Calibrate a boom sprayer: get gallons per acre (GPA) from nozzle output, ground speed, and nozzle spacing — or the nozzle flow you need to hit a target GPA.
Use this before the season, after changing nozzles or tires, and any time product per acre matters. Measure nozzle output with a catch test rather than trusting the chart for worn tips.
Ground speed has to be real field speed. Time the tractor over a measured distance in the field at the gear and RPM you will spray with.
The constant 5,940 converts gallons per minute, miles per hour, and inches of nozzle spacing into gallons per acre for a broadcast boom. Ounces per minute from a catch test divide by 128 to give GPM.
GPM = oz/min ÷ 128
GPA = GPM × 5,940 ÷ (mph × spacing_in)
GPM_needed = GPA × mph × spacing_in ÷ 5,940
A catch test collects 38.4 oz/min per nozzle → 38.4 ÷ 128 = 0.30 GPM. At 6 mph with 20-inch spacing:
GPA = 0.30 × 5,940 ÷ (6 × 20) = 14.85 GPA.
Reverse: to apply 15 GPA at the same speed and spacing you need 15 × 6 × 20 ÷ 5,940 = 0.303 GPM per nozzle (38.8 oz/min). Slowing to 5 mph with the same nozzles would raise the rate to 17.82 GPA.
Assumptions: broadcast boom with uniform nozzles; output measured at the spraying pressure; steady ground speed.
Limits: band and directed spraying need the band width instead of nozzle spacing. Worn or mixed nozzles give uneven output that an average hides. Carrier density other than water changes flow.
Responsibility: follow the product label for rate, carrier volume, nozzle type, and personal protective equipment.
Run the sprayer at spraying pressure with water, hold a measuring cup under a nozzle for exactly one minute, and read the ounces. Check several nozzles and replace any that run more than about 10% off the average.
It is the unit conversion that ties gallons per minute, miles per hour, and nozzle spacing in inches to an acre. It applies to broadcast spraying.
Small changes: adjust speed or pressure. Large changes: change nozzle size. Pressure changes flow only with the square root, so it is a weak lever.