The momme
The momme is a Japanese unit of weight, and in Japan it is still the legal unit for weighing pearls. Japan’s Measurement Units Order, the cabinet order under the Measurement Act that lists which units may be used for what, permits the momme for one purpose only: “the measurement of the mass of pearls”. It defines it as 0.00375 of a kilogram, which is exactly 3.75 grams.
Because Japan has been central to the cultured pearl trade since cultured pearls were first produced commercially there, the momme travels with the pearls. Wholesale lots of loose and strung pearls are often priced and traded per momme, including outside Japan. The unit is also used in a quite different trade, silk, where “momme” describes fabric weight per area; that use is a different measure and is not what this converter does.
The carat and the pearl grain
The metric carat is 200 milligrams, 0.2 gram, a definition adopted internationally in 1907. Japan’s Measurement Units Order lists it in the same table as the momme, for the mass of gemstones, at 0.0002 of a kilogram. Individual pearls of significance are sometimes weighed in carats, like other gems.
The pearl grain is a trade unit equal to one quarter of a carat, so 50 milligrams. It was the traditional unit for weighing natural pearls and still appears in descriptions of historic pearls and in auction catalogues. It is not defined by any statute this page could find, so it is shown here as the trade definition it is: a quarter carat, exactly.
Two grains with one name
The pearl grain should not be confused with the grain used for metals and gunpowder, the troy or avoirdupois grain, which is 64.79891 milligrams. The two differ by about 30%. A weight given simply as “grains” in a description of jewellery needs checking: if it describes a pearl, it is almost certainly the pearl grain; if it describes the metal of a setting, it may be the other one.
The carat used for gems is not the karat that describes gold purity either, although in British English both are spelled carat. The page on what a carat actually is covers that distinction.
How the units relate
Because every unit here is a definition, the conversions are exact and never drift. One momme is 3.75 grams, which is 18.75 carats, which is 75 pearl grains. One gram is 5 carats or 20 pearl grains. One carat is 4 pearl grains. A lot of 10 momme is 37.5 grams.
The converter shows every unit at once, so a weight given in one can be read in any of the others without a second calculation.
What weight does and does not say about pearls
For strands and loose lots, weight is mostly a way of counting material. It says little about quality on its own: size, shape, lustre, surface, nacre thickness, colour and matching all matter, and two strands of the same weight can differ greatly in every one of those. Size is usually given separately, as a diameter in millimetres; the pearl size guide on this site sets out the published size ranges by pearl type.
Weighing also needs care with strung pearls. A strand’s weight includes its silk or wire, knots and clasp, so the weight of the pearls themselves is lower than the weight of the strand. This page converts whatever weight you enter; it does not separate the pearls from the rest.
No prices per momme or per grain appear here. Those are market figures that vary by type and quality, and none of them is a fixed definition.