Why calibrate a sprayer?
Pesticide and fertilizer labels give a rate per acre. If the sprayer applies more water than you think, the product runs out early and the field is over-treated; if it applies less, coverage is poor and control fails. Calibration ties together the three things that set the spray volume: nozzle flow, travel speed and nozzle spacing.
The formula
For a boom sprayer with evenly spaced nozzles:
Gallons per acre (GPA) = 5,940 × nozzle flow (gpm) ÷ (speed (mph) × nozzle spacing (in))
The constant 5,940 converts the units (it equals 60 min per hour × 43,560 ft² per acre × 12 in per ft ÷ 5,280 ft per mile). For a broadcast boom, the spacing is the distance between nozzles. For band or directed spraying, use the band width instead.
Example
Nozzles delivering 0.4 gpm, 20 in apart, at 6 mph:
- GPA = 5,940 × 0.4 ÷ (6 × 20) = 19.8 gal/acre;
- a 300 gal tank covers about 15.2 acres;
- at a label rate of 32 fl oz per acre, add about 485 fl oz (3.8 gal) of product to a full tank.
Checking nozzle flow
- Fill the tank with clean water and set the working pressure.
- Collect the output of one nozzle for exactly one minute in a measuring jug.
- In ounces, the catch for one minute ÷ 128 gives gallons per minute. A 0.4 gpm nozzle fills about 51 fl oz.
- Check every nozzle: replace any that deliver more than 10% above or below the average.
Adjusting the rate
- Speed is the easiest lever, but changing it changes the rate in direct proportion. Keep it constant across the field.
- Pressure changes flow only with the square root: doubling the pressure increases flow by about 41%, and also changes droplet size and drift.
- Nozzle size is the right fix for large changes. The calculator shows the nozzle flow needed for your target volume.
Good practice
- Read the label for the required spray volume, nozzle type, buffer zones and personal protective equipment.
- Measure the actual ground speed over a known distance, for example 200 ft, rather than trusting the tractor display.
- Recalibrate at the start of each season, when changing nozzles and whenever you change products that need a different volume.
- Spray when wind is low and avoid temperature inversions to limit drift.