How much grass seed do I need per square metre?

For a brand-new lawn most general-purpose rye/fescue mixes are sown at about 35 g/m² (roughly 1 oz per square yard). When overseeding an existing lawn you use less — around 25 g/m² — because the established grass already covers much of the ground. Fine ornamental mixes go lighter (~25 g/m²) and hard-wearing family blends heavier (up to 50 g/m²).

What is the difference between a new lawn and overseeding?

A new lawn is sown onto bare, prepared soil and needs full coverage, so it uses a higher rate. Overseeding spreads seed over an existing lawn to thicken it or repair patches, so you only need enough to fill the gaps — typically about two-thirds of the new-lawn rate. This calculator switches the rate automatically when you change the job type.

How do I convert the seed weight to pounds?

The calculator shows both at once. To convert manually, multiply kilograms by 2.205 to get pounds, or divide grams by 28.35 to get ounces. For example, 3 kg of seed is about 6.6 lb.

How is the cost estimate worked out?

Cost equals the seed weight in kilograms multiplied by the price you enter per kilogram. Because seed is usually sold in fixed bag sizes, the tool also rounds up to whole bags and shows that total separately, along with how much spare seed you would have left over.

Should I buy extra grass seed?

A little extra is sensible. Birds, wind, uneven spreading, and patchy germination all waste some seed, and keeping a small reserve lets you touch up thin spots a few weeks later. Buying one extra bag, or adding 5 to 10 percent, is a common rule of thumb — the leftover figure in the breakdown helps you judge this.

When is the best time to sow grass seed?

In temperate climates early autumn is ideal — warm soil and reliable moisture speed germination with less competition from weeds. Mid-to-late spring is the next best window. Avoid the height of summer (drought stress) and deep winter (soil too cold to germinate). Keep newly sown seed consistently moist for the first few weeks.

Grass Seed Calculator

A grass seed calculator that tells you exactly how much seed to buy for your lawn and what it will cost. Enter your lawn’s dimensions in either metric or imperial units, choose a seed mix, say whether you’re starting a new lawn or overseeding an existing one, and the tool returns the quantity in grams, kilograms, pounds, and ounces — plus a price estimate and the number of bags to buy. It’s built for gardeners, landscapers, and groundskeepers who want a defensible number instead of guesswork at the garden centre.

How it works

The calculator does two simple things very carefully. First it works out the area of your lawn. For a rectangle that’s length times width; for a circle it’s π times the radius squared (radius is half the diameter); or you can type a known area directly. Whatever units you pick, the result is converted internally to square metres (1 ft² = 0.0929 m²) so the seed maths is always consistent, and it shows the area back to you in both m² and ft².

Then it applies a sowing rate measured in grams per square metre. Different grasses and different jobs call for different rates: a bare new lawn needs full coverage, while overseeding an existing lawn needs roughly two-thirds as much because established grass already holds the ground. The core formula is:

seed (kg) = area (m²) × rate (g/m²) ÷ 1000

Multiply that weight by your price per kilogram and you get the cost. Because seed sells in fixed bag sizes, the tool also rounds up to whole bags, shows the bag count, the total at that quantity, and how much spare seed you’ll be left with.

Worked example

Say you have a rectangular back garden lawn that is 10 m long and 8 m wide. That’s an area of 80 m². You’re starting from bare soil, so you choose New lawn with a general-purpose mix at 35 g/m².

Seed needed = 80 × 35 ÷ 1000 = 2.8 kg (about 6.2 lb)
At a seed price of £12/kg, the exact cost is 2.8 × 12 = £33.60
Seed is sold in 1.5 kg bags, so you need 2 bags = 3.0 kg, costing £36.00, leaving 0.2 kg spare for touch-ups

Now switch the same lawn to Overseed at 25 g/m² and the requirement drops to 80 × 25 ÷ 1000 = 2.0 kg — a useful saving when you’re only thickening, not starting over.

Reference rates

These are typical domestic sowing rates quoted on UK and US grass-seed bags. One ounce per square yard is very close to 33 g/m², which is why “about 35 g/m²” is the standard new-lawn figure.

Seed mix	New lawn	Overseed
General purpose (rye/fescue)	35 g/m²	25 g/m²
Hard-wearing / family	50 g/m²	35 g/m²
Fine ornamental	25 g/m²	18 g/m²
Wildflower / meadow	4 g/m²	3 g/m²

Always check the rate printed on your specific seed bag — it overrides any general figure, and you can type it into the custom-rate fields. Every calculation here happens in your browser; no measurements or prices are ever uploaded.