SprayMaster How to calibrate a boom sprayer: gallons per acre from nozzle output and speed
GPA equals 5,940 times one nozzle's output in GPM, divided by ground speed in mph times nozzle spacing in inches. A nozzle putting out 0.35 GPM at 5.2 mph on 20 in spacing applies 19.99 gallons per acre.
Why calibrate before you mix
The label tells you how much product per acre. Only calibration tells you how much water per acre your sprayer really puts down, and every tank-mix figure depends on that number. Worn tips, a different gear, a new pressure setting — any of it changes the real GPA. Recalibrate every time you swap tips, even on days when nothing else seems to have moved.
Where does the 5940 come from?
It folds the unit conversions into one constant: 43,560 sq ft per acre, 12 inches per foot, and 88 feet per minute at 1 mph. 12 × 43,560 ÷ 88 = 5,940. The formula is:
- GPA = 5940 × GPM per nozzle ÷ (mph × nozzle spacing in inches)
- Metric: L/ha = 60,000 × L/min per nozzle ÷ (km/h × nozzle spacing in cm)
Measure the three inputs
- Nozzle output: at your spraying pressure, catch one nozzle for exactly one minute in a measuring jug. Divide the fluid ounces by 128 to get GPM; the app does not do this step. 44.8 fl oz in a minute is 0.35 GPM.
- Ground speed: time the tractor or ATV over a measured distance in the field, at the gear and throttle you spray with, rather than trusting the dash.
- Nozzle spacing: the distance from one nozzle to the next on the boom, not the width of the whole boom.
Calculate GPA in SprayMaster
- Open Sprayer Calibration from Pro Calculators (it needs SprayMaster Pro).
- Choose Units, then set Method to Nozzle & Speed.
- Under Measured Values, type Output per Nozzle (GPM or L/min), Ground Speed (mph or km/h) and Nozzle Spacing (in or cm). Spacing starts at 20 in or 50 cm.
- Read the Application Rate and the Same rate in line, which shows the other unit system.
- Take that rate into Tank Mix as your Application Rate, as the screen suggests.
Worked examples
| Output per Nozzle | Ground Speed | Nozzle Spacing | Application Rate | Same rate in |
|---|---|---|---|---|
| 0.35 GPM | 5.2 mph | 20 in | 19.99 GPA | 186.99 L/ha |
| 1.2 L/min | 8 km/h | 50 cm | 180 L/ha | 19.24 GPA |
If the result is far from the spray volume the label calls for, change one thing at a time: speed, pressure or nozzle tips, then catch and calculate again.
What this calculator does not cover
- Broadcast booms only: no banded or directed spraying, airblast or orchard sprayers, or granular spreaders.
- No nozzle tip charts, pressure calculations or tip sizing.
- No stopwatch: use your own watch or phone clock.
- Prefer not to convert ounces to GPM? The 1/128 Acre catch method reads GPA straight from the ounces you catch.
Questions and answers
What is the formula for gallons per acre on a sprayer nozzle?
Can I use boom width instead of nozzle spacing?
Does SprayMaster convert ounces per minute to GPM?
Is sprayer calibration free in the app?
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