Overseeding Calculator: Grass Seed for an Existing Lawn

This calculator works out how much grass seed to buy when you overseed an existing lawn. Enter the lawn area, the overseeding rate from the seed label in pounds per 1,000 ft², and the bag size; you get the pounds and kilograms of seed and the number of bags.

Formula

Seed (lb) = lawn area (ft²) ÷ 1,000 × rate (lb per 1,000 ft²). Kilograms = pounds × 0.45359237. Bags = seed ÷ bag size, rounded up. The metric rate is rate × 453.59237 g ÷ 92.90304 m² (the area of 1,000 ft²).

• a: lawn area in square feet
• rate: overseeding rate printed on the seed bag, in pounds per 1,000 ft²
• bag: weight of one bag in pounds

Worked Example

A 5,000 ft² lawn, 4 lb per 1,000 ft², 20 lb bags.

1. Seed = 5,000 ÷ 1,000 × 4 = 20.0 lb
2. In kilograms: 20 × 0.45359237 = 9.07 kg
3. Bags = 20 ÷ 20 = 1 bag
4. Metric rate = 4 × 453.59237 ÷ 92.90304 = 19.53 g/m²

How to use the result

Set your spreader to the label’s setting for the product, split the seed into two halves and spread them in two directions at right angles for even cover. Keep the seedbed moist until the new grass is established.

Limits and common mistakes

• Rates differ a lot between grass types: fine-seeded grasses use far less seed by weight than coarse ones. Always take the rate from the bag.
• Overseeding rates are usually lower than rates for a bare new lawn; do not mix them up.
• Measure only the area you will seed, not the whole yard.

Frequently Asked Questions

How much seed do I need to overseed 1,000 square feet?

Exactly the label rate: at 4 lb per 1,000 ft² it is 4 lb.

Is overseeding the same as seeding a new lawn?

No. A new lawn on bare soil usually needs a higher rate; the label gives both.

Can I use too much seed?

Yes. Too much seed makes seedlings compete for light and water and can thin the stand.

What if my bag is in kilograms?

Enter the bag size in pounds (1 kg = 2.2046 lb).

in a | Lawn area | ft² | 5000 | 0
in rate | Overseeding rate (from the label) | lb per 1,000 ft² | 4 | 0 | 20
in bag | Bag size | lb | 20 | 0.1
out lb | Seed needed | lb | a / 1000 * rate | 1
out kg | Seed needed | kg | lb * 0.45359237 | 2
out bags | Bags (rounded up) | bags | ceil(round(lb / bag, 6)) | 0
out gm2 | Rate in metric | g/m² | rate * 453.59237 / 92.90304 | 2