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.
A reproducible calculation with complete inputs
For a reproducible lawn fertilizer calculator example, use the metric input system and enter Lawn area: 200 m²; Nitrogen per application: 0.5 kg N/100 m²; Nitrogen content of the product: 20 % N; Bag weight: 10 kg; Feeds per season: 3 feeds (spring, early summer, autumn). These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Fertilizer per application | 5 | 5.0 | kg |
| Bags per application | 1 | 1 | bags |
| Spreading rate | 25 | 25.0 | g/m² |
| Nitrogen applied | 1 | 1.00 | kg N |
| Fertilizer per season | 15 | 15.0 | kg |
How should I prepare these entries?
Lawn area: enter the value in m². Nitrogen per application: Usual single dose: 0.25–0.5 kg N per 100 m² (0.5–1 lb N per 1,000 ft²). Nitrogen content of the product: The first number of the N-P-K on the bag: 20-5-10 means 20% nitrogen. Bag weight: enter the value in kg. Feeds per season: choose among 3 feeds (spring, early summer, autumn), 1 feed, 2 feeds, 4 feeds (high-maintenance lawn). Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
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.