Grass Seed Calculator

What the grass seed calculator does

Sow too little grass seed and the lawn comes up thin and patchy; sow too much and the seedlings crowd and compete. So the amount matters, and it is not the same for every grass. This works it out. You give it the lawn size, whether you are starting fresh or thickening an existing lawn, and the grass type, and it returns the pounds of seed you need.

Below is how it gets there, the difference between the two project types, and why each grass has its own rate.

How to use it

1. Enter the length and width. The lawn area, each with its own unit.
2. Choose the project type. New Lawn for bare ground, or Overseeding to thicken grass that is already there.
3. Pick the grass type from the list, then press Calculate, or Reset to clear it.

How the seed amount is worked out

Seeding rates are quoted in pounds per 1,000 square feet, so the calculator works out your area in those units and multiplies by the right rate:

Seed needed = (lawn area ÷ 1,000) × the rate for your grass

Each grass type carries its own rate, and the calculator also halves it when you choose overseeding rather than a new lawn. It shows the total pounds along with the rate it used, so you can see where the figure comes from, and the area in both square feet and acres.

New lawn or overseeding

The two project types use different rates for a simple reason. A new lawn is bare ground, so the seed has to cover every inch and establish a full stand from nothing, which takes the full rate. Overseeding is sowing into a lawn that is already mostly there, just thin in places, so it needs roughly half as much seed to fill the gaps.

Choosing the wrong one throws the amount off in an obvious way: the full rate over an existing lawn wastes seed, while a half rate on bare ground leaves it sparse. So it is worth being honest about which job this really is.

A worked example: a 2,000 sq ft lawn

Say you are seeding a new lawn of 2,000 square feet with tall fescue.

Tall fescue for a new lawn goes down at about 10 pounds per 1,000 square feet, so that is (2,000 ÷ 1,000) × 10 = 20 pounds of seed. If you were overseeding an existing fescue lawn instead, it would be half that, about 10 pounds.

So the same lawn needs twice as much seed to start from scratch as it does to thicken up, which is the whole reason the project type is a question.

Why the rate depends on the grass

Different grasses need very different amounts of seed, and it comes down to seed size and how the plant spreads. A fine seed like Kentucky bluegrass has many more seeds per pound and spreads sideways on its own, so it goes down light. A larger seed like tall fescue has fewer seeds per pound and forms clumps rather than running, so it needs a heavier rate to fill in evenly.

That is why the list matters. Picking the grass you are actually sowing gets you the rate suited to it, rather than a one-size figure that would be too heavy for one grass and too light for another. The rates here sit at the fuller end, which leans toward a thick, even lawn.

Questions people ask

How much grass seed do I need?

Work out your area in thousands of square feet and multiply by the rate for your grass. A new 2,000 square foot tall fescue lawn needs about 20 pounds.

How much seed for overseeding?

Roughly half the new-lawn rate, since you are filling gaps in a lawn that is already there rather than covering bare ground.

Why does bluegrass need less seed than fescue?

Bluegrass has a much finer seed, so a pound holds far more of it, and it spreads sideways to fill in. Tall fescue has a larger seed and clumps, so it needs more.

Can I use too much grass seed?

Yes. Oversowing makes the seedlings crowd and compete, which slows them and weakens the stand, so heavier is not better past the recommended rate.

References

A quick note on where these figures come from. The seeding rates, and the rule of roughly halving the rate for overseeding, follow the turfgrass guidance published by university cooperative extension services. The area and acre conversions follow the US National Institute of Standards and Technology guide.

1. University of Missouri Extension, Managing Lawns and Turfgrass (seeding rates by species). https://extension.missouri.edu/publications/mg10
2. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811