Case 1
- Lawn area
- 100m²
- Sowing rate
- 35g/m²
3.50kg
Open with these valuesGrass Seed Calculator
3.50kg
Result: 3.50 kgArea times sowing rate, divided by 1000 for kilograms. A 100 m² lawn at the new-lawn rate of 35 g/m² takes 3.5 kg of seed. Overseeding an existing lawn uses far less — around 20 to 25 g/m², because the grass that is already there does most of the covering.
The numbers at a glance
Held fixed: Lawn area 100.0 m².
| Sowing rate (g/m²) | Result (kg) |
|---|---|
| 20.0 | 2.00 |
| 30.0 | 3.00 |
| 35.0Your value | 3.50 |
| 40.0 | 4.00 |
| 50.0 | 5.00 |
Worked examples
Case 2
- Lawn area
- 50m²
- Sowing rate
- 25g/m²
1.25kg
Open with these valuesCase 3
- Lawn area
- 200m²
- Sowing rate
- 40g/m²
8.00kg
Open with these valuesHow it's calculated
kg = area × sowing rate ÷ 1000
- StepEnter the lawn area in square metres — length × width for a rectangle.
- StepSet the sowing rate: about 35 g/m² for a new lawn, 20–25 to overseed.
- StepArea × rate gives grams; divide by 1000 for kilograms.
- ResultBuy 5–10 % more to cover thin patches and losses to birds and wind.
What this number means
New lawn or overseeding decides the rate
The same 100 m² takes 3.5 kg at the new-lawn rate of 35 g/m² but only 2 kg at the 20 g/m² overseeding rate. Choosing the wrong one is the largest error this calculation can make, and no amount of care with the area repairs it. Settle the rate before you read the result.
Bare soil has to close up on its own
A new lawn goes onto prepared soil with nothing else covering it, so the rate has to be dense enough to close the surface. Overseeding drops seed into grass that is already growing and only fills what is thin.
Where 35 and 20 g/m² come from
The breeder's sowing guidance behind this calculator puts ryegrass and fescue mixtures at 35 g/m² and overseeding at 20 g/m². Ornamental blends, hard-wearing utility mixes and shade mixes each sow differently, so read the bag you bought.
Grams for the spreader, kilograms for the shop
Seed is sold in kilogram packs, which is why the result is in kilograms. The grams column of the table is the figure you set on a spreader.
Commonly misread
I used 35 g/m² to overseed the lawn I already have.
35 g/m² is the rate for bare soil, and overseeding wants 20 to 25 g/m². Over 100 m² that is 2 to 2.5 kg instead of 3.5 kg.
My bag quotes 35 kg per hectare, so I entered 35.
This field is grams per square metre, and 35 kg/ha works out at 3.5 g/m². Entering 35 buys ten times the seed you need.
I bought exactly the 3.5 kg the calculator gave me.
That is the exact figure for the area and rate you entered, but birds, wind and uneven spreading each take a share. Around 5 to 10 % more leaves you something for the thin patches.
Reference table
| Area, sowing rate | Grams | Kilograms |
|---|---|---|
| 50, 25 | 1250 g | 1.25 |
| 75, 35 | 2625 g | 2.625 |
| 100, 25 | 2500 g | 2.5 |
| 100, 35 | 3500 g | 3.5 |
| 200, 40 | 8000 g | 8 |
Questions
How much grass seed do I need?
Multiply the area you want to sow by the sowing rate. For a 100 m² lawn at the new-lawn rate of 35 g/m² that is 3500 g, or 3.5 kg of seed.
What sowing rate should I enter?
It depends on the job. Sowing a new lawn onto bare soil takes about 35 g/m² for full cover, while overseeding an existing lawn needs only 20 to 25 g/m². The default of 35 g/m² suits a fresh lawn.
What is the difference between a new lawn and overseeding?
A new lawn goes onto bare, prepared soil and needs a dense rate to close up completely. Overseeding adds seed to grass that is already there, filling thin patches, so it uses roughly a third less per square metre.
Where do these rates come from?
The source behind this calculator is a grass breeder's sowing guidance, which puts ryegrass and fescue mixtures at 35 g/m² and overseeding at 20 g/m². Rates differ by mix — fine ornamental blends, hard-wearing utility mixes and shade mixes each sow a little differently, so check your own bag.
Should I buy a little extra?
Yes. The result is the exact amount for the area and rate you enter, but some seed is lost to birds, wind and uneven spreading. Around 5–10 % more lets you go back over thin patches.
Sources and last check
Information, not professional advice.