How We Calculate
Quick answer
Weight = size term x depth x a factor. Diamonds use trade constants (round: average diameter squared x depth x 0.0061). Colored stones use a shape factor x specific gravity (round: x 0.0018 x SG). Every result is shown with a range built from source disagreement, density spread and a proportion allowance.
Applies to every calculator and chart on the site. Data verified .
The core formula
A gem's weight is its volume times its density. Volume cannot be measured directly with calipers, so the trade uses a shortcut: multiply a size term by the depth, then by a factor that accounts for the shape's geometry and typical proportions. There are two families of factors.
Colored stones (and anything without a diamond constant)
carats = size term x depth x SG x shape factor
The shape factors come from the International Gem Society's guidance on estimating gem weight and the Dendritics standard formula. For an 8 x 6 x 4 mm oval sapphire: 8 x 6 x 4 x 4.00 x 0.0020 = 1.54 ct.
Diamond, lab-grown diamond and moissanite
carats = size term x depth x trade constant, with the constant multiplied by (SG / 3.52) for moissanite.
The constants already include diamond's density. Some shapes use a ratio table: the constant changes with the length to width ratio, and we interpolate linearly between published points, holding the end values outside the table. For a 6.5 mm round, 4.0 mm deep: 6.5 x 6.5 x 4.0 x 0.0061 = 1.03 ct.
Known discrepancy between the families
For a round stone, the colored-stone factor times diamond's SG gives 0.0018 x 3.52 = 0.00634, about 4% more than the diamond constant of 0.0061. The two were derived from stones with different typical proportions. We use 0.0061 for diamonds and include 0.00634 as an alternative, so the displayed range covers both.
Shape factors and constants
| Shape | Colored-stone formula | Alternatives | Diamond formula | Typical depth | Typical ratio |
|---|---|---|---|---|---|
| Round | (average of L and W) squared x depth x SG x 0.0018 | none | (average of L and W) squared x depth x 0.0061 | 58 to 65% | 1 |
| Oval | L x W x depth x SG x 0.002 | 0.0021 | (average of L and W) squared x depth x 0.0062 | 58 to 65% | 1.3 to 1.5 |
| Cushion (square) | (average of L and W) squared x depth x SG x 0.0018 | none | none (uses colored factor x SG 3.52) | 65 to 75% | 1 to 1.05 |
| Cushion (rectangular) | (average of L and W) squared x depth x SG x 0.0022 | none | none (uses colored factor x SG 3.52) | 65 to 75% | 1.1 to 1.4 |
| Emerald cut (rectangular step) | L x W x depth x SG x 0.0025 | none | L x W x depth x ratio table: 1 gives 0.008, 1.5 gives 0.0092, 2 gives 0.01, 2.5 gives 0.0106 | 55 to 68% | 1.3 to 1.6 |
| Square step cut (Asscher style) | (average of L and W) squared x depth x SG x 0.0023 | none | L x W x depth x ratio table: 1 gives 0.008, 1.5 gives 0.0092, 2 gives 0.01, 2.5 gives 0.0106 | 58 to 70% | 1 to 1.05 |
| Rectangle with cut corners (radiant style) | L x W x depth x SG x 0.0026 | 0.0025 | L x W x depth x ratio table: 1 gives 0.008, 1.5 gives 0.0092, 2 gives 0.01, 2.5 gives 0.0106 | 60 to 70% | 1.15 to 1.4 |
| Princess (square brilliant) | (average of L and W) squared x depth x SG x 0.0023 | none | L x W x depth x 0.0083 | 64 to 76% | 1 to 1.05 |
| Pear | L x W x depth x SG x 0.0018 | none | L x W x depth x ratio table: 1.25 gives 0.00615, 1.5 gives 0.006, 1.66 gives 0.0059, 2 gives 0.00575 | 58 to 65% | 1.45 to 1.75 |
| Marquise | L x W x depth x SG x 0.0017 | 0.0016 | L x W x depth x ratio table: 1.5 gives 0.00565, 2 gives 0.0058, 2.5 gives 0.00585, 3 gives 0.00595 | 56 to 65% | 1.85 to 2.1 |
| Heart | L x W x depth x SG x 0.0021 | 0.0018 | L x W x depth x 0.0059 | 56 to 64% | 0.9 to 1.05 |
| Triangle (trillion) | L x W x depth x SG x 0.0018 | none | L x W x depth x 0.0057 | 42 to 55% | 1 to 1.05 |
| Baguette | L x W x depth x SG x 0.0025 | none | L x W x depth x 0.00915 | 55 to 68% | 2 to 4 |
| Cabochon, high dome | L x W x depth x SG x 0.0026 | none | none (uses colored factor x SG 3.52) | 50 to 80% | 1 to 1.5 |
| Cabochon, low dome | L x W x depth x SG x 0.0029 | none | none (uses colored factor x SG 3.52) | 25 to 45% | 1 to 1.5 |
Notes: the princess colored factor uses the square step value, because our sources publish none for princess. The baguette diamond constant expects the width to be the average of the two parallel edges for tapered baguettes. The marquise ratio table follows the published tables, in which 0.00565 belongs to a 1.50 ratio.
Weight corrections
Corrections are optional and only apply to the shapes they are relevant to. Each adds its midpoint to the estimate and its full span to the range.
- Thick girdle: +3% to +8%
- High or lumpy crown: 0% to +10%
- Wide wings or high shoulders: 0% to +10%
- Straight (flat) shoulders: -5% to -1%
- Very short keel or no keel: -3% to -1%
- Bulging pavilion: 0% to +18%
- Straight sides (not curved): -10% to 0%
- Wide cut corners: -5% to 0%
How the range is built
- Compute the estimate with the primary factor and with every alternative published factor. Take the lowest and highest.
- Scale the low end by the material's minimum SG and the high end by its maximum SG (divided by the default SG). A measured custom SG removes this step.
- Apply a proportion allowance of plus or minus 4% for diamond formulas, 6% for colored-stone faceted formulas and 10% for cabochons. This is our editorial judgement of how much typical proportions vary, not a published figure.
- Apply the minimum and maximum of any corrections ticked.
The carat to mm finder solves the same formula for width in closed form (a cube root) and turns the weight range into a size range.
Diamond equivalent weight
Same size, different material: weight = diamond weight x (SG / 3.52). Moissanite sellers often publish a lower real weight than this ratio gives (for example 0.88 ct for a 6.5 mm round). We could not verify that convention independently, so we show it as a published convention next to our density figure.
Units
1 carat = 200 mg exactly. Point = 2 mg. Pearl grain = 50 mg. Troy grain = 64.79891 mg. Pennyweight = 1,555.17384 mg. Avoirdupois ounce = 28,349.523125 mg. Troy ounce = 31,103.4768 mg. 1 inch = 25.4 mm.
Sources
- IGS-EST: International Gem Society, How to Estimate Gem Weight (colored stone shape factors, used with SG)
- DENDRITICS: Dendritics standard gem weight formula (published shape factor set, cross-check)
- ESSLINGER: Esslinger, Stone Weight Estimation Formula (diamond trade constants and ratio tables)
- OMNI: Omni Calculator, Diamond Weight Calculator (trade constants and ratio tables, cross-check)
- OCTONUS: Octonus, Mounting diamonds weight determination (weight estimation formulas)
- ABSURD: absurdtools.com Gem Carat Weight Estimator (colored stone factor cross-check)
- IGS: International Gem Society, Specific Gravity Values of Selected Gems (gemsociety.org/article/select-gems-ordered-density)
- IGS-LOOKALIKE: International Gem Society, Diamond Lookalikes Listed by Specific Gravity (gemsociety.org/article/diamond-look-alike-comparison-chart)
- USGS: U.S. Geological Survey, Natural Gemstones (pubs.usgs.gov/gip/7000029)
- GEMSELECT: GemSelect, Specific Gravity Chart (gemselect.com/gem-info/specific-gravity.php)
- MOG: Ministry of Gems, Gem Data Tables (ministryofgems.com.au/pages/gem-data-tables)
- GIA-RANGE: Standard gemological reference ranges in the GIA style (default value with plus and minus tolerance), used as a cross-check
Values confirmed by two or more independent sources are marked cross-checked. Others are marked approximate on the specific gravity chart and in the calculators, which widen their ranges for them. We cite these organisations as sources only. We are not affiliated with any of them.
Testing
The formula engine is tested automatically on every build: reference cases (for example a 6.5 x 4.0 mm round diamond near 1.03 ct and a 1 ct round diamond at 61% depth between 6.3 and 6.6 mm), every shape and material combination, round trips from weight to size and back within 1%, unit conversions, invalid inputs, and checks that every chart on the site matches the calculators exactly.
Limits
- Results are estimates, not appraisals, grading reports or valuations. Only a calibrated scale gives true weight.
- Formulas assume reasonably well-proportioned stones. Unusual cuts, very deep native cuts, and stones with large cavities or fillers can fall outside the range.
- The tool cannot identify a gem. If the material is wrong, the estimate is wrong.
- Data last verified 2026-11-04.
Related questions
Why two formula families?
Diamond trade constants were fitted to real diamonds and their typical proportions. The colored-stone factors are general shape factors multiplied by SG. Each is best in its own area, so we use each where it fits.
Why not give one exact number?
Because measurements cannot see hidden weight and sources disagree slightly. A range is the honest answer, and the point estimate is the most likely value inside it.
How often is the data reviewed?
The data was last verified on 2026-11-04. We review it when sources change and at least once a year.
Can I use these formulas myself?
Yes. The formulas and physical constants are published facts. Everything you need is on this page.
Found an error?
Please tell us through the contact page. Corrections are logged and dated.