Fertiliser Rate Calculator
How much fertiliser product delivers a target nutrient rate over a paddock, lawn or bed, from the product’s analysis.
A fertiliser label gives the nutrient as a percentage: urea is 46% nitrogen, so 100 kg of urea carries 46 kg of N.
How the fertiliser rate calculator works
A fertiliser label gives the nutrient as a percentage: urea is 46% nitrogen, so 100 kg of urea carries 46 kg of N. To apply a rate of R per hectare over A hectares you need R × A ÷ (percentage ÷ 100) of product. Enter a price per bag-kilogram for the cost.
Formula: product = rate × area ÷ (nutrient % ÷ 100)
Worked examples
| Inputs | Product needed | Note |
|---|---|---|
| 50 kg N/ha from urea on 10 ha | 1,087 | 1,087 kg |
| 45 lb N/acre from 34% ammonium nitrate on 25 acres | 3,308.8 | 3,309 lb |
| A lawn: 2 kg N over 0.05 ha with a 20% blend | 10 | 10 kg |
FAQFrequently asked questions
How do I read a fertiliser label?
The three numbers are percentages of nitrogen, phosphorus (as P₂O₅ in most countries) and potassium (as K₂O): a 10-5-8 bag is 10% N. Use the nutrient you are targeting.
Why is the rate per hectare when I have a lawn?
Lawn advice is usually per 100 m² — divide a per-hectare rate by 100 — or enter the lawn area in hectares (200 m² is 0.02 ha) as the examples do.
What nitrogen rate do crops need?
It varies enormously: 20–50 kg N/ha for legumes and pasture top-ups, 100–200 for cereals and maize, 200 or more for intensive vegetables, split into several applications. Follow local agronomy advice and soil tests.
Should I spread it all at once?
Nitrogen is best split across the season to reduce leaching; phosphorus and potassium are usually applied once. The calculator gives the total — divide it across applications.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- Wellcome Sanger Institute — a leading UK genomics institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.