How much grass seed you need
Grass seed is sold by the pound and sown by the thousand square feet, so the math is short: work out how many thousands of square feet you have and multiply by the seeding rate for your grass. The rates on this page come from university extension services: Penn State for the cool-season grasses and mixtures that fill Northern and transition-zone lawns, and NC State for the warm-season grasses of the South. Each publishes a range, and we use the middle of it and show you both ends.
The step most calculators skip is the one on the seed tag. A pound of grass seed is not a pound of live seed of the grass you want. Some of it is other seed, weed seed, chaff, or coating, and some of the real seed will never sprout. The tag tells you how much, and it can change the answer by a fifth or more.
The formula
pure live seed (PLS) = purity % × germination % ÷ 100
seed to buy (lb) = live seed needed ÷ PLS
Area is in square feet and the seeding rate is in pounds per 1,000 square feet. Purity is the "pure seed" percentage on the tag and germination is the share of that seed that sprouted in the lab test; both are printed on the seed tag of every bag. Penn State's guidance treats the recommended rate as pounds of pure live seed, so dividing by your bag's PLS turns it into pounds of what is actually in the bag.
Worked example
A new 30 by 25 ft tall fescue lawn is 750 sq ft, or 0.75 thousand. Penn State recommends 6 to 8 lb per 1,000 sq ft, so at the middle, 7 lb, the lawn calls for 0.75 × 7 = 5.25 lb of pure live seed.
The bag's tag says 97% pure seed and 85% germination, so it is 97 × 85 ÷ 100 = 82.5% PLS. Dividing, 5.25 ÷ 0.825 = 6.37 lb, so you buy 6.4 pounds: 21.3% more than the rate alone would suggest.
Pure live seed: the number that decides whether a bag is a bargain
Penn State Extension illustrates PLS with two seed lots, and running them through this calculator makes the point better than any warning could. A cheap lot at $0.95 a pound with 85% purity and 60% germination is only 51% PLS. A regular lot at $1.50 a pound with 99% purity and 90% germination is 89.1% PLS. For the 750 sq ft lawn above, you would need 10.29 lb of the cheap seed, costing $9.78, or 5.89 lb of the regular seed, costing $8.84. The bargain bag costs more, and nearly half of what you spread from it is not live seed of the grass you chose, which can include the weed seed you were trying to crowd out.
So compare bags by price per pound of pure live seed, which is the price divided by the PLS: $1.86 for the cheap lot against $1.68 for the regular one. Coated seed follows the same rule. NC State notes that seed coatings add weight and call for higher rates, and on the tag the coating is counted as inert matter, so it shows up as a lower purity and the PLS math takes care of it for you.
Seeding rates by grass
| Grass | New lawn (lb per 1,000 sq ft) | Overseeding (lb per 1,000 sq ft) | Source |
|---|---|---|---|
| Tall fescue | 6 to 8 | 3 to 6 | Penn State |
| Kentucky bluegrass | 2 to 3 | 1 to 2.25 | Penn State |
| Perennial ryegrass | 4 to 5 | 2 to 3.75 | Penn State |
| Fine fescue | 4 to 5 | 2 to 3.75 | Penn State |
| Bluegrass and ryegrass mix | 2 to 3 | 1 to 2.25 | Penn State |
| Bluegrass, fine fescue, and ryegrass mix | 3 to 4 | 1.5 to 3 | Penn State |
| Shade mix | 4 | 2 to 3 | Penn State |
| Bermudagrass | 1 to 2 | 0.5 to 1.5 | NC State |
| Zoysiagrass | 1 to 2 | 0.5 to 1.5 | NC State |
| Centipedegrass | 0.25 to 0.5 | 0.125 to 0.375 | NC State |
The overseeding column applies NC State's advice that seeding into an existing lawn may use a 25 to 50 percent lower rate, so it runs from half the low rate to three quarters of the high one. Extension services in different states do publish somewhat different figures for the same grass: Purdue gives Kentucky bluegrass 1.5 to 2 lb, NC State gives it 1 to 2, and UF/IFAS gives bermudagrass 1 to 4 lb against NC State's 1 to 2. That spread is the honest state of the science rather than anyone being wrong, so if your own state's extension office publishes a rate, it knows your climate best, and the rate box takes it. Higher is not better, either: NC State warns that seeding above these rates can produce weak, thin stands that are more prone to disease.
When to seed, and how to keep it alive
Timing matters as much as the pounds. Cool-season grasses (tall fescue, bluegrass, ryegrass, fine fescue) do best sown in late summer to early fall, when the soil is still warm and the weeds are slowing down. Purdue puts the best window at August 15 to September 15 in the northern halves of Indiana and Illinois and September in the southern halves, and notes that waiting even a week can add two to four weeks to how long the lawn takes to fill in. Warm-season grasses such as bermudagrass and zoysiagrass are the opposite: NC State recommends seeding them from about April to July.
Once it is down, spread half the seed walking one way and half at right angles, rake it in lightly, and do not let the seedbed dry out. Purdue's guidance is to water two to four times a day, keeping the top half inch to inch of soil moist without flooding it, then water less often but more deeply once the seedlings are about two inches tall. If you would rather skip the wait, the sod calculator prices out the instant version, and the soil calculator handles any topsoil you add first.