Millimetres to carats
Estimate carat weight from measured dimensions. Diamonds use the standard lab estimation formulae; coloured stones use density-based formulae with depth defaults matched to how each species is actually cut. Measure with callipers or a leaf gauge, pick the cut and species, and read off the estimate.
Stone list and gem report
Build the full stone schedule for a piece: add the stone above with a role, repeat for each size, and read the totals. Covers the common setting patterns (solitaire, three-stone, halo with melee, eternity band) through the roles, and totals are kept per gem type: a diamond total and a coloured-stone total are never combined into one figure.
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Carats to millimetres
Reverse-estimate face-up size from a known carat weight, for any of the main cuts and species. Pick the cut, the species and, for elongated cuts, the length-to-width ratio you want. Denser species come out smaller at the same weight, so a one carat sapphire faces up smaller than a one carat diamond. Trillions and cabochons are not offered here: their depth varies too much to assume.
Honest limits
These formulae estimate the weight of a typically 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, which is what the likely range shows. Published mm-to-carat charts themselves disagree by 20 to 30% for some cuts, so treat any single chart figure as a midpoint, not truth. Always weigh the stone itself when it matters: setting quotes, insurance valuations or resale.