What this calculator does
The Fertilizer Calculator figures out how many pounds of a fertilizer product to spread over a lawn or garden area to deliver a specific target amount of nitrogen, using the product’s guaranteed analysis percentage.
How the formula works
Nitrogen need is first calculated as total N = (area ÷ 1000) × target rate, expressed in pounds of actual nitrogen. That figure is then divided by the fertilizer’s nitrogen percentage (the first of the three N-P-K numbers on the bag) to get the pounds of product required: product = total N ÷ (N% ÷ 100).
- A common lawn target is about 1 lb of actual nitrogen per 1,000 sq ft per application
- A higher nitrogen percentage on the bag means less product needed for the same nitrogen dose
A note on accuracy
Actual fertilizer needs vary by grass or plant species, soil test results, and season; this calculator is a general planning tool and does not replace a soil test or local extension office recommendation.
Last reviewed August 2026