Carat Weight Calculator.

Convert stone dimensions in millimetres to estimated carat weight for 11 diamond and gemstone shapes: round brilliant, princess, oval, cushion, emerald, pear, marquise, asscher, radiant, heart and trillion. Plus a total carat weight (TCW) counter for multi-stone compositions. Inside the Carit app on iOS and Android.

How mm-to-carat works

Carat weight is an estimate. Stones are weighed on a scale for exact figures, but before a stone is cut - when all you have are millimetre measurements - the estimated weight follows a formula that's been used industry-wide since the 1950s. For a round brilliant:

For fancy shapes, the formula is: length × width × depth × shape-specific factor. The factor varies by cut because the volume ratio of a princess, oval, cushion or pear differs from a round.

Shape-specific factors

These are the industry-standard factors used by every major lab and by Carit:

These factors assume the stone has typical cut proportions. A very deep or very shallow stone will weigh more or less than the formula predicts, by 5-10% in most cases. For a final precise weight, always weigh on a calibrated scale.

Total carat weight (TCW)

Carit's TCW counter is the tool you reach for when you have 40 melee stones to set and the client wants the total carat weight on the receipt. You enter stones grouped by shape and size - "6 × 1.2 mm rounds, 12 × 1.5 mm rounds, 4 × 2.0 mm princess, 2 × 3.0 mm ovals" - and the calculator returns the total. No running tally in your head, no adding 40 small numbers.

Round brilliant mm-to-carat reference chart

The most common mm sizes for round brilliants, with their estimated carat weight (assuming standard 0.61 depth-to-diameter ratio and 0.6043 factor):

Melee diamond chart

Melee diamonds are the small accent stones (0.005 to 0.15 carat, roughly 1.0 to 3.5 mm) that fill pavé, halo and channel-set fields. Carit's melee reference chart is organised by mm size and sieve plate number - the sieve-plate system is how melee is graded and sold at most trade suppliers.

A "pointer" refers to 0.01 ct. A "two-pointer" is 0.02 ct, a "five-pointer" is 0.05 ct. One carat = 100 points. So a 1.3 mm round brilliant is approximately a 1-pointer (0.01 ct), a 2.0 mm round is a 3-pointer (0.03 ct).

Coloured gemstones

The factors above are derived for diamond density. Coloured gemstones have different specific gravities:

• Diamond: specific gravity 3.52
• Sapphire / Ruby (corundum): 4.00 - factor is 14% higher than diamond
• Emerald (beryl): 2.72 - factor is 23% lower
• Aquamarine (beryl): 2.72 - same as emerald
• Tourmaline: 3.06 - 13% lower
• Topaz: 3.53 - same as diamond
• Spinel: 3.60 - 2% higher
• Garnet (varies): 3.60 to 4.30 depending on variety
• Moissanite: 3.22 - 9% lower than diamond (a 6.5 mm moissanite round is only ~0.90 ct, not 1.00 ct)

Carit handles the specific-gravity adjustment automatically when you select a coloured stone. The app's gemstone reference also includes refractive index, Mohs hardness, and basic identification notes.

Why the calculator is in the app

Setting a pavé piece, you need the mm-to-carat for each stone, the TCW across all stones, and occasional look-ups for coloured-stone specific gravity - all at the bench, all in seconds, often with wax on your fingers. The web version of this information is fine for reference; the Carit app gives you the working tool.

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