Surface cleaner size calculator
A surface cleaner is the single biggest speed gain in flatwork, and the most common way to buy the wrong tool. The disc has to be matched to flow, not to pressure and not to ambition. Too big for the machine and the bar slows, the coverage thins, and the slab comes out striped.
Surface cleaner sizing
Largest disc your machine will actually drive
20 in
Needs about 4.0 GPM to keep the bar spinning
Fit a disc bigger than the flow supports and the spray bar slows, the passes stripe, and you spend the time you thought you were saving going back over them.
| Disc | Minimum GPM | Verdict at your flow |
|---|---|---|
| 15 in | 3.0 | Driveable |
| 16 in | 4.0 | Driveable |
| 20 in | 4.0 | Driveable |
| 24 in | 5.5 | Under-fed — will stripe |
| 28 in | 8.0 | Under-fed — will stripe |
| 30 in | 8.0 | Under-fed — will stripe |
PSI & nozzle quick reference
- 0 degRed
- Spot jet. Etches most surfaces.4000 PSI
- 15 degYellow
- Stripping bare concrete.4000 PSI
- 25 degGreen
- General concrete and walkways.3500 PSI
- 40 degWhite
- Siding, wood, vehicles.1500 PSI
- ~65 degBlack
- Detergent draw, low pressure.500 PSI
Why flow, not pressure
The spinning bar is driven by the water leaving its two or four jets. Pressure decides how hard each jet hits; flow decides how much water there is to spread across the disc's circumference. A wide disc on a low-flow machine has the same water spread over a longer arc, so each square inch of concrete gets less of it and the bar turns more slowly — both of which show up as stripes.
The stripes appear a day later
What a correctly matched disc buys you
- Speed. Roughly three to four times a wand on open flatwork, which is the whole argument for owning one.
- Evenness. Fixed stand-off means fixed pressure at the surface, which a hand-held wand never quite manages.
- Containment. The skirt keeps most of the spray and debris off the siding and the windows you would otherwise be re-cleaning.
- Your wrists. Genuinely. A full day on a wand and a full day behind a disc are different jobs.