How to Calibrate a Boom Sprayer (L/ha and GPA)
An uncalibrated sprayer either wastes product or under-doses it. Both cost money — and over-application can harm crops, people and water. Calibration takes about half an hour and should be repeated every season and whenever nozzles change.
The formula
Application rate depends on three things: nozzle flow, travel speed and nozzle spacing.
L/ha = 600 × flow per nozzle (L/min) ÷ (speed (km/h) × nozzle spacing (m))
Example: nozzles delivering 1.2 L/min, spaced 0.5 m, at 8 km/h → 600 × 1.2 ÷ (8 × 0.5) = 180 L/ha. The sprayer calibration calculator does this in metric or US units (GPA).
Measuring in the field
- Speed: mark 100 m in the field and time the tractor at the gear and RPM you'll spray with, with a partly full tank. Speed (km/h) = 360 ÷ seconds.
- Flow: at spraying pressure, collect water from each nozzle for one minute in a measuring jug.
- Uniformity: replace any nozzle more than 10% off the average, and all of them if many are worn.
- Calculate the rate and compare it with the label recommendation. Adjust pressure (small changes) or nozzle size (large changes).
Mixing the tank
Once you know the L/ha, the area one tank covers is tank volume ÷ rate. A 600 L tank at 180 L/ha covers 3.33 ha, so add 3.33 times the per-hectare product dose to each full tank.
Safety
- Calibrate with clean water only, wearing gloves and eye protection.
- Always follow the product label and local rules on drift, buffer zones and protective equipment.
Planning fertilizer too? The NPK fertilizer calculator turns a soil recommendation into kg of product per hectare.