About the Diamond Carat Size Calculator
This diamond carat size calculator estimates how much a diamond weighs from its measurements. Choose the cut shape, enter the length, width and depth in millimetres (as printed on a grading report or measured with a gauge), and it applies the standard trade estimation formula for that shape to give an approximate carat weight, plus depth percentage and length-to-width ratio.
It is handy for estimating the weight of a mounted stone that cannot be put on a scale, checking that a listing’s measurements match its stated weight, or seeing how a millimetre or two changes carat weight. A reference chart shows typical round brilliant sizes from 3 to 11 mm.
Estimates assume a well-proportioned diamond with a medium girdle and are usually within about 5–10% of the true weight. Thick girdles, deep pavilions or bulging sides add weight that measurements cannot see, so only a calibrated scale on an unmounted stone gives the exact figure. The formulas are for diamond (specific gravity 3.52); other gemstones need a different factor.
With the default inputs, the estimated carat weight is 1.03 ct. Change any value above to recalculate instantly.
How to use the diamond carat size calculator
- 1Choose the diamond’s shape.
- 2Enter the length and width in millimetres (for a round, its two diameters).
- 3Enter the total depth from the table to the culet.
- 4Read the estimated carat weight, depth percentage and ratio.
- 5Use the size chart to see typical round diameters for each carat weight.
Formula and method
Gemologists estimate the weight of a diamond from its measurements in millimetres using shape factors k derived from the volume of a typically cut stone and diamond’s density (3.52 g/cm³). For round brilliants the average girdle diameter is squared and multiplied by the depth and 0.0061; ovals use 0.0062. Fancy shapes use length × width × depth × k, with k from 0.0058 (marquise) to 0.0092 (emerald).
Fancy-shape factors change slightly with the length-to-width ratio — longer emerald cuts and marquises run a little heavier per cubic millimetre — and a thick or very thick girdle can add several percent. The calculator uses mid-range factors for well-proportioned stones with medium girdles; one carat is 0.2 grams.
- L, W
- Length and width (or two diameters) in mm
- D
- Total depth in mm, table to culet
- k
- Shape factor (0.0061 for round)
Worked examples
6.5 mm round brilliant, 4.0 mm deep
6.5² × 4.0 × 0.0061 = 1.03 ct. The depth is 4.0 ÷ 6.5 = 61.5% of the diameter, a typical proportion, so a 6.5 mm round is the classic one-carat size.
Oval 8 × 6 mm, 3.7 mm deep
The average diameter is (8 + 6) ÷ 2 = 7 mm. 7² × 3.7 × 0.0062 = 1.12 ct. The length-to-width ratio is 8 ÷ 6 = 1.33.
Princess 5.5 × 5.5 mm, 4.0 mm deep
5.5 × 5.5 × 4.0 × 0.0083 = 1.00 ct. Princess cuts are deeper than rounds (here 72.7%), so a one-carat princess looks smaller face-up than a one-carat round.
Frequently asked questions
What size is a 1 carat diamond in mm?+
A well-cut 1-carat round brilliant measures about 6.4–6.5 mm across with a depth near 3.9–4.0 mm. Princess cuts of 1 carat are about 5.5 mm square, and ovals roughly 7.7 × 5.7 mm.
How accurate is estimating carat weight from measurements?+
For well-proportioned stones the formulas are usually within 5–10% of the true weight. Unusual proportions, thick girdles or bulging pavilions add hidden weight, so the only exact method is weighing the unmounted stone.
Can I use this for gemstones like sapphire or emerald?+
Not directly. The factors assume diamond’s specific gravity of 3.52. Sapphire and ruby (about 4.0) are heavier for the same size and emerald (about 2.7) is lighter; use the gemstone carat weight calculator instead.
Does carat measure size?+
No — carat is a unit of weight equal to 0.2 grams. Two diamonds of the same weight can look different in size depending on shape and how deep they are cut.
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