Seeds per Unit of Seed Weight

Seed is bought by weight and planted by count, and the thousand-kernel weight is what converts between them. The equation is a proportion so simple it barely deserves a page — if a thousand seeds weigh TKW grams, then M grams hold $1000 M / TKW$ of them — but the conversion is done wrong often enough, and in units confusing enough, to be worth writing down.

Wheat with a TKW of 40 g gives $1000 \times 1000/40 = 25{,}000$ seeds per kilogram, so a 25 kg bag holds 625,000 seeds. Corn at a TKW of 300 g works out per pound, since that is how the bag is still weighed in North America: a pound is 453.59237 g exactly, so $1000 \times 453.59237/300 = 1512$ seeds. That is why a standard 80,000-seed unit of corn weighs about 53 lb, and why corn is sold by seed count rather than weight at all — the buyer wants a fixed number of plants, and the weight of them changes with the seed lot.

The mistake to watch for is the per-thousand basis itself. Thousand-kernel weight is quoted in grams per 1000 seeds worldwide, including in countries that measure nothing else in grams, and the factor of 1000 in the equation is part of the quantity's definition rather than a unit conversion. Drop it and the answer is out by three orders of magnitude, which at least announces itself. The subtler version is confusing TKW with the older "thousand seed weight" or with a per-100 count from a small sample, which is out by a factor of ten and looks entirely plausible.

Do not confuse any of this with test weight, which is a different measurement answering a different question. Test weight is bulk density — kilograms per hectolitre, or pounds per bushel — and it describes how tightly seed packs in a container, which depends on shape and surface as much as on the mass of individual kernels. A plump, round sample and a shrivelled one can share a test weight while their thousand-kernel weights differ by a third. Test weight grades grain for sale; thousand-kernel weight sets a seeding rate.