NPK Fertilizer Amount Calculator
Convert a target nitrogen rate into pounds of fertilizer to apply — for any N-P-K analysis and any area.
How it works
Actual N needed = target N × (area ÷ 1000) · Fertilizer to apply = actual N ÷ (N% ÷ 100) · P applied = fertilizer × P% · K applied = fertilizer × K%
The three numbers on a fertilizer bag are the percentages of nitrogen (N), available phosphate (P₂O₅) and soluble potash (K₂O) by weight. Rates are quoted as pounds of actual nitrogen per 1000 ft², so you first work out how much nitrogen the area needs, then divide by the bag’s nitrogen fraction to get the product weight. A higher first number means less product for the same nitrogen — and more risk of burning if over-applied.
Worked example
To deliver 1 lb of N per 1000 ft² over a 1000 ft² lawn with a 24-4-12 fertilizer: nitrogen needed = 1 lb, product = 1 ÷ 0.24 = 4.17 lb. That also lays down 4.17 × 0.04 = 0.17 lb of P and 4.17 × 0.12 = 0.50 lb of K.
Frequently asked questions
What do the three N-P-K numbers mean?
They are the percentage by weight of nitrogen, phosphate and potash in the bag. A 24-4-12 fertilizer is 24% nitrogen, 4% phosphate and 12% potash; the rest is filler and secondary nutrients.
Why base the rate on nitrogen?
Nitrogen drives leafy growth and is the nutrient most easily lost and most easily overdone. Turf and vegetable programs are almost always written as pounds of actual N per 1000 ft², so nitrogen is the anchor for the math.
Can I apply too much?
Yes. Exceeding about 1 lb of nitrogen per 1000 ft² in a single application can burn foliage and leach into groundwater. Split higher seasonal totals into several lighter feedings, and always water in as directed.
Related calculators
Educational estimate — follow the product label and local guidance for exact rates.