Landscaping
Fertilizer Calculator
Work out fertiliser quantity from area and the nutrient rate being applied.
Result
Rates are based on nitrogen, the first NPK number. Over-application burns turf and runs off into waterways, and many regions regulate application rates and timing.
Get a Free Business AuditWhat moves the number
- Nutrient analysis on the bag, since the rate is nutrient-based rather than product-based.
- Area treated, measured rather than paced.
- Whether the product is slow or quick release, which changes the application rate.
Where this one goes wrong
Applying by product weight rather than by nutrient rate. Two bags with different analyses applied at the same weight per area deliver very different amounts of nutrient, and over-application scorches the grass and runs off.
Before you quote
- The analysis on the specific product rather than a remembered figure.
- Whether local rules restrict application timing or nutrient content.
- Whether the spreader is calibrated for this product.
Questions
About this calculator
Why does the analysis matter more than the weight?
Because the rate is set by nutrient delivered, not by product spread. A higher-analysis product applied at the same weight delivers substantially more nutrient and can scorch the lawn.
Does spreader calibration matter?
Considerably. The same setting delivers different rates for different granule sizes, which is why calibration is per product rather than once per spreader.
Are there restrictions on fertiliser use?
In many areas yes, on timing, on nutrient content and near watercourses. The rules are local and worth checking rather than assuming.
The formula
product per 1,000 sq ft = target nitrogen ÷ (nitrogen % ÷ 100)
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How to use this calculator
- Lawn area (sq ft) (sq ft). Enter your own figure; the page starts at 5,000 sq ft.
- Target nitrogen (lb per 1,000 sq ft). Enter your own figure; the page starts at 1.
- Nitrogen in the bag (first NPK number) (%). Enter your own figure; the page starts at 24%.
- Bag weight (lb). Enter your own figure; the page starts at 40.
- Cost per bag ($). Enter your own figure; the page starts at $32.
The result updates as you type, so you can try a figure, see what it does to product needed (lb), and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Lawn area (sq ft) | 5,000 sq ft |
| Target nitrogen (lb per 1,000 sq ft) | 1 |
| Nitrogen in the bag (first NPK number) | 24% |
| Bag weight (lb) | 40 |
| Cost per bag | $32 |
What comes out
| Result | Value |
|---|---|
| Product needed (lb) main result | 20.8 |
| Bags needed | 1 |
| Product per 1,000 sq ft (lb) | 4.2 |
| Cost | $32.00 |
To apply 1 lb of nitrogen per 1,000 sq ft from a 24% product, spread 4.2 lb per 1,000 sq ft, 20.8 lb total, or 1 bags.
How sensitive the answer is
Holding everything else steady and changing only cost per bag, this is what happens to product needed (lb). It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per bag ($) | Product needed (lb) |
|---|---|
| $16 | 20.8 |
| $24 | 20.8 |
| $32 example above | 20.8 |
| $48 | 20.8 |
| $64 | 20.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Lawn area (sq ft) (sq ft)
- Measured in sq ft, starting at 5,000 sq ft.
- Target nitrogen (lb per 1,000 sq ft)
- A number, between 0 and 10, starting at 1.
- Nitrogen in the bag (first NPK number) (%)
- A percentage, between 0 and 100, starting at 24%.
- Bag weight (lb)
- A number, between 0 and 500, starting at 40.
- Cost per bag ($)
- A money amount, starting at $32.
What the results tell you
| Result | What it is |
|---|---|
| Product needed (lb) | The headline result. A decimal figure. |
| Bags needed | A number. |
| Product per 1,000 sq ft (lb) | A decimal figure. |
| Cost | A money figure. |
Seasonality is not in this
Coverage and material figures assume the area you enter. Slope, access for machinery, existing growth and the time of year all affect both quantity and labour, and none of them appear in a calculation like this.
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