How much grass seed do I need?

How the math works

pounds of seed = square feet ÷ 1,000 × rate

Overseeding an existing lawn uses roughly half the new-lawn rate, because the established grass is already holding the ground. The standard rates built into this calculator:

Tall fescue 8 lb, perennial ryegrass 8 lb, fine fescue 5 lb, Kentucky bluegrass 3 lb, hulled bermuda 2 lb, zoysia 2 lb, centipede 0.5 lb — all per 1,000 square feet for a new lawn.

Timing matters more than quantity

Getting the amount right and the timing wrong is worse than the reverse.

Cool-season grasses — fescue, bluegrass, ryegrass — go down in late summer to early fall. The soil is still warm, the weeds are slowing down, and the seedlings get a full cool season to establish before summer stress. Spring is the distant second choice.

Warm-season grasses — bermuda, zoysia, centipede — go down in late spring to early summer, once soil temperatures are consistently in the 65 to 70 degree range.

Seed needs consistent moisture at the surface until germination, which can mean light watering twice a day for two to three weeks. If you cannot commit to that window, wait for a better one — or consider sod, which establishes faster and is less timing-dependent.

Background

Seeding rates are per thousand square feet, and they vary enormously by species — tall fescue goes down at 8 pounds per thousand while centipede goes down at half a pound. Using a fescue rate on centipede seed wastes an extraordinary amount of money; using a centipede rate on fescue gives you a thin lawn.