Smaller adjacent orifice
More restrictive. It raises the theoretical pressure for the same flow. If that result exceeds a system rating, it is not an acceptable choice.
Home Pressure Washer Nozzle Calculator
GPM / PSI sizing tool
Pressure Washer Nozzle Calculator
Calculate the theoretical pressure-washer nozzle size from flow and pressure, or reverse the relationship to estimate pressure or flow. Multi-nozzle setups are handled by splitting total flow evenly.
Manufacturer-published relationship
Nozzle Size = GPM × √(4000 ÷ PSI). For multiple identical nozzles, divide machine GPM by the nozzle count first.
The calculator uses the standard nozzle-number convention documented by General Pump and Cat Pumps.
Pressure washer nozzle calculator
Choose what you want to solve. Results update automatically as the inputs change.
How the calculator handles standard nozzle sizes
The theoretical result can fall between manufactured sizes. In “Find nozzle size” mode, the calculator shows the adjacent standard sizes used in General Pump's pressure-washer nozzle chart and estimates the pressure each would produce at the entered per-nozzle flow.
For a rotating attachment, use the attachment's actual nozzle count. The Surface Cleaner guide and embedded tool applies this split-flow calculation while also checking documented cleaner operating ranges. The Foam Cannon guide uses the single-orifice relationship to compare documented MTM orifice mappings.
Larger adjacent orifice
Less restrictive. It lowers the theoretical pressure for the same flow. This can sacrifice pressure but reduces the risk of selecting an orifice that is too small.
Formula used
Find nozzle size: Nozzle # = per-nozzle GPM × √(4000 ÷ PSI)
Estimate flow: GPM per nozzle = Nozzle # × √(PSI ÷ 4000)
Estimate pressure: PSI = (GPM per nozzle ÷ Nozzle #)2 × 4000
The 4000 PSI value is the reference built into the nozzle-number convention; it is not an assumption that your machine is rated for 4000 PSI. A #4 nozzle is approximately a 4 GPM nozzle at 4000 PSI, and its flow changes with pressure according to the relationship above.
Worked examples
4 GPM at 3000 PSI
4 × √(4000 ÷ 3000) ≈ 4.62. A #4.5 tip is slightly more restrictive; a #5.0 is less restrictive.
4 GPM at 3000 PSI
Split 4 GPM into 2 GPM per nozzle. 2 × √(4000 ÷ 3000) ≈ 2.31 per nozzle. The accessory's own nozzle and operating-range requirements still control.
#5 nozzle with 4 GPM
(4 ÷ 5)2 × 4000 ≈ 2560 PSI theoretical pressure before real-system losses and limitations.
Important safety boundary
This calculator does not determine whether a pressure washer, pump, unloader, hose, gun, wand, fitting, turbo nozzle or surface cleaner is safe at the calculated pressure. Never use a smaller orifice to exceed a manufacturer's pressure, flow, temperature or power rating.
Technical sources
- General Pump — Nozzle Chart (2025) — standard nozzle sizes and GPM at common pressures.
- General Pump — catalog reference formulas — Nozzle # = GPM × √(4000/PSI), plus inverse GPM and PSI formulas.
- Cat Pumps — Standard Nozzle Selection Chart — manufacturer confirmation of the 4000-PSI nozzle-number convention and formulas.
- General Pump — FAQ — multi-gun sizing guidance: divide total flow by the number of guns/nozzles and size for each branch flow and desired pressure.
Frequently asked questions
How do I calculate pressure washer nozzle size?
Use Nozzle Size = GPM × √(4000 ÷ PSI). For a multi-nozzle accessory, divide total machine flow by the number of identical nozzles first, then calculate the size for each nozzle.
What nozzle size do I need for 4 GPM at 3000 PSI?
The theoretical size is about 4.62. Standard stocked sizes around that value include 4.5 and 5.0. A 4.5 is more restrictive and theoretically produces about 3160 PSI at 4 GPM; a 5.0 is less restrictive and produces about 2560 PSI. Do not exceed system ratings.
How do I size two nozzles on a surface cleaner?
Divide the machine flow by two before sizing each identical nozzle. A 4 GPM machine sends about 2 GPM to each nozzle in an ideal two-nozzle split; then use the target pressure to calculate each nozzle size.
Should I round pressure washer nozzle size up or down?
Do not round blindly. A smaller orifice is more restrictive and raises theoretical pressure; a larger orifice lowers it. Compare both adjacent standard sizes and stay within the pressure washer and accessory ratings. When avoiding an over-pressure condition is the priority, the larger-orifice side is the safer direction.
Is the calculated PSI the exact pressure at my spray tip?
No. The formula is a nozzle-flow relationship. Actual open-trigger pressure also reflects pump condition, engine or motor capability, unloader setting, hose and fitting losses, water supply and component tolerances. Use a suitable pressure gauge when exact system pressure matters.