MM to Carat Conversion Calculator — Gemstone Diameter to Weight

Formula

How this is calculated

Gemstone weight in carats depends on the stone's density and precise proportions — it cannot be measured from a single diameter alone. For round brilliant diamonds, the jewellery industry uses the approximation Carat = diameter² × depth × 0.0061. The constant 0.0061 combines the average diamond density (≈ 3.52 g/cm³), a shape correction for the round brilliant's crown-and-pavilion profile, and the unit conversions needed to go from mm³ to grams to carats (1 carat = 0.2 g exactly by international definition).

The ideal-cut depth for a round brilliant is 61–63% of the girdle diameter. For a 6.5 mm diameter stone at 62% depth (4.03 mm): 6.5² × 4.03 × 0.0061 ≈ 1.04 ct. A shallower cut produces fewer carats for the same face-up diameter; a deeper cut produces more, but reduces brilliance. The default depth of 4.0 mm for a 6.5 mm stone (61.5%) is close to ideal-cut proportions.

This formula is an industry standard, not an exact physical derivation. Actual carat weight for a specific stone may differ by 5–10% depending on crown height, girdle thickness, and culet size. For an exact weight, a certified gemological report or a precision scale is needed. This calculator applies to round brilliant diamonds only — princess, oval, cushion, and other cuts have different shape factors and correction constants.