How it works
Every nozzle sprays a fixed flow at a given pressure. How much lands on each acre depends on how fast you drive and how far apart the nozzles are. The constant 5,940 converts gallons per minute, miles per hour, and inches to gallons per acre.
Catch the output of several nozzles for one minute to get the true flow per nozzle rather than the rated flow. Then use the tank volume and gallons per acre to find how many acres a tank covers, and how much chemical to add per tank.
Worked example
A boom with 20-inch nozzle spacing, 0.4 gpm per nozzle, driven at 5 mph with a 200 gal tank and a 32 fl oz per acre product:
- GPA = 0.4 gpm × 5,940 ÷ (5 mph × 20 in) = 23.8 gal/ac
- Acres per tank = 200 gal ÷ 23.8 = 8.4 ac
- Product per tank = 8.4 ac × 32 per ac = 269.4
| Input | Value |
|---|---|
| Flow per nozzle | 0.4 gpm |
| Ground speed | 5 mph |
| Nozzle spacing | 20 in |
| Tank volume | 200 gal |
| Product rate per acre | 32 /ac |
| Result | Value |
|---|---|
| Application rate | 23.8 gal/ac |
| Area covered per tank | 8.4 ac |
| Product per tank (rate units) | 269.4 |
Assumptions and limits
- Use the measured flow per nozzle, not the catalog flow; wear and pressure change it.
- Speed is actual ground speed, checked over a measured distance.
- Follow the herbicide label for rates, carrier volume, drift limits, and buffer requirements.
Common questions
How do I calibrate a boom sprayer?
Measure a course, time your tractor over it to get true speed, catch the flow from each nozzle for a minute at working pressure, then calculate GPA with this formula and adjust pressure, nozzle size, or speed to hit the target rate.
How do I change my gallons per acre?
Slow down, or raise pressure (gives only a modest increase), or change to larger nozzles. Doubling the flow needs about four times the pressure, so nozzle size is the better lever.
Where does 5,940 come from?
It is 43,560 sq ft per acre × 60 min per hr × 12 in per ft ÷ 5,280 ft per mile, which works out to exactly 5,940.
Sources
- Standard sprayer calibration formula: GPA = GPM × 5,940 ÷ (MPH × nozzle spacing in inches), as published in land-grant university extension guides.
Updated 2026-09-30