Overseeding Calculator

How much seed to overseed a lawn

An overseeding calculator tells you how many pounds of grass seed to spread over an existing lawn. Enter the length and width of the area in feet. The tool divides the area by 500 square feet per pound and multiplies by your price per pound.

Overseeding means adding seed to grass that is already growing, to thicken thin spots and fill bare patches. Because the old grass is already there, you need less seed than on bare ground. The rate on this page is half the new-lawn rate.

The formula in plain words

1. Area = length × width.
2. Pounds of seed = area ÷ 500.
3. Cost = pounds × price per pound.

For comparison, a brand-new lawn on bare soil uses 250 square feet per pound in this site’s grass seed calculator. Overseeding takes half as much.

Worked example: the default lawn

A lawn measures 50 ft by 20 ft. Seed costs $4 per pound.

• Area: 50 × 20 = 1,000 sq ft
• Seed: 1,000 ÷ 500 = 2.00 lb
• Cost: 2 × $4 = $8.00

Worked example: an odd-sized yard

A side yard is 45 ft by 35 ft.

• Area: 45 × 35 = 1,575 sq ft
• Seed: 1,575 ÷ 500 = 3.15 lb
• Cost: 3.15 × $4 = $12.60

You would round up to whatever bag you can buy. A 5 lb bag would cover this yard with some left over for bare patches.

Seed needed by lawn size

Lawn area	Pounds of seed
1,000 sq ft	2
2,500 sq ft	5
5,000 sq ft	10
10,000 sq ft	20

If a lawn has a few bad patches, you do not need to seed the entire area at the full rate. Seed the patches heavier and the rest lightly, or enter only the thin areas.

How to measure the area

Walk the lawn with a tape or a measuring wheel. Break it into rectangles and add them up. Skip beds, driveways, and the house. If a section is a rough circle, multiply the diameter by itself and then by 0.785 for the area.

Enter the length in the first field and the width in the second. For several rectangles, run each one and add the pounds.

Getting the best results

Seed has to touch soil to germinate. Mow the lawn short, rake out loose thatch, and consider a pass with a core aerator before you spread. After seeding, keep the surface evenly moist until the new grass is up. Walking on seeded ground and heavy use will cut into your results.

Timing matters as well. Cool-season grasses are often seeded in early fall, while warm-season grasses go in during late spring and early summer. Ask your local extension office what window fits your grass.

A thin layer of compost spread across the surface can help hold moisture. The lawn topdressing calculator estimates how much you would need.

What the tool does not cover

The result is only the seed. Starter fertilizer, compost, straw cover, and any rental of a spreader or aerator are separate costs. Add a few dollars for those if you want a full budget, and check your seed bag for the recommended starter product.

Common mistakes

• Using the new-lawn rate. That wastes seed and can crowd seedlings.
• Treating 500 as exact for every blend. Check the bag, since seed size varies.
• Seeding without preparing the soil. Seed sitting on thick thatch rarely takes.
• Forgetting to water. Seed that dries out after sprouting will die.
• Spreading unevenly. A drop or broadcast spreader gives better coverage than hand throwing.

Related calculators

Start with the grass seed calculator for a bare lot, use the lawn topdressing calculator for a compost layer, and the lawn aeration cost calculator if you want a professional to open the soil before you seed.