Grass Seed Calculator

How to use the grass seed calculator

Measure the lawn area in feet — length times width — and split odd shapes into rough rectangles, running the calculator per section. For established yards, pacing it off is fine; seed quantities are forgiving at the edges. The number that is NOT forgiving is the rate, which is why the grass-type select does the real work here.

Seeding rates differ wildly by species, and it isn’t marketing — it’s seed size. Tall fescue seeds are big (about 230,000 per pound), so covering 1,000 sq ft takes roughly 9 lbs. Kentucky bluegrass seeds are tiny (over 1.4 million per pound), so 3 lbs blankets the same ground. Bermudagrass is tinier still, hence 1.5 lbs. The rates in this calculator are midpoints of US university extension-service recommendations for NEW lawns; your seed bag will print its own range, and if it differs meaningfully from the preset, trust the bag.

The project select cuts the rate in half for overseeding — spreading seed into an existing lawn to thicken it. Existing grass already occupies most of the soil contact a seedling needs, so full-rate overseeding just feeds birds and washes into gutters. New-lawn rate is for bare or nearly bare soil: a fresh grade, a renovation after killing off the old turf, or repair patches raked to dirt.

Resist the "double it for insurance" instinct — over-seeding a new lawn is a real failure mode, not extra credit. Seedlings packed too densely compete for the same light, water, and nitrogen; they come up thin-bladed and weak, stay vulnerable to damping-off fungus, and the stand often looks worse at week six than a correctly seeded lawn. If you want insurance, spend it on soil contact (rake the seed in lightly and roll it) and on watering discipline, not on extra pounds of seed.

Timing beats quantity, too. Cool-season grasses — tall fescue, Kentucky bluegrass, perennial ryegrass, fine fescue — establish best seeded in late summer to early fall, with early spring a second choice. Warm-season bermudagrass wants late spring through early summer, once soil reliably tops 65°F. Seed a cool-season lawn in June or bermuda in October and no rate will save it; seed in the right window and these rates produce a full stand in one season.

The formula

Area in thousands of square feet, times the species rate, halved for overseeding:

lbs of seed = (length × width ÷ 1,000) × rate × project factor
rates (new lawn, lbs/1,000 sq ft): tall fescue 9 · KBG 3 · perennial rye 8 · fine fescue 5 · bermuda 1.5
project factor: new lawn = 1 · overseeding = 0.5

Worked example with the defaults: a 25 × 40 ft area is exactly 1,000 sq ft. Tall fescue at the new-lawn rate is 9 lbs; switch the project to overseeding and the same area needs 4.5 lbs. The same 1,000 sq ft in Kentucky bluegrass would take just 3 lbs — smaller seed, more seeds per pound.

Buy to the nearest bag size at or above the number — and check the label’s pure-live-seed math (germination % × purity %) on bargain mixes; a cheap bag that’s 30% filler isn’t cheap.

Frequently asked questions

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