Grass Seed Calculator — Pounds, Bags and Starter Feed

How much grass seed for 5,000 square feet?

For a new lawn in tall fescue at 7 lb per 1,000 sq ft, that is 35 pounds — two 20 lb bags, or one 50 lb bag with some left over. In Kentucky bluegrass it is only 12.5 lb, because bluegrass seed is far smaller and there are many more seeds to the pound.

Overseeding the same 5,000 sq ft with fescue takes half: 17.5 pounds. You will also want about 25 lb of starter fertilizer, and for a new lawn roughly seven bales of straw to cover it.

Why the rates differ so much

Seeding rates look arbitrary until you realize they are about seed count, not weight. A pound of Kentucky bluegrass contains something like two million seeds. A pound of tall fescue contains around 220,000 — nearly ten times fewer, each one much larger.

Both need a similar number of seedlings per square inch to make a lawn, so the fescue rate has to be far higher by weight. This is also why bluegrass is slow: small seeds carry little energy, so they take two to four weeks to emerge and longer still to fill in. Ryegrass is at the other end — big seed, up in five to seven days, which is why it is in almost every quick-repair blend.

A blend of species is usually the right answer for a home lawn. The ryegrass holds the soil while the bluegrass or fescue establishes underneath it.

More seed does not make a better lawn

The instinct when seeding is to be generous, and it backfires. Seedlings that come up too close together compete for water, light and nutrients exactly when they are least able to cope with competition. The result is a thick flush of thin, weak grass that thins out badly in the first hot spell.

Going over the rate also wastes a genuinely expensive input. Good seed is not cheap, and doubling the rate does not double the lawn.

Where extra effort actually pays is in the seed-to-soil contact. Raking lightly so the seed sits in the top quarter inch of soil, or rolling after spreading, does far more for germination than adding another ten pounds.

Starter fertilizer and why it is different

Starter fertilizer is high in phosphorus — the middle number — because phosphorus drives root development, and a new seedling's first job is to make roots. A typical analysis is something like 12-24-12, applied at around 5 lb of product per 1,000 square feet.

It is not the same as regular lawn feed, which is high in nitrogen to push top growth. Nitrogen on a new seeding produces soft leaf on a plant with no root system to support it.

Note that several states restrict phosphorus on established lawns, with an exemption for new seeding. If you are overseeding rather than starting from bare soil, check your local rules — and in most cases a soil test will tell you whether you needed the phosphorus at all.

Straw, and what it is actually for

A light straw cover over new seed does three things: it keeps the surface from drying out between waterings, it stops seed washing away in heavy rain, and it moderates the soil temperature.

The key word is light. You want to still see soil through it — roughly a bale per 750 square feet. A thick mat smothers the seedlings and has to be raked off, which pulls seed with it.

Use certified weed-free straw, not hay. Hay is cut with the seed heads on and you will be planting a field's worth of weeds along with your lawn. Straw is the stalk left after grain harvest and carries far less. Pelletized paper mulch is an alternative that costs more and does not need removing.

Overseeding does not need straw — the existing grass does the same job.

Watering: the part that decides everything

You can get every number on this page right and still fail by letting the seed dry out once. A germinating seed that dries at the wrong moment is dead, not dormant.

For the first two to three weeks the top half inch of soil has to stay damp — that usually means light watering two or three times a day, not one long soak. Short and frequent is the rule, and it reverses completely once the grass is up.

After germination, shift to deeper and less often to pull the roots down. By about the fourth week you should be watering every other day; by the sixth, twice a week and deeply.

First mow when the grass reaches about three inches, taking off no more than the top third, with a sharp blade. A dull blade tears young grass out of soft ground by the roots.