How much grass seed for 1000 square feet?

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:

Grass	lb per 1,000 sq ft	Source
Turf-type tall fescue	6–8	Penn State Extension
Perennial ryegrass	4–5	Penn State Extension
Fine fescue	4–5	Penn State Extension
Kentucky bluegrass	2–3	Penn State Extension
Creeping bentgrass	1	Penn State Extension
Bermudagrass (hulled)	1–2	NC State Extension
Zoysiagrass	1–2	NC State Extension
Centipedegrass	0.25–0.5	NC State Extension

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.