How much grass seed does the lawn need?
Area times the rate on the packet, which is usually 35 grams a square metre for a new lawn.
2.8 kg of seed
2800 grams over 80 square metres.
- 10 m by 8 m is 80 square metres.
- 80 square metres at 35 grams a square metre is 2800 grams.
- 2800 grams is 2.8 kg.
Working it out on paper
The sum is short enough to do by hand, and it is the same sum the tool does.
- Multiply the length by the width to get the area in square metres.
- Multiply the area by the rate in grams a square metre.
- Divide by 1000 for kilograms.
| Area | New lawn, 35 g | Overseeding, 25 g |
|---|---|---|
| 10 square metres | 350 g | 250 g |
| 25 square metres | 875 g | 625 g |
| 50 square metres | 1.75 kg | 1.25 kg |
| 100 square metres | 3.5 kg | 2.5 kg |
| 250 square metres | 8.75 kg | 6.25 kg |
The rate on the packet wins
There is no single correct sowing rate for grass seed, and anyone who gives you one without asking what mix you have bought is guessing. A fine ornamental mix with a lot of bent and fescue in it has far more seeds per gram than a hard wearing ryegrass mix, so the two are sown at different weights to put roughly the same number of seeds on the ground. The Royal Horticultural Society is explicit about this: the weight per square metre is given on the packet and varies with the mix.
The figures this tool starts with, 35 grams a square metre for a new lawn and 25 for overseeding, are the common trade numbers and a reasonable default. Change the rate to whatever your packet says and the answer changes with it. That is the whole reason the rate is an input rather than something hidden inside the sum.
New lawn or overseeding
A new lawn is sown onto bare, prepared soil, so every seed has room and light and the rate is set by how thick you want the finished sward. Overseeding goes into an existing lawn to thicken it, and the rate is lower because the grass that is already there takes up most of the space. Overseeding into a lawn you have scarified and aerated first works. Overseeding into an untouched lawn mostly feeds birds, because the seed never reaches soil.
Preparation is most of the job
The seed is the cheap part and the ground is the work. Dig or rotovate to about twenty centimetres, take out perennial weed roots properly rather than chopping them up, and add compost to light soil. Then leave it for five or six weeks to settle before you do anything else, which is the step everybody skips and the reason so many new lawns end up with dips in them. Firm the surface with short shuffling steps, rake it in several directions to a fine tilth, and only then sow.
Sow half the seed walking up and down and the other half walking side to side. It is a better way to get an even spread by hand than any amount of care in one direction, and it is what the RHS recommends. Rake lightly, water with a fine rose, and keep the surface damp until the grass is up. A dry week at the wrong moment costs you the whole sowing.
When to sow
Early autumn is the best window in a temperate climate, roughly September to November in the north and March to May in the south, because the soil is still warm, rain is more reliable and the seedlings get established before winter. Mid spring is the second window. Summer sowing is possible if you can water every day without fail, and it usually fails on the day you forget.
Questions from the margin
How much grass seed do I need per square metre?
About 35 grams a square metre for a new lawn on bare soil and about 25 for overseeding an existing one. Those are the usual trade figures, but the weight is printed on the packet and varies with the mix, so use the packet's number if it differs.
When is the best time to sow a lawn?
Early autumn is the best, from September to November, because the soil is warm, there is moisture and the seedlings establish before the first hard frosts. Mid spring, March and April, is the other window. Avoid high summer.
Can I use more seed to get a thicker lawn?
Not really. Sowing far above the rate gives crowded seedlings that compete with each other, and the result is thinner rather than thicker, as well as more prone to damping off. Prepare the ground properly instead.