Grass Seed Calculator
The sum is area times grams per square metre, but that rate is what decides the whole answer, which is why it is an input here rather than a built-in constant. Species with larger seeds need more weight to reach the same plant density, so the published rate can differ two or three fold between them — copy the figure from the bag. On top of that, the same species is sown at roughly double the rate onto bare soil as it is for overseeding an existing lawn. A hundred square metres at 35 g/m² comes to 3.85 kg once 10% is added for waste, and one 5 kg bag covers 143 m².
Your numbers
Seed needed
3.85kg
Bags needed
1bags
Area one bag covers
142.9m²
What it means
You need 3.85 kg of seed and one bag covers 142.9 m². Sowing heavier does not give a denser lawn — the seedlings crowd each other out instead.
Formula
Seed needed = Area × Sowing rate (g/m²) × (1 + Waste allowance ÷ 100) ÷ 1000, Bags needed = ⌈Seed needed ÷ Bag weight (kg)⌉
Leftover seed loses germination every year it sits, so there is little point buying a large surplus. Rake it in lightly so the seed touches soil, then keep the surface damp until everything is up — that matters more to the result than getting the rate exactly right.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Area (m²)The space a shape covers — length squared. | 100 | 0 and up |
| Sowing rate (g/m²)Grams sown per square metre; the figure on the bag varies two- or three-fold by species. | 35 | 0 ~ 100 |
| Waste allowance (%)The extra bought to cover breakage and offcuts. | 10 | 0 ~ 50 |
| Bag weight (kg)The weight of one bag; 20, 25 and 40 kg are common and printed on the front. | 5 | 0 and up |
Step by step
Quick reference table
Results when only Area (m²) changes and everything else stays put.
| Area (m²) | Seed needed (kg) | Bags needed (bags) | Area one bag covers (m²) |
|---|---|---|---|
| 50 | 1.93 | 1 | 142.9 |
| 75 | 2.89 | 1 | 142.9 |
| 100 | 3.85 | 1 | 142.9 |
| 150 | 5.78 | 2 | 142.9 |
| 200 | 7.7 | 2 | 142.9 |
What each result means
| Result | At default values |
|---|---|
| Seed needed (kg)The seed to buy with waste added; leftovers lose germination every year they sit. | 3.85 |
| Bags needed (bags)How many bags to buy; they are not sold in fractions, so this rounds up. | 1 |
| Area one bag covers (m²)How much area one bag covers at that spread rate — a sanity check on the count. | 142.9 |
Common mistakes
Leftover seed loses germination every year it sits, so there is little point buying a large surplus. Rake it in lightly so the seed touches soil, then keep the surface damp until everything is up — that matters more to the result than getting the rate exactly right.
Glossary
- Area
- The space a shape covers — length squared.
- Sowing rate (g/m²)
- Grams sown per square metre; the figure on the bag varies two- or three-fold by species.
- Waste allowance
- The extra bought to cover breakage and offcuts.
- Bag weight (kg)
- The weight of one bag; 20, 25 and 40 kg are common and printed on the front.
- Seed needed
- The seed to buy with waste added; leftovers lose germination every year they sit.
- Bags needed
- How many bags to buy; they are not sold in fractions, so this rounds up.
- Area one bag covers
- How much area one bag covers at that spread rate — a sanity check on the count.
Frequently asked questions
Q. How is Grass Seed Calculator calculated?
Seed needed = Area × Sowing rate (g/m²) × (1 + Waste allowance ÷ 100) ÷ 1000, Bags needed = ⌈Seed needed ÷ Bag weight (kg)⌉ — The sum is area times grams per square metre, but that rate is what decides the whole answer, which is why it is an input here rather than a built-in constant. Species with larger seeds need more weight to reach the same plant density, so the published rate can differ two or three fold between them — copy the figure from the bag. On top of that, the same species is sown at roughly double the rate onto bare soil as it is for overseeding an existing lawn. A hundred square metres at 35 g/m² comes to 3.85 kg once 10% is added for waste, and one 5 kg bag covers 143 m².
Q. Can you walk through an example?
With Area 100m², Sowing rate (g/m²) 35, Waste allowance 10%, Bag weight (kg) 5, the answer is Seed needed 3.85kg.
Q. What do I need to enter?
Enter Area, Sowing rate (g/m²), Waste allowance, Bag weight (kg). The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Area (m²) moves the answer to Area (m²) 50 → Seed needed (kg) 1.93 and Area (m²) 200 → Seed needed (kg) 7.7. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Leftover seed loses germination every year it sits, so there is little point buying a large surplus. Rake it in lightly so the seed touches soil, then keep the surface damp until everything is up — that matters more to the result than getting the rate exactly right.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.