About this tool
Fertilizer / NPK Calculator is a free, in-browser tool that turns a target nitrogen rate into the amount of a specific fertilizer to spread. Enter the area, the target nitrogen in pounds of N per 1,000 square feet, and the three N-P-K percentages printed on the bag — it returns the pounds of product to apply and how much actual nitrogen, phosphorus and potassium that delivers. Nothing is uploaded; the math runs on your device.
Product per 1,000 sq ft = target N ÷ (N% ÷ 100), because only the nitrogen fraction of the bag counts toward the rate. Total product = product per 1,000 × (area ÷ 1000). The nutrients actually applied are total product × each percentage: nitrogen = product × N%, phosphorus = product × P% and potassium = product × K%. A 10-10-10 fertilizer at 1 lb N per 1,000 over 1,000 sq ft, for example, needs 10 lb of product.
Use it to compare fertilizers fairly: a higher first number means fewer pounds of product for the same nitrogen rate. Lawn programs commonly target about 1 lb of nitrogen per 1,000 sq ft per feeding — check your local extension guidance for the right seasonal rate.