This grass seed calculator works out how much grass seed you need from your lawn's length and width, the grass type you are sowing, and whether you are starting a new lawn or overseeding an existing one. It returns the total in pounds and kilograms, along with the seeding rate it used in pounds per 1,000 square feet.
Quick answers by lawn size: 1,000 sq ft · 1 acre · 2 acres · ½ acre · 100 m² · per sq ft · per m² · to overseed · new lawn vs overseeding
Grass seed is sold by weight but sown by rate — pounds per 1,000 square feet of ground. The calculator does that conversion for your lawn.
Enter length and width in feet, yards or metres and the tool converts to square feet, since one square yard is 9 square feet and one square metre is 10.76 square feet. For an L-shaped or irregular lawn, split it into rectangles, calculate each one and add the totals. Leave out paths, beds and anything else you will not be sowing.
Seeding rates vary with seed size and growth habit: fine, spreading grasses cover ground with far less seed than coarse bunch grasses. The new-lawn presets are 2.5 lb per 1,000 sq ft for Kentucky bluegrass, 1.5 for bermuda, 5 for a generic mix, 7 for tall fescue and 8 for perennial ryegrass. These are mid-range values from typical U.S. cooperative-extension recommendations, and you can override the rate if your seed bag states its own.
Overseeding uses roughly half the new-lawn rate, because the ground is already covered and the new seedlings have to compete with established turf rather than colonise bare soil. Tall fescue drops from 7 to 3.5 lb per 1,000 sq ft, perennial ryegrass from 8 to 4, and Kentucky bluegrass from 2.5 to 1.5. The new lawn vs overseeding comparison sets the two side by side.
The output gives the total seed in pounds and kilograms plus the rate it applied, so you can check it against the bag before buying. Sowing well over the rate crowds seedlings and invites disease, while sowing under it leaves thin patches that weeds fill. For common lawn sizes there are ready-made answers for 1,000 square feet, 1 acre and 100 square metres.