fall outside it. Applies to solid stones only, not hollow, drilled, doublet, triplet, backed, assembled, matrix-bearing or composite stones, or coated, fracture-filled, impregnated or stabilised stones. |
Supported shape and cut factors
These eleven outline and cut pairs come from Carmona's GIA article, figures 5–10 and Box A. Faceted and cabochon factors are separate. Only these pairs can be chosen, and each factor assumes the typical profile described; other shapes are not estimated by borrowing a similar shape's factor.
| Outline and cut | Factor | Reference identity | Required profile and limitations |
|---|---|---|---|
| RoundFaceted brilliant | 0.00200 | shape-round-faceted-average gia-carmona-1998.v1 GIA source | Average brilliant; two perpendicular diameters plus depth No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires approximately 30% crown and 70% pavilion, a thin-to-medium girdle and no culet. |
| OvalFaceted brilliant | 0.00210 | shape-oval-faceted-average gia-carmona-1998.v1 GIA source | Average brilliant; non-transitional oval No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires approximately 30% crown and 70% pavilion, a thin-to-medium girdle and no culet. |
| Square cushionFaceted brilliant | 0.00235 | shape-square-cushion-faceted-average gia-carmona-1998.v1 GIA source | Average brilliant square-cushion outline No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires approximately 30% crown and 70% pavilion, a thin-to-medium girdle and no culet. |
| Rectangular cushionFaceted brilliant | 0.00235 | shape-rectangular-cushion-faceted-average gia-carmona-1998.v1 GIA source | Average brilliant, visibly shouldered rectangular cushion No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires approximately 30% crown and 70% pavilion, a thin-to-medium girdle and no culet. |
| Rectangular emerald cutFaceted step | 0.00270 | shape-rectangular-emerald-step-average gia-carmona-1998.v1 GIA source | Average step profile; keel about one-third of length No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires a keel about one-third of the length. |
| PearFaceted brilliant | 0.00200 | shape-pear-faceted-average gia-carmona-1998.v1 GIA source | Average pear; not broad-shouldered or transitioning to heart No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires approximately 30% crown and 70% pavilion, a thin-to-medium girdle and no culet. |
| RoundCabochon | 0.00270 | shape-round-cabochon-average gia-carmona-1998.v1 GIA source | Average dome and shoulder profile No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires an average dome and shoulders. |
| OvalCabochon | 0.00270 | shape-oval-cabochon-average gia-carmona-1998.v1 GIA source | Average oval dome and shoulders No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires an average dome and shoulders. |
| Square cushionCabochon | 0.00300 | shape-square-cushion-cabochon-average gia-carmona-1998.v1 GIA source | Average square-cushion dome and shoulders No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires an average dome and shoulders. |
| Rectangular cushionCabochon | 0.00290 | shape-rectangular-cushion-cabochon-average gia-carmona-1998.v1 GIA source | Average rectangular-cushion dome and shoulders No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires an average dome and shoulders. |
| PearCabochon | 0.00245 | shape-pear-cabochon-average gia-carmona-1998.v1 GIA source | Average pear cabochon; not transitioning to heart No interpolation or closest-profile substitution. Not valid for guessed or obstructed depth, or for shallow, deep, flattened, bulged or asymmetric profiles. Requires an average dome and shoulders. |
What the calculator doesn't cover
Can't be estimated here
- An unknown material, or a pearl.
- Quartz, generic garnet, zircon, jade or aggregates, cubic zirconia, moissanite and unlisted laboratory-grown materials.
- Marquise, heart, Radiant, Princess, square emerald, rectangular step, triangle or trillion, baguette, fantasy and freeform shapes, or any other shape and cut not in the table above.
- A missing measurement, or any measurement outside 0.10–20.00 mm.
Where the estimate won't be reliable
The calculator can't check the stone itself. Don't rely on its estimate for:
- Beads, rough (uncut) stones, carvings and freeforms.
- Drilled or hollow stones; doublets, triplets and other backed, assembled, matrix-bearing or composite stones.
- Coated, fracture-filled, impregnated or stabilised stones.
- Stones that are unusually shallow, deep, flat, bulging or lopsided, or whose outline is partway to another shape.
- A depth that was guessed or measured through a setting.
What the range means
- The actual weight can fall outside the range. The range comes from the gemstone's typical SG range and the typical profile for its shape; it isn't a statistical confidence interval.
- The calculator doesn't identify, grade, value, certify or appraise a gemstone. Unusual proportions, a setting that hides the stone, treatments and assembled stones can all change the actual weight.
- Measurements must be between 0.10 and 20.00 mm. The upper limit matches the sizes the cited tables cover; the lower limit is a practical cut-off, not a claim of accuracy for tiny stones.
- No professional weight-correction factor is applied, because choosing one takes a jeweller's experience and judgement.
Sources and review
- Gem-A Foundation in Gemmology Specifications and Syllabus — 2017 constants sheet. Specific-gravity references, revision gem-a-foundation-2017.v1; reviewed 31 August 2026.
- Charles I. Carmona, Estimating Weights of Mounted Gemstones, GIA (1998). Shape factors and average-profile guidance, revision gia-carmona-1998.v1; reviewed 31 August 2026.
Reference revisions and their limitations are listed under “How we worked this out” with each result, and are included in its copied text.
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