Nozzle flow at pressure calculator
Short answer
Nozzle flow changes with the square root of pressure: flow at new pressure equals rated flow multiplied by the square root of new pressure divided by rated pressure. Enter a known flow and its test pressure to convert it to your operating pressure.
Key takeaways
- Flow scales with the square root of pressure, so doubling pressure raises flow by only about 41 percent.
- A nozzle rated 0.025 GPM at 1,000 psi flows about 0.022 GPM at 800 psi and 0.018 GPM at 500 psi.
- Pressure has a much bigger effect on droplet size than on water use, which is why a weak pump makes wet mist.
- The rule assumes a fixed orifice; anti-drip check valves and worn or scaled orifices shift real results.
The square root rule
Q2 = Q1 x √(P2 / P1), where Q is flow and P is pressure in the same units. The rule falls out of Bernoulli's equation for a fixed orifice: the speed of water leaving the hole is proportional to the square root of the pressure drop across it, and flow is speed multiplied by the area of the hole. Because the orifice and its discharge coefficient stay roughly constant, only the pressure term changes.
| Pressure | Flow vs 1,000 psi | 0.012 in nozzle (GPM) |
|---|---|---|
| 250 psi | 50% | 0.013 |
| 500 psi | 71% | 0.018 |
| 700 psi | 84% | 0.021 |
| 800 psi | 89% | 0.022 |
| 1,000 psi | 100% | 0.025 |
| 1,200 psi | 110% | 0.027 |
| 1,500 psi | 122% | 0.031 |
Why this matters in practice
Diagnosing pressure problems
If your pump is rated 0.5 GPM and the nozzles need more than that at 1,000 psi, the pump cannot hold 1,000 psi. The system settles at whatever pressure makes total nozzle flow equal pump output. For example, 26 nozzles of 0.012 in need 0.65 GPM at 1,000 psi. A 0.5 GPM pump drives them to about (0.5 / 0.65)² x 1,000 = 590 psi, below the point where high-pressure mist stays dry. That calculation explains many cases in low pressure and pressure loss.
Droplet size changes faster than flow
Lower pressure means slower jets that break up into larger drops. Dropping from 1,000 to 500 psi cuts flow by 29 percent but makes the mist visibly coarser and wetter. That trade is covered in droplet size explained.
Reconciling spec sheets
Different sellers rate nozzles at different pressures. Converting every rating to 1,000 psi makes them comparable, and comparing them against the planning flow table shows which charts are optimistic.
When the rule breaks down
- Near the check-valve opening pressure. Anti-drip nozzles stay shut below their spring setting and open gradually, so flow near that point is lower than the rule predicts. See anti-drip nozzles.
- Worn or scaled orifices. Erosion enlarges the hole (more flow); scale narrows it (less flow). Either changes Q1.
- Pressure at the nozzle, not the pump. Use the pressure at the nozzle. On long runs or undersized tubing it can be lower than the gauge at the pump.
Frequently asked questions
Why doesn't doubling pressure double the flow?
Water leaving an orifice speeds up until its kinetic energy matches the pressure energy. Velocity rises with the square root of pressure, and flow is velocity times orifice area, so flow also rises with the square root. Twice the pressure gives about 1.41 times the flow.
Can I run high-pressure nozzles at mid pressure?
They will spray, but flow drops and droplets grow. At 250 psi a nozzle flows about half its 1,000 psi rate, and the mist is noticeably coarser. Nozzles designed for mid-pressure systems are usually a better match for booster pumps.
Does the square root rule work for low-pressure hose nozzles?
Approximately, as long as the nozzle is a fixed orifice. Some low-pressure nozzles include flow-compensating or check-valve parts that behave differently near their opening pressure, so use the rule for estimates and trust a measured flow over it.
How do I find a nozzle's rated flow?
Check the manufacturer's chart for the orifice size and the pressure it was tested at, usually 1,000 psi for high-pressure nozzles. If there is no chart, measure it: collect the spray from one nozzle for 5 minutes at a known pressure and convert to gallons per hour.
Sources and further reading
- Orifice and nozzle flow equations (Bernoulli and discharge coefficient), standard fluid mechanics references
- Spray nozzle manufacturer engineering guides describing flow proportional to the square root of pressure
Figures on this page are cross-checked against the shared planning values in our research methodology. Where manufacturers publish different numbers, your equipment's data sheet takes precedence.
Spotted an error or an outdated figure? Tell us and see how we handle corrections.