Lawn Seed & Fertiliser Calculator
Enter the lawn area, grass type and job to get the seed in kilograms, the bags to buy, the fertiliser and the exact nitrogen, phosphorus and potassium, plus germination and watering guidance.
Seed is sold by the bag and spread by the square metre, and the rate changes with the grass, the job and whether you are starting fresh. Enter the area and the job; the page returns the seed in kilograms, the bags to buy, the fertiliser and its nitrogen, phosphorus and potassium, and what to do about water until the seed comes up.
The lawn
Use the area you will actually sow, not the whole garden. Paths, beds and buildings do not need seed.
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Seed rate by job
| Job | Cool-season | Warm-season |
|---|---|---|
| New lawn | 35 g/m² | 25 g/m² |
| Overseeding | 20 g/m² | 15 g/m² |
| Patching | 45 g/m² | 35 g/m² |
What the numbers on the bag mean
| Analysis | Nitrogen (N) | Phosphate (P₂O₅) | Potash (K₂O) | Best for |
|---|---|---|---|---|
| 20-10-10 | 200 g/kg | 100 g/kg | 100 g/kg | New seed and bare patches |
| 10-10-10 | 100 g/kg | 100 g/kg | 100 g/kg | General feeding, unknown soil |
| 15-5-10 | 150 g/kg | 50 g/kg | 100 g/kg | Established lawns, low run-off |
Quick reference — cool-season seed
| Area | New lawn | Overseeding | Fertiliser (new) |
|---|---|---|---|
| 50 m² | 1.75 kg | 1.00 kg | 1.25 kg |
| 100 m² | 3.50 kg | 2.00 kg | 2.50 kg |
| 200 m² | 7.00 kg | 4.00 kg | 5.00 kg |
| 500 m² | 17.50 kg | 10.00 kg | 12.50 kg |
Five rules before you buy
- New lawn needs 35 g/m² of cool-season seed or 25 g/m² of warm-season seed. Sowing thin is the most common cause of a patchy lawn.
- Overseeding uses less — 20 g/m² cool-season, 15 g/m² warm-season — because the existing grass already covers the ground.
- Patching bare spots needs the most seed per square metre: 45 g/m² cool-season, 35 g/m² warm-season, so the patch competes with the surrounding grass.
- Fertiliser is 25 g/m² for a new lawn, 15 g/m² when overseeding and 30 g/m² for patches. More is not better; excess nitrogen burns the seedlings.
- Buy by weight, not by coverage claims. Divide the total seed by the bag size and round up; half a bag left over is normal.
Practical notes
- Water lightly twice a day for the first two weeks. The top 2–3 cm must stay moist; a single heavy soak runs off before the seed swells.
- After germination, switch to a deep watering once or twice a week — about 15–20 mm each time — to push roots down.
- Spread half the seed walking north–south, then the other half east–west. Two passes cover the edges and overlaps that one pass misses.
- Lightly rake or roll after sowing. Seed needs soil contact; seed sitting on thatch or on top of the blades dries out.
Four common mistakes
- Sowing in the heat of summer. Cool-season seed germinates poorly above 25 °C; early autumn is the reliable window.
- Applying a high-nitrogen fertiliser at sowing. It burns the young roots; use a starter analysis and keep the rate low.
- Letting the surface dry for a single day. Germinating seed dies fast; once it has started, it cannot pause.
- Buying only the exact area. Edges, slopes and overseeding overlaps always take 5–10% more than the measured area.
Rates are general guidelines for homeowner lawns. Follow the seed and fertiliser label, test the soil when possible, and keep children and pets off freshly treated areas until the label says it is safe.