Lawn Fertiliser Calculator
How much product to spread for a target nitrogen dose, the rate per square metre, bags needed and cost.
Results
What this tool does
Fertiliser is sold by product weight but applied by nutrient, and mixing the two up is how lawns get scorched. A common target is around 3 g of nitrogen per square metre per feed, and the last result halves the amount for the safer practice of two lighter passes at right angles.
Formula
product = wanted nitrogen ÷ N percentage × area
Variables
| Symbol | Meaning | Unit |
|---|---|---|
a | Area | m² |
nt | Nitrogen wanted | g/m² |
np | Nitrogen in the product | % |
bp | Price per bag | — |
bw | Cement bag weight | kg |
PD | Product to apply | kg |
RA | Application rate | g/m² |
NB | Cement bags | — |
CO | Cost | — |
PH | Product to apply | kg |
Worked example
- Area200 m²
- Nitrogen wanted3 g/m²
- Nitrogen in the product20 %
- Price per bag24
- Cement bag weight10 kg
- Product to apply3.000 kg
- Application rate15.00 g/m²
- Cement bags1
- Cost7.20
- Product to apply1.500 kg
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
What do the three NPK numbers mean?
They are the percentages by weight of nitrogen, phosphorus and potassium in the bag, always in that order. A 20-5-10 fertiliser is twenty percent nitrogen, so ten kilograms of it carries two kilograms of nitrogen. The first number is the one that drives leaf growth and the one lawn feeding is usually planned around; enter it as the nitrogen percentage here.