Flow moves the dirt, pressure only loosens it
This is the one thing worth internalising about pressure washing, and it is the thing most often got backwards. Pressure — the PSI number on the box — determines whether the nozzle can break the bond between the dirt and the surface. Once that bond is broken, what carries the dirt away and rinses the surface behind the nozzle is water volume, measured in gallons per minute. A machine with twice the flow cleans roughly twice the area per hour. A machine with twice the pressure cleans a little faster and gouges softer materials.
The model here reflects that. Working rate is the surface base rate multiplied directly by GPM, then adjusted by a pressure factor that runs only from 0.9 at 1,500 PSI to 1.0 at 3,000 and 1.1 at 4,500, where it is capped. Double the flow and the estimate halves. Double the pressure and it moves by a few percent. That is not a modelling convenience, it is what happens on site, and it is why contractors buying a machine ask about GPM first and treat PSI as a threshold to clear rather than a number to maximise.
The practical version: an 8 GPM belt-drive unit is not four times the machine a 2 GPM consumer unit is because it costs four times as much. It is four times the machine because it puts four times the water on the ground every minute.
Surface cleaners, and where the rest of the time goes
A surface cleaner is a shrouded disc with two or four rotating jets that sweeps a fixed width at a consistent standoff. On flat work it roughly doubles the rate against a wand, and it does so while producing an even result instead of the striping a wand leaves when the overlap is not perfect. It also needs flow to feed it: a surface cleaner splits the machine output between its jets, so a low-GPM machine driving a wide surface cleaner produces a disc that turns slowly and cleans badly.
| Surface | Base rate, sq ft per hour per GPM | Notes |
|---|---|---|
| Concrete flatwork | 250 | Surface cleaner applies |
| Pavers, stamped concrete | 200 | Joint sand blows out; plan to re-sand |
| Vinyl or painted siding | 350 | Low pressure, detergent does the work |
| Brick or masonry | 180 | Mortar erodes; test an out-of-sight patch |
| Wood deck or fence | 200 | Softest material here by a wide margin |
None of these rates include setup, masking, moving furniture, running hose, dealing with a hose bib that will not sustain the flow, or the time spent on the two square feet of oil stain that takes as long as the rest of the driveway. The setup field exists so those hours land in the quote rather than in your evening.
The injuries nobody expects
A pressure washer at 3,000 PSI produces a high-velocity injection injury if the stream contacts skin. The entry wound often looks like a small puncture and hurts less than it should, and the damage is deep in the tissue where nothing is visible. These injuries can lead to amputation when they are treated as minor cuts, and they are a genuine emergency department visit the same day, with the pressure of the machine reported to the treating clinician. That is the entire message; there is no technique in it.
The second hazard is the ladder. Working a pressure washer from a ladder is a documented fall mechanism, because the wand produces a reaction force that pushes back against you and a ladder does not object until it is already moving. The third is water intrusion: water driven upward under lap siding, into a window assembly or past a soffit does not drain, it sits in the wall. If the work needs height, it needs an approach that is not a person on a ladder holding a wand.
Chemistry, water supply and what to do afterwards
Most of what people credit to pressure is actually dwell time with a detergent. Applied at low pressure, given time to work and rinsed off, a cleaner does the job on siding, roofs and organic growth at pressures that do no damage. Use one product at a time at the dilution its own label specifies. Never combine cleaning chemicals: bleach with an ammonia-containing cleaner produces a toxic gas, and bleach with an acid releases chlorine. That warning belongs in every discussion of exterior cleaning because the products involved are the exact pair most likely to be sitting next to each other in the truck.
Check your supply before you promise a schedule. A machine rated at 4 GPM fed by a hose bib delivering 3 will cavitate and damage the pump. Fill a five gallon bucket from the tap and time it — five gallons in a minute is 5 GPM. On the finishing end, pavers lose joint sand and want re-sanding, and a washed deck is the start of a job rather than the end of one; carry on into the deck cleaning and sealing calculator for the cleaner, brightener and sealer quantities.
Questions people ask
Is GPM or PSI more important on a pressure washer?
GPM, by a long way, once you have enough PSI to break the dirt loose. Pressure is a threshold: below roughly 1,500 PSI many outdoor jobs become slow regardless of flow, and above about 3,000 the extra pressure mostly buys you the ability to damage things. Flow is a rate: it is the water that carries the loosened dirt off the surface and rinses behind the nozzle, so it maps almost linearly onto square feet per hour. If you are choosing between a 4,000 PSI at 3 GPM machine and a 3,000 PSI at 5 GPM machine for production work, the second one finishes first.
How much water does pressure washing use?
Flow times running minutes, and nothing else. A 4 GPM machine running for two hours of actual trigger time uses 480 gallons. That is why the estimate here reports water directly off the time: there is no efficiency term to argue about. It matters in three situations — you are on a well, you are on a water meter and the bill is per gallon, or you are working from a tank on a trailer and need to know whether you can finish without a refill. If you want the cost side of a metered supply, the water bill calculator handles the tariff arithmetic.
Can I pressure wash my siding myself?
House washing is normally done at low pressure with a detergent rather than by force, and that part is within reach. The parts that are not are height and water intrusion. Water driven upward under lap siding or into a window assembly ends up inside the wall where it stays, and the resulting damage shows up as a stain or a smell months later. Working a wand from a ladder is a known fall mechanism because the wand pushes back. Ground-level walls with a wide tip and patience are one thing; second storey work is a different job.
Why did my deck come out fuzzy and striped after washing?
Because wood is soft and it erodes. Softwood has alternating bands of soft spring growth and hard summer growth, and the pressure cuts the soft bands away first, leaving raised hard grain and a furred surface. The stripes are wand overlap. Neither can be washed out afterwards; they are sanded out or lived with. It is the reason wood is cleaned with a wide fan tip at low pressure and a wood cleaner doing the work, and the reason a surface cleaner has no place on a deck.
Do I need hot water?
For grease, oil, gum and food-service work, yes, and the difference is not marginal. Hot water changes the viscosity of oils and lets them release at pressures cold water cannot achieve. For general dirt, algae and mildew on concrete, siding and fences, cold water with the right detergent and enough dwell time does the job, and a hot water unit is a large capital and maintenance cost to justify. Judge it by whether the soil on your typical job is organic or petroleum-based.