Grass seed calculator

One rate for every grass is the mistake

The most common answer online is "5 lb per 1,000 sq ft". There is no grass for which that is universally right, and for several it is dangerous.

Published rates per 1,000 sq ft for a new lawn:

Tall fescue and bermudagrass differ by a factor of four to eight. The reason is seed size and habit: there are on the order of two hundred thousand tall fescue seeds in a pound and roughly ten times as many bermudagrass seeds, and bermuda then spreads by stolons and rhizomes to fill in. Put 5 lb of bermuda on 1,000 sq ft and you get a mat of seedlings competing for the same light and water; they damp off, and the stand thins to bare ground. Centipedegrass at that rate is money set on fire — a quarter pound does the same job.

Purdue Extension's AY-20 seeding table independently lists 100% tall fescue at 8.0 lb and 100% Kentucky bluegrass at 2.0 lb per 1,000 sq ft, which brackets the Penn State figures from the other side.

Overseeding takes about half

Clemson's Home & Garden Information Center puts ryegrass at 5 lb per 1,000 sq ft into an established lawn and 10 lb on ground with no permanent grass — a clean 2:1. The limiting factor when overseeding is not seed, it is light and water at the soil surface, both of which the existing turf is already using. Doubling the seed does not double the take; it wastes the seed and can smother what you have.

Buying it

Bags come in a wide, uneven spread — 3, 7, 10, 20, 25 and 50 lb — so the bag count often forces you to overshoot. A quarter acre (10,890 sq ft) of tall fescue is 87.1 lb, which is two 50 lb bags with 13 lb spare. Store the remainder cool, dry and sealed; germination falls off noticeably after a year.

Two things that are not seed but decide whether it works: seed-to-soil contact, and water. Broadcast onto unprepared thatch, even a correct rate germinates in patches.

Where the published rate stops applying

Every rate in the table is for a pure stand of one species on a prepared seedbed. Four ways your bag is not that:

• Most bags are mixtures, not pure stands. A sun-and-shade mix, a contractor's mix or a bluegrass blend has its own rate printed on it, and that rate is not the rate of any single grass inside it. If the bag states one, use the bag's. It also knows things this page cannot — whether the seed is coated, whether there is a mulch carrier in the weight, and what its germination percentage is. Two bags of equal weight are not necessarily equal amounts of live seed.
• The half rate for overseeding is one species' published pair. Clemson gives it for ryegrass — 5 lb into established turf against 10 lb on bare ground. This page applies the same halving to every grass in the list, which is a reasonable extension and not a published figure for each of them. Where your extension service publishes an overseeding rate for your grass, prefer it.
• The defaults are the top of each range, on purpose. They suit bare soil, a rough seedbed and spring sowing. There is no control here for the favourable case — a well-prepared bed seeded in early autumn under irrigation takes the low end of the published range, and to buy at that you have to work it out yourself from the table above.
• Lawn rates only. Pasture, erosion-control and wildflower seeding are different crops at different rates, and the numbers here will be badly wrong for them.

And the quantity is the smallest part of the outcome. Timing in the season, soil contact, and keeping the surface damp until germination decide whether any of this seed becomes lawn. None of them are on this page.