A loose gemstone can simply be weighed. A stone already set in a piece of jewellery cannot, which is why the trade uses published formulas that estimate carat weight from millimetre measurements, the cut outline and the stone's specific gravity. This gemstone carat weight calculator applies those formulas, states which factor it is using, and takes specific gravity as an input rather than assuming it.
Arb Digital publishes this among the other measurement and conversion tools our team builds. It is an estimation aid. It is not an appraisal, it is not a valuation, and it does not identify what a stone is — all three of those are jobs for a qualified gemmologist with the stone in hand.
What This Gemstone Carat Weight Calculator Does
Enter the outline dimensions and depth in millimetres, pick a cut shape, and set the specific gravity. The tool multiplies an area term by depth, by specific gravity, and by the shape factor published for that cut, then applies any girdle-thickness adjustment you specify. It reports the result in carats, in points, and in grams.
Specific gravity can be typed directly or filled from a short list of mineral species, each figure taken from a named mineralogical database. No specific gravity table is invented here, and no gem variety is assigned a number that its species does not carry.
How to Use It
- Pick the cut shape. The factor shown beside each is the published multiplier for that outline, and it appears in the results so you can check it.
- Measure the outline in millimetres. A round brilliant needs only its diameter; other shapes need length and width across the widest points.
- Measure the depth from table to culet, through the stone rather than across it. In a setting this is the hardest measurement to take and the largest source of error.
- Set the specific gravity for the species, either from the list or by typing a figure from your own reference.
- Apply a girdle adjustment if the girdle is visibly thicker or thinner than normal, using the published percentage ranges.
The Formula: Volume, Density and a Shape Factor
Every version of this calculation is the same physical statement — mass equals volume times density — dressed in trade units. Millimetre dimensions give an approximate volume, specific gravity converts volume to mass relative to water, and the shape factor absorbs both the unit conversion into carats and the fact that a faceted stone occupies only part of the box its outline describes.
The International Gem Society's guide How to Estimate Gem Weight in a Jewelry Setting publishes the standard set: round brilliant is diameter² × depth × SG × 0.0018; oval is diameter² × depth × SG × 0.0020, where the diameter is taken as an average of the outline measurements; emerald cut and rectangles use length × width × depth × SG × 0.0026; marquise uses 0.0017; pear uses 0.0018; heart uses 0.0021; and a square cushion uses diameter² × depth × SG × 0.0018. The same guide publishes the girdle corrections this tool applies: subtract 1 to 2 per cent for a thin girdle, add 1 to 2 for slightly thick, 3 to 4 for thick, 5 to 6 for very thick and 7 to 10 for extra thick.
Worked through with the defaults — a round brilliant 6.5 mm across and 4.0 mm deep, at diamond's specific gravity of 3.52 — the calculation is 6.5² = 42.25, times 4.0 = 169, times 3.52 = 594.88, times 0.0018 = 1.071 carats. That is 107.1 points, or 0.214 grams. Apply a thick-girdle correction of 3.5 per cent and the estimate rises to 1.108 carats.
Where Specific Gravity Comes From
Specific gravity is a property of the mineral species, not of the gem name attached to it. Ruby and sapphire are both corundum and share its density. Emerald and aquamarine are both beryl. Amethyst and citrine are both quartz. That is why the list on this page is organised by species with the familiar gem names beside them rather than the other way round.
The figures used are those listed in the Webmineral mineralogy database, which publishes a specific gravity for each species — for example on its Corundum Mineral Data page, which gives corundum a specific gravity of 4.01 with a measured density range of 4 to 4.1. Real stones vary within a range around the species figure because of trace element substitution and inclusions, which is one reason an estimate is an estimate.
Two consequences follow. First, an unidentified stone cannot be weighed by this method at all, because you do not know which density to use — and getting the species wrong by even a small margin scales the answer by the same margin. Second, synthetic and natural material of the same species share a density, so this calculation says nothing whatever about origin.
The Carat Itself Is an Exact Unit
The metric carat is defined as exactly 200 milligrams, or 0.2 grams, and it has been since the definition was adopted internationally in the early twentieth century. NIST's Precious Metals Conversion Information page records the carat's origin in carob seeds and its standardisation at 0.2 grams. One carat divides into one hundred points, so a stone described as 75 points weighs 0.75 carats, or 150 milligrams.
The word is also used for something entirely different. Carat as a measure of gold purity — 18 carat, 24 carat — is a fraction of twenty-four parts fine, not a weight at all. The two meanings share a spelling in British usage and are usually distinguished as carat and karat in American usage, and confusing them is the most common misunderstanding in this whole area.
Why the Estimate Misses
The shape factors assume a stone cut to reasonably normal proportions for its outline. Three things break that assumption regularly.
Depth. A stone cut deep to preserve rough weight holds more material than its outline suggests, and a shallow one holds less. Depth is also the measurement you cannot take accurately in a closed-back setting, which is why estimates on mounted stones are much rougher than estimates on loose ones.
Girdle thickness. An extra-thick girdle can add most of a tenth to a stone's weight without adding anything visible from the face. That is precisely what the published percentage corrections exist for, and skipping them is the most common avoidable error.
Outline irregularity. A cushion that is not quite square, a pear with a long shoulder or an oval with an unusual length-to-width ratio all deviate from the model the factor was fitted to. In those cases take the measurements carefully, apply the factor, and treat the answer as an approximation rather than a number to quote.
What This Tool Is Not For
It is not an appraisal. Value depends on species, colour, clarity, cut quality, treatment, origin and market, none of which are dimensions. It is not an identification: density is one property among many that a gemmologist uses, and it cannot be inferred from a photograph or a ring size. And it is not a substitute for a scale — a loose stone should always be weighed, because a balance reading to a hundredth of a carat is both faster and vastly more accurate than any formula.
Where dimensions and density genuinely help is in checking that a described stone is plausible, in estimating a set stone for insurance documentation prepared by a professional, and in planning a setting. For the related arithmetic, the density calculator handles mass, volume and density directly, the weight converter and the grams to ounces converter move the result between systems, and the volume converter covers the volume side. For precious metal rather than stone, the gold melt value calculator works from weight and purity, and the money weight calculator applies the same mass-from-dimensions thinking to coin and note quantities.
Arb Digital builds free calculators that name their sources and state their limits. Browse the library, or get in touch about a set for your audience.
Browse Free Tools Get in TouchCommon Mistakes to Avoid
- Using one specific gravity for every stone — density is a property of the species, and getting it wrong scales the answer by the same proportion.
- Skipping the girdle correction — an extra-thick girdle can add up to ten per cent that the outline measurements never see.
- Guessing depth on a mounted stone — depth is the hardest measurement to take in a setting and the one the estimate is most sensitive to.
- Confusing carat with karat — one is a weight of exactly 200 milligrams, the other is a fraction of gold fineness in twenty-fourths.
- Treating the estimate as a weight — a loose stone belongs on a balance, where the answer is exact rather than modelled.
Related Free Tools From Arb Digital
Work with density directly using the density calculator, convert the result with the weight converter or the grams to ounces converter, and price precious metal with the gold melt value calculator. The free online tools hub lists everything else.
Frequently Asked Questions
Multiply an area term from the outline by the depth, by the stone's specific gravity, and by a shape factor published for that cut. For a round brilliant that is diameter squared times depth times specific gravity times 0.0018.
Exactly 0.2 grams, or 200 milligrams. The metric carat has been defined at that value internationally since the early twentieth century, and it divides into one hundred points.
Because the formula estimates volume from dimensions and volume alone cannot give weight. Density converts one to the other, and it is a property of the mineral species rather than of the gem name.
Yes. Both are the mineral corundum, listed at 4.01 in the Webmineral database, so they share a density. The same is true of emerald and aquamarine, which are both beryl, and of amethyst and citrine, which are both quartz.
It is an approximation that assumes normal proportions for the cut. Unusual depth, an unusually thick girdle or an irregular outline will all move the real weight away from it, and a mounted stone whose depth cannot be measured properly is rougher still.
Carat is a unit of weight equal to 200 milligrams. Karat, spelled that way in American usage, describes gold purity in twenty-fourths, so 18 karat gold is eighteen parts gold in twenty-four. They are unrelated quantities.
No. It estimates weight from dimensions and an assumed density. Identification, treatment detection, quality grading and valuation all require a qualified gemmologist, and a laboratory weighs a loose stone rather than estimating it.
Figures produced by this tool are estimates for reference only. They are not an appraisal, a valuation or an identification, they assume a stone of normal proportions and a specific gravity you supply, and a gemmological laboratory weighs a loose stone on a balance rather than calculating it from measurements.
