Fertilizer Calculator

Enter target nutrients and fertilizer NPK to find application rate per area.

Target nitrogen (lb N per 1,000 sq ft)

Fertilizer N (%)

Fertilizer P (%)

Fertilizer K (%)

Area to cover (sq ft)

Application rate —

Total fertilizer needed —

Phosphorus applied —

Potassium applied —

Rate = Target N (lb) / (Fertilizer N% / 100). A 10-10-10 fertilizer at 1 lb N/1000 sq ft requires 10 lb of product per 1,000 sq ft.

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How to Calculate Fertilizer Application

The three numbers on a fertilizer bag (like 10-10-10) represent the percentage by weight of nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O).

To apply 1 pound of actual nitrogen per 1,000 square feet using a 10-10-10 fertilizer, you need 10 pounds of product (1 ÷ 0.10 = 10). This also delivers 1 lb of P and 1 lb of K.

Fertilizer bags list three numbers: N-P-K (nitrogen, phosphorus, potassium) as percentages by weight. A 50-lb bag of 10-10-10 contains 10% × 50 = 5 lbs each of N, P, and K. To apply 1 lb of nitrogen per 1,000 sq ft using 10-10-10, you need 10 lbs of fertilizer per 1,000 sq ft.

Formula: lbs of fertilizer needed = (lbs of nutrient needed × 100) / (nutrient percentage). For 0.5 lb N per 1,000 sq ft from a 20-5-10 fertilizer: (0.5 × 100) / 20 = 2.5 lbs per 1,000 sq ft.

Typical nitrogen application rates (approximate, verify with a soil test): lawns 0.5–1 lb N per 1,000 sq ft per application; vegetables 0.1–0.2 lb N per 100 sq ft per season; fruiting plants (tomatoes, peppers) benefit from lower N once flowering starts to avoid excess leafy growth at the expense of fruit.

Soil testing is the most reliable way to determine what nutrients are needed. University extension services and private labs test for $15–$30 and provide crop-specific recommendations. Applying fertilizer without a test risks both under-supply and over-supply; over-application causes runoff and can burn plants.