Vegetable Seed Calculator
Buy for the germination on the packet, not for the plants you want.
The formula
seed needed = area * seeding rate
Rates are quoted per unit area, and they vary
Grass seed is sold by a rate per thousand square feet or per hundred square metres, and the right rate depends on whether you are sowing bare ground or overseeding an existing lawn — overseeding usually takes about half. Species differ more than that: fine fescues are sown at three or four times the rate of ryegrass because the seeds are so much smaller.
Germination is not 100 %
A seed label carries a germination percentage and a purity percentage, and the product of the two is what actually comes up. At 85 % germination and 95 % purity, only about four seeds in five are viable plants — so a target plant population needs a quarter more seed than the arithmetic suggests.
More is not better
Oversowing produces a thick, weak stand that competes with itself, dries out fast and is prone to disease. Grass in particular thins itself back to what the site will support, so seed sown beyond that rate is simply wasted.
Rules of thumb, and the field
Every figure here describes an average season on an average site. Soil type, aspect, drainage, variety and weather each move the answer by more than the arithmetic does, and a grower who has kept records for a few years on one piece of ground knows it better than any formula. Use these to plan an order or size an implement, then trust what the ground has actually done before.