Sprayer Calibration Calculator (GPA, Nozzle Size, Pressure & Tank Mix)
Calibrate a boom sprayer: gallons per acre from nozzle output, speed and spacing, the nozzle flow (and ISO colour size) needed for a target GPA, the pressure change to hit it, the 1/128-acre catch test distance and time, acres per tank, product per tank and per load for a rate in ounces, pints or quarts per acre, and tanks needed for the field.
How it works
GPA = 5940 × gpm per nozzle ÷ (mph × nozzle spacing in inches) · gpm needed = GPA × mph × spacing ÷ 5940 · flow ∝ √pressure: psi₂ = psi₁ × (gpm₂ ÷ gpm₁)² · 1/128 acre: distance ft = 340.3 ÷ spacing ft; oz caught in that time = GPA · acres per tank = gallons ÷ GPA; product per tank = rate × acres per tank
A boom sprayer applies a volume per acre set by three things — how fast each nozzle flows, how far apart the nozzles are and how fast the machine moves — combined in the 5,940 formula that is the backbone of every calibration sheet. Flow rises only with the square root of pressure, so pressure is a fine trim while speed and tip size are the coarse controls, and tips are chosen from standardised sizes by colour for the flow that gives the target volume near the middle of their pressure range. The same volume per acre fixes how many acres a tank covers, and from that the product measured into each load, which is where most application errors actually happen.
Worked example
Nozzles catching 40 oz/min at 40 psi on 20-inch spacing at 10 mph put out 9.3 GPA — 38% under a 15 GPA target. Hitting 15 needs 0.505 gpm per nozzle: 104 psi with the same tips (too high), slowing to 6.2 mph, or — the right answer — switching to a brown ISO 05 tip, which gives 0.505 gpm at about 41 psi. The 1/128-acre course is 204 ft (about 14 s at 10 mph); a 1,000-gal tank covers 66.7 acres and takes 16.7 gal of a 32 oz/ac product, so 320 acres is 4.8 tanks and 80 gallons of product.
Frequently asked questions
How often should I calibrate?
At the start of the season, whenever tips, speed or pressure change, and after every 50–100 hours — worn tips flow 10–20% more within a season. Replace any nozzle more than 10% off the average of the boom.
Why not just raise the pressure to increase volume?
Doubling pressure only raises flow 41% and makes droplets finer and more drift-prone. Use pressure for ±20% adjustments, keep within the tip's rated range, and change tip size or speed for larger changes.
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