Nozzle Flow Rate from Pressure Calculator
Find nozzle flow in gallons per minute from rated flow, rated pressure and operating pressure, then feed the result into a sprayer GPA calibration.
How This Calculation Works
Nozzle flow scales with the square root of pressure: new flow equals rated flow times the square root of operating pressure divided by rated pressure. Doubling pressure increases flow by about 41 percent, not 100 percent. The resulting gpm then feeds the standard GPA formula with speed and spacing.
Field Calibration & Rate Reference Chart
| Ground speed | Nozzle pressure | Typical GPA | Droplet class |
|---|---|---|---|
| 4 mph | 20 psi | 12 | Coarse |
| 5 mph | 25 psi | 14 | Coarse |
| 6 mph | 30 psi | 15 | Medium |
| 7 mph | 35 psi | 16 | Medium |
| 8 mph | 40 psi | 18 | Medium |
| 9 mph | 45 psi | 19 | Fine |
| 10 mph | 50 psi | 20 | Fine |
| 12 mph | 55 psi | 22 | Fine |
| 14 mph | 60 psi | 24 | Very fine |
| 16 mph | 65 psi | 26 | Very fine |
Common Questions
How does pressure affect nozzle flow rate?
Flow is proportional to the square root of pressure, so raising pressure from 30 to 40 psi increases flow by about 15 percent. Doubling pressure raises flow by roughly 41 percent. This square-root relationship is why small pressure changes have a smaller effect than expected.
What pressure should I run my spray nozzles at?
Most flat-fan nozzles are rated for 15 to 40 psi, with 30 to 40 psi giving a good balance of coverage and droplet size. Air-induction nozzles often run higher, up to 60 psi or more. Stay within the range printed on the nozzle chart.
How do I know when a nozzle is worn out?
Catch-test each nozzle and compare it with the rated flow at your pressure. A nozzle flowing more than 10 percent above rated is worn and should be replaced. Worn tips over-apply product and create uneven patterns.
Does nozzle flow change with the product being sprayed?
Yes. Denser or more viscous solutions flow less than water at the same pressure, so a water calibration can overstate output for a heavy fertilizer. Recalibrate with the actual product or apply a density correction. Temperature also affects viscosity.