Fertilizer Calculator
Calculate fertilizer amount for lawn or garden from area, NPK ratio, and target application rate.
Returns lbs per 1,000 sq ft and total bags needed.
Fertilizer application is based on the desired nutrient rate and the fertilizer’s NPK ratio.
NPK ratio represents the percentage (by weight) of:
- N = Nitrogen (promotes leaf growth)
- P = Phosphorus (promotes roots and flowers)
- K = Potassium (overall plant health)
Imperial (lbs per 1,000 sq ft):
Fertilizer (lbs) = (Desired N rate per 1,000 sq ft / N percentage) × (Area sq ft / 1,000)
For example, to apply 1 lb of nitrogen per 1,000 sq ft with a 10-10-10 fertilizer:
Fertilizer = (1 / 0.10) × (Area / 1,000) = 10 lbs per 1,000 sq ft
Metric (kg per 100 m²):
Fertilizer (kg) = (Desired N rate per 100 m² / N percentage) × (Area m² / 100)
For example, to apply 0.5 kg of nitrogen per 100 m² with a 10-10-10 fertilizer:
Fertilizer = (0.5 / 0.10) × (Area / 100) = 5 kg per 100 m²
Common lawn fertilizer rates:
- Spring: 0.5–1 lb N per 1,000 sq ft (25–50 g N per m²)
- Summer: 0.5 lb N per 1,000 sq ft (25 g N per m²)
- Fall: 1–1.5 lb N per 1,000 sq ft (50–75 g N per m²)
Common NPK ratios:
- 10-10-10: Balanced, all-purpose
- 20-5-10: High nitrogen for lawns
- 5-10-10: High P and K for flowers/fruit
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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