Grass Seed Calculator
How much grass seed your lawn needs: rectangles, circles, triangles, a known area or a list of areas, seeding rates for 11 grass types (new lawn or overseeding) or the rate on your bag, pure-live-seed adjustment, bags to buy and cost.
How it works
area: rectangle L × W · circle π × d² ÷ 4 · triangle b × h ÷ 2, minus areas left out · seed (lb) = area ÷ 1,000 × rate (lb per 1,000 sq ft), or seed (g) = area (m²) × rate (g/m²) · with PLS: seed ÷ (purity × germination) · bags = seed ÷ bag size, rounded up
Lawn area in square feet, divided by 1,000, gives the number of 1,000-sq-ft units, and multiplying by the seeding rate gives pounds of seed (in metric, square meters times grams per square meter). Rectangles, circles and triangles each have their own area formula, and the list mode adds several areas together. Pick a grass type to use a typical extension-service rate for a new lawn or for overseeding, which is about half, or enter the rate printed on your seed bag. If the rate is given as pure live seed, the purity and germination on the label tell you how much bagged seed delivers that much live seed. The result is rounded up because seed is sold in bags.
Worked example
A 50 × 40 ft lawn is 2,000 sq ft. At the standard new-lawn rate of 5 lb per 1,000 sq ft, that’s 2,000 ÷ 1,000 × 5 = 10 lb of seed, or two 7 lb bags with 4 lb left over. As tall fescue at 7 lb it would need 14 lb, and overseeding it at 4 lb would need 8 lb.
Sources: Penn State Extension — Turfgrass seed and seed mixtures (rates, PLS) · NC State Extension Gardener Handbook — Lawns (seeding rates and timing)
Frequently asked questions
What’s the difference between new-lawn and overseeding rates?
Seeding a bare area from scratch uses the full rate (4–6 lb per 1,000 sq ft for most cool-season grasses) so the ground fills in. Overseeding thickens an existing lawn, so you use about half that — the established grass is already doing most of the covering.
Does the seed type change the rate?
A lot. Tall fescue and perennial ryegrass have big seeds and need 5–8 lb per 1,000 sq ft, Kentucky bluegrass 2–3 lb, and tiny-seeded centipedegrass only ¼–½ lb. The grass-type list fills in a typical rate, but the bag’s label rate always wins — choose “Use the rate on my seed bag” and enter it.
What is pure live seed (PLS)?
The share of a bag that is seed of the right kind and will actually sprout: purity × germination. A lot with 85% purity and 72% germination is 61% PLS, so you need about 1.6 lb of it for every pound of live seed a recommendation calls for.
When is the best time to seed?
For cool-season grasses, early fall is ideal — warm soil, cooler air and fewer weeds. Spring is the second-best window. Warm-season grasses go down in late spring to early summer once the soil has warmed.
Can I seed St. Augustine grass?
Not practically. St. Augustine and hybrid Bermudagrass produce little or no viable seed and are planted as sod, plugs or sprigs. Use a sod calculator for those.
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This calculator is for educational purposes. Double-check important results before acting on them.