About the Fertilizer Calculator

Fertilizer math is nitrogen math: the product's N% tells you how much product delivers the target. For 1 lb of N per 1,000 sq ft over 5,000 sq ft with a 20-5-10 blend: 5 lb of N needed ÷ 0.20 = 25 lb of fertilizer, which also delivers 1.25 lb of P and 2.5 lb of K.

Reading the N-P-K

The analysis is percent by weight, so a 40 lb bag of 20-5-10 contains 8 lb of N, 2 lb of P, and 4 lb of K. Matching the analysis to the season — higher N in spring, higher K in fall for cold tolerance — is the difference between feeding the lawn and just buying fertilizer.