OneTaskBy Tim King · Updated
Grass Seed Calculator
How Much Seed for a New Lawn or Overseeding
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How the Grass Seed Calculator works
This grass seed calculator gives how much seed to buy for a lawn. Enter the length and width of the area, pick the kind of lawn and whether you are starting from bare soil or thickening an existing one, and the calculator gives the weight of seed, the number of bags, the sowing rate in grams per square metre and how much to sow in each direction for an even finish. The table compares the rates for every mix so you can see the difference between a fine lawn and a hard-wearing one before you buy.
Grass seed calculator: area times sowing rate
Grass seed is sold by weight and sown by area, and the rate depends on what you are sowing and why. Sow too thinly and weeds fill the gaps; too thickly and the seedlings crowd each other, damp off and you have paid for seed that never grew. Working out the quantity from the area and a sensible rate is the whole trick.
Sowing rates. A new lawn from bare soil needs 30 to 40 grams per square metre for most mixes. Ryegrass-based family and hard-wearing mixes have larger seed, so they sit at the top of that range; fine fescue and bent mixes have tiny seed and go on at 25 to 30 g/m². Overseeding a thin but living lawn needs roughly half to two-thirds of the new-lawn rate because established grass fills in around the new seedlings. Wildflower meadow mixes are sown very thinly, around 4 to 5 g/m², because flowers need space and poor competition, and a pure clover lawn needs only a couple of grams per square metre.
Measuring the lawn. Multiply length by width for a rectangle. For an L-shape, split it into two rectangles and add them; for a circle use 3.14 × radius². Irregular borders can be approximated by pacing the average length and width. The fields accept metres or feet and inches, so 40'6" works in the imperial view. Add about 10 percent for awkward shapes and for patching, which the bag count usually absorbs through rounding up.
Bag sizes. Seed comes in boxes and bags from 500 g to 20 kg; 10 kg is a common size for whole lawns. The result shows how many bags of your chosen size you need and how far one bag goes at the selected rate, which is useful when comparing prices: a bag covering 250 m² of fine lawn covers only 200 m² of ryegrass mix.
Sowing evenly. Weigh out half the seed and walk it up and down, then the other half across, which is what the split line gives you. For small areas mix the seed with dry sand or compost to bulk it up. Rake lightly, firm, and keep the surface damp until the seed is well up, typically two to three weeks in spring or autumn. Sow when soil temperatures are above about 8 °C and rain is likely: mid-spring and early autumn are the best windows in a temperate climate.
After sowing. Do not feed a new lawn for the first six weeks; a pre-seeding fertiliser worked into the soil before sowing is better. Mow for the first time when the grass is 5 to 7 cm tall, taking off no more than a third, and keep the blades sharp. Overseeded lawns benefit from scarifying first so the seed reaches soil rather than sitting on thatch.
Grass seed by lawn size
A general-purpose lawn mix at 35 g per m², sold in 10 kg bags.
Lawn area, seed needed and bags by lawn size
| Lawn | Area | Seed | 10 kg bags |
|---|---|---|---|
| 4 × 4 m | 16 m² | 0.56 kg | 1 |
| 6 × 4 m | 24 m² | 0.84 kg | 1 |
| 6 × 6 m | 36 m² | 1.26 kg | 1 |
| 8 × 4 m | 32 m² | 1.12 kg | 1 |
| 8 × 6 m | 48 m² | 1.68 kg | 1 |
| 8 × 8 m | 64 m² | 2.24 kg | 1 |
| 10 × 6 m | 60 m² | 2.1 kg | 1 |
| 10 × 8 m | 80 m² | 2.8 kg | 1 |
| 10 × 10 m | 100 m² | 3.5 kg | 1 |
| 12 × 6 m | 72 m² | 2.52 kg | 1 |
| 12 × 8 m | 96 m² | 3.36 kg | 1 |
| 12 × 10 m | 120 m² | 4.2 kg | 1 |
| 12 × 12 m | 144 m² | 5.04 kg | 1 |
| 14 × 8 m | 112 m² | 3.92 kg | 1 |
| 14 × 10 m | 140 m² | 4.9 kg | 1 |
| 16 × 8 m | 128 m² | 4.48 kg | 1 |
| 16 × 10 m | 160 m² | 5.6 kg | 1 |
| 16 × 12 m | 192 m² | 6.72 kg | 1 |
| 20 × 8 m | 160 m² | 5.6 kg | 1 |
| 20 × 10 m | 200 m² | 7 kg | 1 |
| 20 × 12 m | 240 m² | 8.4 kg | 1 |
Grass seed formula and worked examples
The formula: seed (kg) = lawn area (m²) × sowing rate (g/m²) ÷ 1,000; bags = seed ÷ bag size, rounded up. A general-purpose mix is 35 g/m² for a new lawn and 20 g/m² for overseeding.
First example. A new 10 m × 8 m family lawn is 80 m²; at 35 g/m² it needs 2.8 kg, one 10 kg bag.
Second example. Overseeding a 50 m² lawn at 20 g/m² needs 1 kg, sown as 0.5 kg lengthways and 0.5 kg across.
FAQ
How much grass seed do I need per square metre?
About 35 g/m² for a new general-purpose lawn, 25 to 30 g/m² for a fine ornamental lawn and 40 g/m² for a hard-wearing ryegrass mix. Overseeding needs roughly half to two-thirds of those figures. The calculator uses these rates and gives the total for your lawn.
How much seed for an 80 m² lawn?
Around 2.8 kg for a new general-purpose lawn at 35 g/m², or 1.6 kg to overseed. A fine lawn mix needs 2.4 kg new and 1.2 kg to overseed. One 10 kg bag covers a lawn of about 285 m² at the family-lawn rate.
Can I sow more seed to get a thicker lawn?
Not usefully. Above the recommended rate, seedlings compete with each other and are more prone to damping off, so the lawn ends up patchier, not thicker. Sow at the right rate, keep it watered and feed later once the grass is established.
When is the best time to sow grass seed?
Early autumn is ideal in most temperate climates: the soil is warm, rain is reliable and weeds are slowing down. Mid-spring is the second choice. Avoid high summer unless you can water daily and avoid sowing into cold soil below about 8 °C.
What is the difference between overseeding and a new lawn?
Overseeding adds seed to a living but thin lawn after scarifying and aerating, so the existing grass helps fill in and about half the seed is needed. A new lawn starts from prepared bare soil and needs the full rate to give complete cover.
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