Fertilizer Calculator

Fertilizer recommendations are given as pounds of nutrient, not pounds of product. Enter the three numbers from the bag and your target rate to find exactly how much product to spread.

View formula

Product per 1,000 sq ft = target nutrient rate ÷ (nutrient % ÷ 100) Total product = product per 1,000 × area ÷ 1,000 Metric: g product per m² = g nutrient per m² ÷ (nutrient % ÷ 100)

How to use the Fertilizer Calculator

Enter the area you will fertilize.
Type the N-P-K numbers printed on the bag (e.g. 20-5-10).
Choose the nutrient your recommendation is based on (usually nitrogen) and the rate.
Press Calculate to see product needed and the N, P and K actually applied.

Reading N-P-K numbers

The three numbers on a fertilizer bag are the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 20 lb bag of 20-5-10 contains 4 lb N, 1 lb phosphate and 2 lb potash; the rest is carrier material and other nutrients.

Product needed = nutrient rate ÷ nutrient % (as a decimal)

Worked example

Applying 1 lb of nitrogen per 1,000 sq ft to a 2,000 sq ft lawn with 20-5-10:

Product per 1,000 sq ft: 1 ÷ 0.20 = 5 lb
Total: 5 × 2,000 ÷ 1,000 = 10 lb of product
Also applied: 0.5 lb phosphate and 1 lb potash

Good practice

Base rates on a soil test where possible, it tells you whether phosphorus or potassium are needed at all.
Many lawn programs apply no more than about 1 lb of nitrogen per 1,000 sq ft at a time; slow-release products are gentler.
Calibrate your spreader, apply in two passes at right angles and sweep granules off paths and driveways.
Follow the product label and any local rules (some areas restrict phosphorus or set blackout dates).

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