Fertilizer Calculator
Free Fertilizer Calculator for gardening & crops. Enter variables to compute results with formulas and detailed steps.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Fertilizer (lbs/1000 sq ft) = Desired N Rate / (N% / 100)
To calculate fertilizer needed, divide the desired nitrogen application rate (in pounds per 1,000 square feet) by the nitrogen percentage of the fertilizer expressed as a decimal. Then multiply by your total area in thousands of square feet to get total product needed. The phosphorus and potassium amounts are automatically determined by the fertilizer ratio.
Worked Examples
Example 1: Lawn Fertilization
Problem:A 5,000 sq ft lawn needs 1 lb of nitrogen per 1,000 sq ft using 20-5-10 fertilizer. Bags are 50 lbs at $30 each.
Solution:Area: 5,000 sq ft = 5.0 thousand sq ft\nFertilizer per 1,000 sq ft: 1.0 / (20/100) = 5.0 lbs\nTotal fertilizer: 5.0 x 5.0 = 25.0 lbs\nBags needed: ceil(25/50) = 1 bag\nActual N applied: 25 x 0.20 = 5.0 lbs N\nActual P2O5: 25 x 0.05 = 1.25 lbs\nActual K2O: 25 x 0.10 = 2.5 lbs\nCost: 1 x $30 = $30
Result:Need 25 lbs of 20-5-10 (1 bag) | Cost: $30 | Applies 5 lbs N, 1.25 lbs P2O5, 2.5 lbs K2O
Example 2: Vegetable Garden Application
Problem:A 20x30 ft garden needs 1.5 lbs N per 1,000 sq ft using 10-10-10. Bags are 40 lbs at $20 each.
Solution:Area: 20 x 30 = 600 sq ft = 0.6 thousand sq ft\nFertilizer per 1,000 sq ft: 1.5 / (10/100) = 15.0 lbs\nTotal fertilizer: 15.0 x 0.6 = 9.0 lbs\nBags needed: ceil(9/40) = 1 bag\nActual N applied: 9.0 x 0.10 = 0.9 lbs N\nCost: 1 x $20 = $20
Result:Need 9 lbs of 10-10-10 (1 bag) | Cost: $20 | Applies 0.9 lbs each of N, P2O5, K2O
Frequently Asked Questions
How do I read fertilizer numbers (N-P-K)?
The three numbers on a fertilizer bag represent the percentage by weight of three primary nutrients: Nitrogen (N), Phosphorus (as P2O5), and Potassium (as K2O). A 10-10-10 fertilizer contains 10% nitrogen, 10% phosphate, and 10% potash by weight. A 50-pound bag of 10-10-10 contains 5 lbs each of N, P2O5, and K2O, with the remaining 35 lbs being filler or carrier material. Higher numbers mean more concentrated fertilizer, so you need less product per application. A 20-10-10 provides twice the nitrogen per pound compared to 10-10-10.
How much fertilizer do I need per 1,000 square feet?
The amount depends on your desired nutrient rate and the fertilizer analysis. For lawns, a typical nitrogen application rate is 0.5-1.0 lbs of actual nitrogen per 1,000 sq ft per application. To calculate product needed: divide the desired nitrogen rate by the nitrogen percentage as a decimal. For 1 lb N per 1,000 sq ft using 10-10-10: 1.0 / 0.10 = 10 lbs of product per 1,000 sq ft. Using 20-10-10: 1.0 / 0.20 = 5 lbs per 1,000 sq ft. Higher analysis fertilizers require less product, which is more convenient for large areas.
How do I calculate fertilizer application rates?
If a soil test recommends 2 pounds of nitrogen per 1,000 square feet and your fertilizer is 10-10-10 (10% N), you need 2 / 0.10 = 20 pounds of fertilizer per 1,000 square feet. For a 200 square foot bed: 20 * (200/1000) = 4 pounds. Split applications into 2-3 feedings during the growing season for best results.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy