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Grass Seed Quantity Calculator

Enter your lawn size, the sowing rate and the seed price to see how much seed to buy.

How it is calculated

The lawn is treated as a rectangle: length and width in metres, sowing rate in grams per square metre and seed price per kilogram. The tool computes area = length × width (m²), seed in grams = area × rate, seed in kg = grams / 1000 and cost = kg × price per kg. The sowing rate is the number printed on the seed bag for your mix and purpose, which differs between a new lawn and overseeding. No allowance is added for waste, uneven ground or patches you intend to reseed, and the result is not rounded up to whole bags. For an irregular lawn, split it into rectangles and add the areas, or enter an equivalent length and width.

Worked example

A lawn of 10 m by 6 m, sown at 35 g/m², with seed at 9 per kg. Area = 10 × 6 = 60.00 m². Seed = 60 × 35 = 2,100 g, which is 2.10 kg. Cost = 2.10 × 9 = $18.90. You would buy the nearest pack size above 2.10 kg, for example a 2.5 kg bag, and keep the small remainder for patching.

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Common mistakes

  • Entering the sowing rate in pounds per 1,000 sq ft or ounces: the field is grams per square metre.
  • Entering the price per bag instead of per kilogram: the cost is then scaled by the weight in kg, not the bags.
  • Using the rate for overseeding on a bare-soil lawn, which leaves it thin, or the new-lawn rate for overseeding, which wastes seed.
  • Measuring length and width in feet while the area is read as square metres, which overstates the quantity about tenfold.
  • Forgetting flower beds, paths or trees inside the rectangle, which are not lawn and should be subtracted from the area.

About this calculator

Grass seed quantity is a straightforward multiplication of lawn area by the sowing rate recommended for your seed mix and application type, typically expressed in grams per square meter or pounds per 1,000 square feet. Sowing rates differ significantly by purpose: a new lawn sown from bare soil needs a much heavier rate — often 30–50 g/m² — than overseeding an existing lawn to thicken it, which typically needs only 15–25 g/m².

Homeowners and landscapers run this calculation before buying seed because underestimating the rate leaves bare or thin patches prone to weed invasion, while overestimating it wastes seed and can cause overcrowded, weak seedlings competing for light and nutrients. Matching the purchase quantity to the actual area also avoids the common mistake of buying seed sized for a much larger lawn than the one being sown.

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