How the lawn fertilizer calculator works
Turf advice is written in pounds or kilograms of nitrogen, not of product, because nitrogen is the nutrient that drives leaf growth and colour. A bag labelled 20-5-10 holds 20% nitrogen, 5% phosphate and 10% potash by weight, so only one fifth of what you spread is nitrogen. To deliver a given dose, divide the nitrogen you want by that percentage.
Product = area × nitrogen rate ÷ (N % ÷ 100)In metric the rate is kg of N per 100 m²; in US units it is lb of N per 1,000 ft². The two are close: 1 lb N/1,000 ft² equals 0.49 kg N/100 m², or about 4.9 g of nitrogen per square metre. The calculator also turns the dose into a spreading rate you can set on a drop or rotary spreader. No waste margin is added on purpose: over-application scorches grass and washes into drains, so round down rather than up.
Worked example
A 200 m² lawn needs 0.5 kg N per 100 m² this spring, which is 2 × 0.5 = 1 kg of nitrogen. With a 20-5-10 granular feed you spread 1 ÷ 0.20 = 5 kg of product, or 25 g per square metre – half of one 10 kg bag. Three feeds a season use 15 kg. In pounds: a 5,000 ft² lawn at 1 lb N/1,000 ft² needs 5 lb of nitrogen; a 24-0-6 product supplies it in 5 ÷ 0.24 = 20.8 lb, spread at 4.2 lb per 1,000 ft².
Yearly nitrogen needs by grass type
| Grass | lb N/1,000 ft² per year | kg N/100 m² per year | Main feeding season |
|---|---|---|---|
| Kentucky bluegrass | 2–4 | 1.0–2.0 | Autumn, then late spring |
| Perennial ryegrass | 2–4 | 1.0–2.0 | Spring and autumn |
| Tall fescue | 1–3 | 0.5–1.5 | Autumn |
| Fine fescues | 1–2 | 0.5–1.0 | Autumn, light doses |
| Bermudagrass | 2–5 | 1.0–2.4 | Late spring to late summer |
| Zoysia | 1–3 | 0.5–1.5 | Early and mid summer |
| Centipede grass | 0.5–1 | 0.25–0.5 | Early summer only |
Split the yearly total into doses of no more than 1 lb N/1,000 ft² (0.5 kg N/100 m²) of quick-release nitrogen. Slow-release products, marked as coated or water-insoluble nitrogen on the label, tolerate slightly heavier single doses.
Reading the bag and avoiding burn
- Check phosphate rules. Several US states and some municipalities ban phosphorus on established lawns unless a soil test shows a deficiency; the middle number of the analysis must then be 0.
- Spread in two passes. Set the spreader to half the rate and walk the lawn twice at right angles. Stripes of dark and pale grass are the classic sign of a single pass with gaps.
- Water it in. Granules left on dry leaves can scorch. Irrigate lightly afterwards unless the label says otherwise, but never feed right before a forecast downpour.
- Sweep hard surfaces. Pellets on the drive or path end up in storm drains and stain concrete if they contain iron.
- Test the soil every few years. Potash and lime needs vary far more between gardens than nitrogen needs do.
Frequently asked questions
How much 10-10-10 fertilizer do I need per 1000 square feet?
To apply 1 lb of nitrogen per 1,000 ft², divide 1 by 0.10: you need 10 lb of 10-10-10 per 1,000 ft². For half a pound of nitrogen, spread 5 lb.
How many grams of lawn feed per square metre?
Most granular lawn feeds are spread at 20–35 g/m². At 0.5 kg N per 100 m² a 20% nitrogen product works out at 25 g/m²; a 15% product at about 33 g/m².
What do the three numbers on fertilizer mean?
They are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 25 lb bag of 24-0-6 contains 6 lb of nitrogen, no phosphate and 1.5 lb of potash.
How often should I fertilize my lawn?
Cool-season lawns usually need 2–4 feeds a year, with the largest in autumn. Warm-season grasses are fed 2–3 times while actively growing, from late spring to late summer.
Can I apply more fertilizer to get a greener lawn faster?
No. Doses above about 1 lb N per 1,000 ft² (0.5 kg per 100 m²) of fast-release nitrogen can burn the grass, push soft growth that invites disease and wash nutrients into waterways.
Last reviewed: 2026-09-27 · How we calculate