🌾Fertilizer Calculator

About this tool

Enter the fertilizer's three numbers (N-P-K), choose which nutrient you're targeting and the rate you want, and enter the area. The tool calculates the amount of product, the rate per area, all three nutrients delivered and the number of bags.

Reading N-P-K and setting rates

The three numbers on a fertilizer bag, such as 20-5-10, are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Recommendations are given as nutrient per area, so the amount of product is the target rate divided by the nutrient percentage. For lawns a common feeding is about 1 lb of nitrogen per 1,000 sq ft, roughly 5 g per m². With 20-5-10 that means 5 lb of product per 1,000 sq ft, so a 5,000 sq ft lawn needs 25 lb, which also delivers 0.25 lb of phosphate and 0.5 lb of potash per 1,000 sq ft. In metric, 5 g/m² of nitrogen over 200 m² from a 20% product is 5 kg. Switch the target to phosphate or potash when that's what your soil test calls for. Honest limits: follow the product label and local rules, since many places restrict phosphorus on lawns. Too much fertilizer burns grass and washes into rivers, a soil test shows what your soil actually needs, and rates differ for vegetables, trees and slow-release products. The compost calculator and the grass seed calculator are related.

Frequently asked questions