Fertilizer NPK Calculator
Work out how much fertilizer to spread to hit a target nitrogen rate, and see the phosphorus and potassium you apply along with it.
How it works
Product needed (lb) = (target N × area ÷ 1000) ÷ (N% ÷ 100) · P applied = product × P% · K applied = product × K%
Fertilizer is dosed by its nitrogen, and the three numbers on the bag — the NPK ratio — are the percentages by weight of nitrogen (N), phosphate (P) and potash (K). To hit a target nitrogen rate you first find how much actual nitrogen you need (the rate times your area in thousands of square feet), then divide by the bag's nitrogen fraction to get the product weight, because only part of the bag is nitrogen. A 10-0-0 bag needs twice the product of a 20-0-0 bag for the same nitrogen. Whatever product you spread also lays down phosphorus and potassium in proportion to the other two numbers, which is why this calculator reports those too — over-applying phosphorus, in particular, is restricted in many areas for water-quality reasons.
Worked example
For a 1,000 sq ft lawn at a target of 1 lb N per 1,000 sq ft using a 10-10-10 fertilizer: nitrogen needed is 1 × (1,000 ÷ 1,000) = 1 lb, and the product needed is 1 ÷ 0.10 = 10 lb. That 10 lb also delivers 10 × 0.10 = 1 lb of phosphorus and 1 lb of potassium.
Frequently asked questions
What do the three NPK numbers mean?
They are the percentage by weight of nitrogen, phosphate and potash in the bag. A 10-10-10 product is 10% of each; a 46-0-0 (urea) is 46% nitrogen and nothing else. The rest of the bag is filler and other nutrients.
Why dose by nitrogen?
Nitrogen drives leaf and blade growth and is the nutrient plants use most, so lawn and garden programs are built around a nitrogen rate — commonly about 1 lb per 1,000 sq ft per feeding. Phosphorus and potassium then come along in whatever ratio the product carries.
Can I apply too much phosphorus?
Yes. Excess phosphorus runs off into waterways and fuels algae, so many regions limit or ban phosphorus lawn fertilizer unless a soil test shows a deficiency. Check the P you apply — this calculator shows it — and choose a low- or zero-P blend where required.
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This calculator is for educational purposes. Double-check important results before acting on them.