Gem Carat Weight Estimator
A 1ct diamond cut shallow shows 17% more face-up area than the same weight cut deep. Carat is a weight, not a size.
The same weight, cut at different depths
Every row below is the same carat weight. Only the depth changes.
Carat is a weight, and the weight hides underneath
Weight goes as length × width × depth, so a stone can carry a great deal of its carat weight below the girdle where nobody sees it. A one-carat round brilliant cut at 55 per cent depth measures 6.74mm across; the same carat weight cut at 70 per cent measures 6.22mm. That is 17 per cent more face-up area for the same weight, and therefore for roughly the same price.
This is the single most useful thing to know when comparing stones by the number on the ticket. Two one-carat stones are the same weight by definition and can look markedly different across a finger. The table above is the same weight all the way down.
Why the round factor here is not the published one
Nearly every source gives 0.0018 as the round-brilliant factor. Checked against the 11 standard trade diameters — the sizes every jeweller works from, where 3.8mm is a fifth of a carat and 6.5mm is a carat — it overestimates on ten of the eleven and is up to 11.3 per cent high.
Fitting those same eleven sizes gives 0.00169, whose worst error is 6.3 per cent and which is not biased in one direction. That is what this page uses. A number that is reliably wrong the same way every time is worth replacing, and worth saying so rather than quietly repeating.
Even then these are estimates and the page shows a band rather than a figure to two decimals. Girdle thickness, a bulged pavilion and an uneven bottom each move the answer several per cent, and no formula taking three measurements can see any of them.
What the shape factor measures
How completely the shape fills the box its measurements describe — which is not the same as how big it looks.
| Shape | Factor | |
|---|---|---|
| Marquise | 0.0016 | Two points mean it fills its box least, so it weighs least for given measurements — the reason a marquise looks large for its weight. |
| Round brilliant | 0.00169 | Fitted to eleven standard trade diameters rather than taken from the usual 0.0018. |
| Pear | 0.00175 | Between an oval and a marquise for spread. |
| Heart | 0.0018 | Measure the width across the lobes, not the diagonal. |
| Oval | 0.002 | Ovals often carry a bulged pavilion, which adds weight you cannot see. |
| Cushion | 0.0022 | Cushions vary more than any other shape; treat the estimate loosely. |
| Princess / square | 0.0023 | Deep by design, so a princess hides more weight below the girdle than most cuts. |
| Emerald cut | 0.0025 | Fills its box more completely than any other shape here, so it weighs most for given measurements. Usually cut shallow, which offsets that. |
A marquise has two points and fills its rectangle least, so it weighs least for a given set of measurements. An emerald cut is a rectangle with the corners taken off and fills its box most, so it weighs most. Whether a cut looks big for its weight depends on this and on the depth it is cut to, which is why the two are worth keeping apart.
Same size, different stone
Specific gravity is why an identical-looking stone can weigh half as much again.
| Stone | Specific gravity | Weight vs diamond, same size | |
|---|---|---|---|
| Diamond | 3.52 | +0% | The reference stone for every published factor. |
| Moissanite | 3.21 | -9% | Lighter than diamond, so the same size weighs about 10% less. |
| Cubic zirconia | 5.7 | +62% | Much heavier than diamond — the same size weighs about 60% more. |
| Ruby or sapphire | 4 | +14% | Both are corundum; only the colour differs. |
| Emerald or aquamarine | 2.72 | -23% | Both are beryl. Light for their size. |
| Amethyst, citrine or quartz | 2.65 | -25% | All the same mineral, coloured by trace impurities. |
| Topaz | 3.53 | +0% | Almost exactly diamond's weight for the same size. |
| Garnet | 3.9 | +11% | Varies by type; 3.9 is a reasonable middle. |
| Tourmaline | 3.1 | -12% | |
| Peridot | 3.34 | -5% | Gem-quality olivine. |
| Spinel | 3.6 | +2% | |
| Opal | 2.15 | -39% | The lightest common gem — an opal looks far larger per carat. |
How to use
- Pick the shape and the stone, then measure in millimetres.
- Depth is measured from the flat top to the point, not across the face.
- Read the range rather than the middle figure — a girdle can move it several per cent.
- Compare the depth table: same weight, very different face-up size.
Frequently asked questions
How is gem weight estimated from measurements?
Carats equal length times width times depth in millimetres, times the specific gravity of the stone, times a factor for the shape. The factor accounts for how completely the shape fills the box those three measurements describe — a marquise with two points fills far less of it than an emerald cut with the corners taken off.
Why is the round factor here not 0.0018?
Because 0.0018 is reliably too high. Checked against the eleven standard trade diameters — where 3.8mm is a fifth of a carat and 6.5mm is a carat — it overestimates on ten of the eleven and is up to 11.3 per cent high. Fitting those same sizes gives 0.00169, whose worst error is 6.3 per cent and which is not biased one way.
Do two stones of the same carat weight look the same size?
No, and the gap is larger than most people expect. A one-carat round brilliant cut at 55 per cent depth measures 6.74mm across; the same weight cut at 70 per cent measures 6.22mm. That is about 17 per cent more face-up area for the same weight and roughly the same price — the deep stone carries its weight underneath, hidden in the setting.
Where should I measure the depth?
From the flat table on top to the point at the bottom, with the stone on its side — not across the face. Depth is the dimension people leave out, and it is the one that carries the weight. If the stone is set, depth is usually the measurement you cannot take, which is why an estimate on a mounted stone is much rougher.
How accurate is this?
Within about eight per cent, which is why a range is shown rather than a figure to two decimals. Girdle thickness alone moves a round brilliant several per cent, a bulged pavilion adds weight that no external measurement can see, and the fitted factor has its own six per cent worst case. For anything that matters, weigh it.
What is a carat exactly?
Precisely 0.2 grams, fixed by international agreement in 1907. Before that it varied between markets, which made stones hard to trade across borders. It is a weight and nothing else — it says nothing directly about size, and the word is unrelated to the karat that measures gold purity.
Why does the stone type change the answer so much?
Because specific gravity varies widely. Cubic zirconia is 5.7 against diamond's 3.52, so a cubic zirconia cut to exactly the same dimensions weighs about 60 per cent more. That is a genuine identification test: a stone that looks like a one-carat diamond but weighs 1.6 carats is not a diamond.
Which shape looks biggest for its weight?
Generally a marquise, because its two points mean it fills the least of its bounding box. But the shape factor is only half the story — how big a stone looks also depends on how deep it is cut, and step cuts like emeralds are usually cut shallower than their factor alone would suggest. The two effects are worth keeping apart.
Can I use this on a mounted stone?
Partly. Length and width are usually measurable in a setting; depth generally is not, and depth is the dimension carrying the weight. An estimate from a guessed depth inherits that guess entirely, so treat the result as a wide range rather than a number.
Why does a deep stone exist at all if it looks smaller?
Because it can be cut from rough that would not otherwise yield that weight, and weight is what is priced. A cutter choosing between a well-proportioned 0.90 and a deep 1.02 will often take the 1.02, since crossing a round number moves the price per carat. The buyer pays for weight sitting under the setting.
Is specific gravity the same thing as density?
Numerically the same in grams per cubic centimetre, since water is one, but conceptually different: density has units and specific gravity is a ratio without any. The specific gravity calculator on this site measures it by water displacement, which needs only a kitchen scale and a cup of water.
Does this send my measurements anywhere?
No. Everything is computed in your browser, nothing is uploaded, and nothing is stored between visits.
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