Grass Seed Calculator

How much grass seed do I actually need?

Every seeding rate you see on a bag is quoted the same way: pounds of seed per 1,000 square feet. So the whole job is two steps. Measure the lawn, then multiply. The calculator above does the arithmetic, but it is worth seeing it once by hand so you can sanity-check the number on the bag in the shop.

The formula is: pounds of seed = (lawn area in sq ft ÷ 1,000) × seeding rate. For overseeding an existing lawn you halve the result, because you are filling gaps rather than covering bare soil.

Worked example: a 2,000 sq ft tall fescue lawn

Say the back garden measures 50 ft by 40 ft, which is 2,000 sq ft. Tall fescue is sown at 8 lb per 1,000 sq ft for a new lawn. That gives 2,000 ÷ 1,000 = 2, and 2 × 8 = 16 lb of seed. In shop terms that is two 7 lb boxes plus a 3 lb box (17 lb), or a single 20 lb bag if you would rather have spare for touch-ups next spring.

Now overseed the same lawn instead: 16 ÷ 2 = 8 lb. If you switched the species to Kentucky bluegrass at 3 lb per 1,000 sq ft, a new lawn would need only 6 lb, and overseeding just 3 lb. Same patch of grass, a five-fold difference in seed — which is why picking the right species in the dropdown matters more than measuring to the inch.

Typical seeding rates

Cool-season versus warm-season grass

The species list splits along a line that runs roughly through Tennessee and Oklahoma in the US. North of it, and across all of the UK and Ireland, cool-season grasses dominate: tall fescue, Kentucky bluegrass, perennial ryegrass and the fine fescues. They grow hardest in spring and autumn and sulk in mid-summer heat. South of that line, bermudagrass and zoysia take over — they love heat, go dormant and brown after frost, and are usually sown in late spring once soil temperatures hold above 65-70°F.

The transition zone in between is the awkward middle, and most people there hedge with tall fescue, which tolerates both ends better than anything else. That is why tall fescue is the default selection in this tool.

Seedbed preparation, in order

Seed lying on hard ground is bird food. Before you spread anything: mow the existing grass short, at 1 to 1.5 inches, and bag the clippings. Rake or dethatch until you can see soil between the plants — seed needs soil contact, not thatch contact. Core aerate if the ground is compacted, which for most family lawns it is. Level any hollows with a thin topdressing of screened soil or compost, no more than half an inch at a time.

Spread half your seed walking north-south and the other half east-west. A single pass always leaves stripes. Set a broadcast spreader to roughly half the rate the bag suggests and do two passes rather than one heavy one. Finish by raking very lightly so seed settles into the top quarter-inch, and roll or tread it down if the surface is loose.

Watering: the part people get wrong

Germinating seed dies within hours of drying out. For the first two weeks water lightly two or three times a day, just enough to keep the top inch damp — not soaked, which floats seed into drifts. Once seedlings are up and mowed twice, switch to deep watering once or twice a week to push roots downward. Hold off nitrogen-heavy feeds until after the second mow; use a starter fertiliser high in phosphorus at seeding time instead, where local rules allow it.

Limits of this calculator

Rates here are mid-range figures used by seed suppliers in the US and UK. Your bag may state something different because of coating (coated seed weighs more per viable seed), a mixture rather than a single species, or a supplier who assumes irrigation. When the bag disagrees with this tool, follow the bag. Also add 5-10% if your lawn is an irregular shape and you measured a bounding rectangle, or if birds are a known problem and you are not covering the seed with a thin mulch.