Grass Seed Calculator
Enter your lawn area and seed rate to see how much grass seed you need, in kilograms and grams.
Kilograms and grams
Get both figures, so you can buy by the bag and still measure out smaller patches accurately.
Rates vary
A new lawn sows at about 35 g/m²; overseeding an existing lawn needs roughly 25 g/m².
How much grass seed do I need?
From lawn area to kilograms
A grass seed calculator turns a lawn area and a seed rate into the amount of seed to buy. It multiplies the area you want to sow by the grams of seed each square metre needs, then converts the total to kilograms — the figure suppliers sell against.
Enter your lawn area and the seed rate to get the amount of grass seed in kilograms and grams.
The amount of seed is the lawn area multiplied by the seed rate. That gives grams; dividing by 1,000 converts it to kilograms.
Seed = area × seed rateThe seed rate is entered in grams per square metre because that is how seed bags specify coverage. Multiplying by the area gives the total in grams, which the calculator divides by 1,000 to show kilograms.
Suppose you are sowing a new 100 m² lawn from bare soil at the standard new-lawn rate.
Pick the seed rate
A new lawn sows at about 35 g/m².
Multiply by the area
100 m² × 35 g/m² = 3,500 g.
Convert to kilograms
3,500 g ÷ 1000 = 3.5 kg of grass seed.
The seed rate you choose drives the answer. Sowing a new lawn from bare soil sits around 35 g/m² for full coverage, while overseeding an existing lawn to thicken it needs roughly 25 g/m² — according to RHS guidance. Lowering the rate from 35 to 25 g/m² cuts the seed you need by nearly a third for the same area. The kilogram figure tells you how many bags to buy, and the gram figure helps you measure out smaller or patchy areas. Treat both as the minimum the lawn needs.
The formula is exact, but a few real-world factors keep it an estimate.
Seed rates vary by mix
Rates differ by seed mix — fine ornamental blends, hard-wearing utility mixes, and shade mixes each sow at slightly different rates, so check the figure on your seed bag. Over-seeding well above the recommended rate wastes seed and can crowd seedlings rather than thicken the lawn. Some seed is also lost to birds and wind, so buying around 5–10% extra is sensible.