Millimetres to carats
Estimate carat weight from measured dimensions using the standard trade formulae. Measure with callipers or a leaf gauge, pick the cut and species, and read off the estimate.
Carats to millimetres
Reverse-estimate the approximate diameter of a round brilliant from a known carat weight. Round brilliants only: fancy cuts have too many proportion variables to invert reliably.
Honest limits
These formulae estimate the weight of an averagely proportioned stone; they are not a substitute for a scale. Girdle thickness, pavilion bulge and cut quality can each swing the real weight by 10% or more in either direction. Always weigh the stone itself when it matters: setting quotes, insurance valuations or resale.