Diamond Weight Calculator
Estimate a diamond's carat weight from its millimetre measurements. Choose the cut, enter the size, and read off an indicative carat figure — handy for stones already set in a ring.
Calculator
Measurements
- Length
- 6.5 mm
- Width
- 6.5 mm
- Depth
- 4 mm
- Cut factor
- 0.0061
About this calculator
This calculator estimates a diamond's carat weight from its measured size in millimetres. You can't weigh a stone that's already mounted in a setting, but you can measure it with a gauge or loupe — so jewellers use a length-by-width-by-depth formula, scaled by a factor for each cut shape, to read a carat figure straight off the dimensions. Pick the cut (round, princess, oval, cushion, emerald, marquise, pear or radiant), type the millimetre measurements, and the tool returns an estimated weight in carats and points. It is an estimate, not a certificate: true weight shifts with the exact proportions, girdle thickness and any bulge in the stone.
How to read your results
The headline figure is the estimated weight in carats, with the same value restated in points (1 carat = 100 points, so 0.50 ct = 50 points). Below it you can see which cut factor was applied and the three dimensions used. For a round brilliant the diameter is squared, so width mirrors the length unless you measured the stone slightly off-round and enter both. If you leave depth blank, the tool fills it in from a typical ratio (about 62% of the diameter for a round, 65% of the width for fancy cuts) and flags that the depth was estimated — supply a real depth measurement for a tighter result. Treat the answer as indicative: it places the stone in the right weight band, not to the third decimal a scale would give.
How it's calculated
Round brilliant: weight in carats ≈ diameter² (mm) × depth (mm) × 0.0061. The 0.0061 factor bundles together diamond's density (≈3.52 g/cm³) and a typical girdle allowance. For fancy shapes the formula is length × width × depth × a cut-specific factor: princess 0.0083, cushion 0.00815, oval 0.0062, emerald 0.0080, marquise 0.00565, pear 0.00615, radiant 0.0081. If you don't supply a depth, the calculator estimates it (round = diameter × 0.62; fancy = width × 0.65) and says so. Because these factors assume average proportions, the result is an estimate — a stone with a thick girdle or deep pavilion will weigh more than the formula suggests.
Worked example
A round brilliant measured at 6.5 mm across with a 4.0 mm depth.
6.5² × 4.0 × 0.0061 = 1.03 carats — which is why a 6.5 mm round brilliant is the classic "one-carat look". In points that is about 103 points. A 5.0 mm round with a 3.1 mm depth, by contrast, comes out near 0.47 ct (about a half-carat stone).
Frequently asked questions
How accurate is estimating carat weight from millimetre size?
For a well-cut stone of average proportions it is usually within a few percent — close enough to know whether you are looking at a half-carat or a full carat. It is an estimate, not a substitute for a scale or a grading report: the same face-up size can hide a heavier stone if it has a thick girdle or a deep pavilion, or a lighter one if it is cut shallow.
Why is a 6.5 mm round diamond about one carat?
Plugging 6.5 mm into the round-brilliant formula — 6.5² × 4.0 mm depth × 0.0061 — gives 1.03 carats. That is why 6.5 mm is the standard "one-carat look" diameter for a round brilliant. The face-up size grows slowly with weight, so a two-carat round is only about 8.1 mm across, not twice the diameter.
What if I don't know the depth of the diamond?
Leave the depth field blank and the calculator estimates it from a typical ratio — about 62% of the diameter for a round brilliant, 65% of the width for fancy shapes — and flags that the depth was assumed. The estimate is rougher without a real depth measurement, so measure the depth with a gauge or read it off the grading report when you can.
Does the cut shape change the weight for the same dimensions?
Yes. Each shape has its own factor because it fills its bounding box differently: a princess (square) is denser than a marquise (pointed) for the same length and width. That's why the tool asks for the cut first — selecting oval versus emerald with identical millimetre measurements gives a different carat estimate.
Sources
- 4cs.gia.edu/en-us/diamond-carat-weight
- www.bluenile.com/education/diamonds/carat-weight
- blog.esslinger.com/diamond-weight-estimation-chart
Reviewed by the YouCalc Team · Last reviewed
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