What this calculator does
Fertiliser recommendations are given in pounds of nutrient, while the bag is a product containing only some percentage of it. A 20% nitrogen product needs five pounds of product to deliver one pound of nitrogen, so the label percentage is the division that stands between the recommendation and the spreader setting.
The relationship is inverse, and it matters most at the extremes. A 46% urea product delivers the same pound of nitrogen from 2.17 pounds of product, less than half what a 20% product needs. Applying a 46% product at a 20% product rate more than doubles the intended nitrogen.
The formula
The product application rate is the target nutrient rate divided by the label percentage expressed as a fraction. That rate is per thousand square feet, and multiplying by the treated area in thousands of square feet gives the total product needed. The calculation handles one nutrient at a time.
| Term | Meaning |
|---|---|
| Target rate | Pounds of actual nutrient per 1,000 square feet, which is how recommendations are written. |
| Label percentage | The percentage of the bag that is the nutrient in question, from the NPK analysis. |
| Product rate | Pounds of bagged product needed to deliver the target nutrient rate. |
| NPK | The three numbers on a fertiliser bag: nitrogen, phosphorus as P₂O₅ and potassium as K₂O, by percentage. |
The inputs explained
| Field | What to enter |
|---|---|
| Target nutrient rate (lb N/1000 ft²) | Target nutrient rate in pounds per 1,000 square feet. One pound of nitrogen per 1,000 sq ft is a common lawn recommendation. |
| Nutrient % on label (%) | The label percentage for the nutrient being targeted, the relevant one of the three NPK numbers. |
| Area to treat (ft²) | Area to be treated in square feet. |
When to use it
Applying a lawn fertiliser recommendation
Recommendations come in pounds of nitrogen and bags come in percentages, and the conversion between them is what determines the spreader setting.
Comparing two products
A cheaper bag at a lower analysis may cost more per pound of actual nutrient. Converting both to product rate shows how much of each is needed for the same result.
Switching products mid-season
A different analysis needs a different rate for the same nutrient delivery, and carrying over the previous setting is a common way to over-apply.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much product delivers one pound of nutrient?
A fixed target rate and area across a range of product analyses.
| Label percentage | Fertilizer needed | Product application rate | Area treated |
|---|---|---|---|
| 10% | 50.0 lb | 10.00 lb/1000 ft² | 5,000 ft² |
| 20% | 25.0 lb | 5.00 lb/1000 ft² | 5,000 ft² |
| 30% | 16.7 lb | 3.33 lb/1000 ft² | 5,000 ft² |
| 46% | 10.9 lb | 2.17 lb/1000 ft² | 5,000 ft² |
Questions
What do the three numbers on a fertiliser bag mean?
Nitrogen, phosphorus and potassium by percentage, in that order. A 20-5-10 bag is 20% nitrogen, 5% phosphorus as P₂O₅ and 10% potassium as K₂O. Calculate for one nutrient at a time using the relevant number.
How much nitrogen does a lawn need?
Commonly around 1 lb per 1,000 square feet per application, with the annual total split across several applications and varying by grass type, climate and whether clippings are returned. Local extension guidance is worth more than a general figure.
What happens if I over-apply?
Fertiliser burn from salt concentration, excessive soft growth that is more disease-prone, and nutrient running off into waterways. Nitrogen in particular is both the easiest to over-apply and the most environmentally consequential when it leaves the site.
Why is phosphorus expressed as P₂O₅?
It is a historical convention from the days when fertiliser analysis reported oxides. The bag percentage is P₂O₅, not elemental phosphorus, and elemental P is about 44% of that figure. The same applies to potassium as K₂O, where elemental K is about 83%.
For dosing liquid feed by parts per million, see the water-soluble fertilizer calculator. For field coverage rates, see the acres per hour calculator.