Pressure Washer Nozzle Calculator
Enter your pressure washer's GPM and PSI and get the nozzle orifice size, the nearest standard sizes and the pressure each would give, from a pump maker's nozzle chart.
Use the rated GPM and PSI from the machine’s label or manual. The answer updates as you type; nothing is sent anywhere.
4 GPM at 3,000 PSI
Calculated size 4.62
Between the chart's sizes 4.5 and 5.
- Size 4.5 (smaller orifice): 3,160 PSI at your flow (+160 vs rated).
- Size 5 (larger orifice): 2,560 PSI at your flow (-440 vs rated).
| Size | Orifice (ref.) | Pressure at 4 GPM | Flow at 3,000 PSI |
|---|---|---|---|
| 3.75 | 0.045″ | 4,551 PSI | 3.25 GPM |
| 4 | 0.046″ | 4,000 PSI | 3.46 GPM |
| 4.5 | 0.049″ | 3,160 PSI | 3.9 GPM |
| 5 | 0.052″ | 2,560 PSI | 4.33 GPM |
| 5.5 | 0.054″ | 2,116 PSI | 4.76 GPM |
| 6 | 0.057″ | 1,778 PSI | 5.2 GPM |
The arithmetic
Size = GPM × √(4000 ÷ PSI) = 4 × √(4000 ÷ 3000) = 4.619. A nozzle’s size number is its flow in GPM at 4,000 PSI; pressure at a given flow is 4000 × (GPM ÷ size)².
What this page cannot tell you
Size numbers follow General Pump’s convention (GPM at 4,000 PSI); some makers number their tips on a different basis. The chart assumes a nozzle in good condition; worn orifices pass more water at lower pressure. Real pressure at the nozzle also depends on the pump, unloader setting and hose losses. The machine’s manual and the pump and nozzle makers set the limits for your equipment.
How to work out a pressure washer nozzle size
- Find the machine's rating. Read the rated flow (GPM) and pressure (PSI) from the label or manual. Convert bar to PSI first if needed (1 bar ≈ 14.5 PSI).
- Enter GPM and PSI. The calculator works out size = GPM × √(4000 ÷ PSI). In General Pump's chart, a nozzle's size number is the flow it passes at 4,000 PSI.
- Compare the nearest sizes. Nozzles come in steps such as 4.0, 4.5 and 5.0. The tool shows the pressure each nearby size would give at your machine's flow.
- Check the spray angle separately. The size sets flow and pressure. The spray angle is a separate choice and does not change the size this calculator gives.
10 situations where the nozzle size matters
Replacing a worn nozzle
When pressure drops, the replacement needs the right orifice. The tool gives the size for the machine's rated GPM and PSI.
Buying a surface cleaner
Surface cleaners use two or more nozzles that share the flow. Entering half the machine's GPM gives each nozzle's size for a two-nozzle bar.
Setting up a new pump
After a pump swap, the nozzle has to match the new pump's flow and pressure.
Using a long hose run
The chart shows pressure at the nozzle for a given flow, which helps when comparing against what reaches the gun.
Reading a nozzle chart
The table lists the pressure each nearby size gives at your flow, the same figures a printed nozzle chart holds, worked for your machine.
Planning a turbo or rotary nozzle
Rotary nozzles are also sold by orifice size. The calculated size is the figure to compare with the maker's sizing for that nozzle.
Downstreaming chemicals
Soap nozzles have larger orifices to drop pressure. The table shows how much a larger size lowers pressure at your flow.
Fleet washing rigs
Contractors with several machines can work out the nozzle for each GPM and PSI combination.
Diagnosing low pressure
If a machine reaches less than its rated pressure with its fitted nozzle, the chart's pressure for that size is a reference point.
Metric machines
Machines rated in litres per minute and bar can be converted (1 US gal = 3.785 L, 1 bar ≈ 14.5 PSI) and entered.
Where the sizes come from
The figures come from General Pump's 'General Pump Nozzle Chart' (2021), which lists the flow in gallons per minute for 31 nozzle sizes from 2 to 60 at pressures from 40 to 5,000 PSI, with a reference orifice diameter for each size. In the chart, each size's flow at 4,000 PSI equals its size number, and every other column follows GPM = size × √(PSI ÷ 4000).
We checked that formula against every printed cell: 490 of the 496 are within 0.015 GPM. The six that differ are printing slips — for example size 7 at 2,000 PSI is printed 1.95 GPM where the rest of its row and the formula give 4.95. The calculator uses the formula, so it is not affected by those slips. Rearranged, it gives size = GPM × √(4000 ÷ PSI) and pressure = 4000 × (GPM ÷ size)².
The orifice diameters are shown as the chart prints them; the chart says they are for reference only. Nothing is fetched while you use the page.
The limits: size numbers are a maker's convention. In this chart they mean GPM at 4,000 PSI, but not every catalogue uses that basis — Spraying Systems, for example, lists its WashJet MEG 2540 tip as 4 GPM at 40 PSI — so match the convention of the nozzle you are buying. The chart describes new nozzles. Worn nozzles flow more at lower pressure, and real pressure depends on the pump, unloader, hose and fittings. The equipment makers' ratings and manuals set what a machine can safely run at.
Sources
- Spraying Systems Co. — WashJet MEG 1/4MEG-2540 — Rated 4 gpm at 40 psi — a different size convention, cited as a caution
- General Pump — Nozzle Chart (2021, PDF) — GPM by nozzle size at 40–5,000 PSI; reference orifice diameters
Frequently asked questions
What size nozzle for a 4 GPM 3000 PSI pressure washer?
The formula gives 4 × √(4000 ÷ 3000) = 4.62. The nearest chart sizes are 4.5, which gives about 3,160 PSI at 4 GPM, and 5.0, about 2,560 PSI.
What size nozzle for 2.5 GPM at 3000 PSI?
2.5 × √(4000 ÷ 3000) = 2.89, between sizes 2.5 (about 4,000 PSI at 2.5 GPM) and 3.0 (about 2,780 PSI).
What does the nozzle size number mean?
In General Pump's chart, a nozzle's size is the flow in GPM it passes at 4,000 PSI — size 4 passes 4.00 GPM at 4,000 PSI and 2.00 GPM at 1,000 PSI. Not every maker numbers tips this way: Spraying Systems, for example, rates its WashJet MEG 2540 tip at 4 GPM at 40 PSI. Check which convention your nozzle's maker uses.
Does a smaller nozzle increase pressure?
At the same flow, yes: pressure = 4000 × (GPM ÷ size)². That is why the table shows a higher PSI for each smaller size. The pump and unloader limit what the machine actually reaches.
How do I size nozzles for a surface cleaner?
The nozzles share the flow, so divide the machine's GPM by the number of nozzles and enter that with the machine's PSI. A 4 GPM, 3,000 PSI machine with two nozzles: 2 GPM each, size 2.31.
My machine is rated in bar and L/min. What do I enter?
Convert first: GPM = L/min ÷ 3.785 and PSI = bar × 14.5. For example 15 L/min at 200 bar is about 3.96 GPM at 2,900 PSI.
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