Fertigation Injection Rate Calculator

This calculator sizes a fertigation injector system: given the nutrient concentration you want in the irrigated water (target PPM) and your injector's dilution ratio (1:100, 1:128, and 1:200 are the common presets), it works out how concentrated the stock tank solution must be, since a 1:100 injector dilutes the stock 100-fold, so the tank itself must hold 100 times the target PPM. From system flow (GPM) it derives the injector's required output both in gallons per hour (flow × 60 ÷ ratio) and in milliliters per minute for smaller metering pumps, using the standard 3,785.41 mL-per-gallon conversion. It then converts that target concentration into ounces of actual fertilizer product needed per gallon of stock solution, based on the product's nutrient analysis percentage — a 20%-N product needs five times the product weight of a 100%-N product to hit the same PPM.

Multiplying by your irrigation run time gives total water applied, total nutrient pounds delivered (using the standard 8.34 lbs-per-million-gallons-per-PPM conversion for water-based concentrations), and a recommended stock tank size with a 10% buffer so you don't run the tank dry mid-cycle. The biggest practical caveat is that this model assumes a liquid or fully dissolved product and a well-calibrated injector; it does not account for solubility limits (some dry fertilizers won't fully dissolve at very high stock concentrations), water pH or hardness interactions, or injector calibration drift, all of which matter in a real fertigation setup. Always verify the injector's actual output against a stopwatch-and-graduated-cylinder test before relying on the calculated rate in production.