Pressure washer buffer tank size calculator

A pressure washer pump wants its full rated flow every second the trigger is held, and plenty of house spigots cannot give it. Time a 5-gallon bucket at the end of your hose, enter the machine’s GPM, and this works out the shortfall, the smallest tank that covers it, how long a given tank lasts, and how much water and weight you haul to a site with no water at all.

Buffer tank size

Pre-filled with an example 4 GPM rig. Put in your own plate figure and bucket time.

Water on siteMachine flow (data plate)GPM

Seconds to fill a 5-gal bucketsec

Timed at the end of the hose you will actually use.

Longest stretch with the trigger heldminReserve on top%A tank you own or are pricinggal

Pump4.0 GPM

Spigot3.0 GPM

Minimum buffer tank

48 gal

Covers 1.0 GPM short for 40 min, plus 20%

Your 100 gal tank covers this stretch. Release the trigger and the spigot refills it while you move.

Spigot supply (bucket test): 3.0 GPM
Water through the gun in 40 min: 160 gal
Shortfall the tank must cover: 1.0 GPM
A 100 gal tank lasts, trigger held: 100 min
Refill 100 gal from empty, trigger off: 33 min
100 gal of water weighs: 834 lb

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

What a buffer tank does

A buffer tank sits between the water supply and the pump. A float valve lets the spigot fill it at whatever rate the spigot can manage, and the pump draws from it at its full rated flow. When the gun is open, the tank covers the difference; when you release the trigger to move a hose or a ladder, the spigot tops it back up. The pump never sees a supply weaker than it needs.

The spigot fills at whatever it can manage; the pump draws its full rating from the tank.

Flow is the number that matters here, not pressure. A 4 GPM machine needs 4 gallons every minute the trigger is held, and the bigger the machine, the harder that is for a house spigot. The flow classes on the cleaning units calculator run up to 8 GPM for trailer rigs, and a large surface cleaner needs that flow to keep its bar spinning, as the surface cleaner size calculator shows.

Why the pump must never run short

Alkota, a pressure washer manufacturer, describes cavitation as the pump being unable to draw enough water to fill its cylinders on each intake stroke. Vapour bubbles form and collapse inside the pump, which makes a chattering noise and pits the valves, plungers and pump housing. Its fix is a supply that delivers at least the machine’s GPM at the inlet, at a minimum of 20–30 PSI, and it names a float tank as the way to buffer an inconsistent supply. Pump maker Pumptec puts it more bluntly: do not run the pump dry.

Listen for the chatter

A pump that pulses, surges or rattles on a job that ran fine last week is often short of water. Alkota lists starvation first among the causes of a pulsing pump, ahead of a blocked inlet strainer. Check the tank level and the strainer before you start replacing parts.

How to use this calculator

Read the data plate. Enter the machine’s rated GPM, not the pressure.
Do the bucket test. Run the spigot fully open through the hose you will actually work from and time how long a 5-gallon bucket takes to fill. A long or narrow hose delivers less than the spigot itself, so test at the end of it.
Enter the longest stretch with the trigger held. A driveway behind a surface cleaner is long and continuous; a house wash with ladder moves is broken up and refills the tank between pulls.
Pick a reserve. 20% is the example; a weak or shared supply deserves more.
Check a tank you own. The tool shows how long it lasts, how long it takes to refill, and what it weighs full.

Bucket fills in	Spigot supply
60 sec	5.0 GPM
75 sec	4.0 GPM
100 sec	3.0 GPM
150 sec	2.0 GPM
200 sec	1.5 GPM

Arithmetic: 5 gallons × 60 ÷ seconds to fill.

Worked examples

The first row is the calculator’s default: a 4 GPM machine on a spigot that gives 3.0 GPM is 1.0 GPM short. Over a 40-minute stretch that is 40 gallons, and 20% on top makes a 48 gal minimum. A 100 gal tank on that job lasts 100 min with the trigger held.

Job	Shortfall	Minimum tank
House wash, 4 GPM, spigot fills 5 gal in 100 s	1.0 GPM	48 gal
Driveway, 20 in disc at 4 GPM, weak spigot (150 s)	2.0 GPM	144 gal
Trailer rig, 28 in disc at 8 GPM, spigot 60 s	3.0 GPM	162 gal
No water on site, 4 GPM, 60 min of spraying	All of it	288 gal

Inputs are examples, with a 20% reserve on every row. The outputs are arithmetic.

Plumbing the tank: float valve, outlet and height

Float valve on the fill. It closes as the water rises, so you can leave the spigot open all job without the tank overflowing.
Outlet low, with a strainer. Draw from near the bottom so the last of the water is usable, and strain it. A partly blocked strainer starves the pump just like a weak spigot does.
Feed line no narrower than the pump inlet. Pumptec stresses properly sized inlet hoses. Your pump manual gives the inlet size; match or exceed it.
Tank above the pump. Gravity feed is what Pumptec recommends. If your manual asks for inlet pressure a gravity tank cannot give, ask the machine dealer how they set that model up.

A dedicated soft-wash system has its own pump and chemical tank; see soft wash system setup. This page is about the water feeding the pressure washer. To match the tip to the machine’s flow and pressure, use the nozzle size calculator.

Weight: the number that decides the trailer

Water weighs about 8.34 lb per gallon (USGS). That is the figure the calculator uses, and it adds up fast:

Water in the tank	Water weight
55 gal	459 lb
100 gal	834 lb
150 gal	1,251 lb
250 gal	2,085 lb
325 gal	2,711 lb

Water first, then everything else

Those figures are water only. The tank, the machine, hose reels and chemical all sit on the same axle. Every trailer carries a certification label with its gross vehicle weight rating (GVWR) and a rating for each axle (GAWR). Add it all up and stay under both, and under what the truck is rated to tow. Where the tank sits relative to the axle decides the hitch load; the homemade pressure washer trailer setup guide walks through it.

Hot-water machines often ride on a skid or trailer, as the hot vs cold water comparison explains, and a large tank is the heaviest thing on one. Fleet work at a customer’s yard is another job where you confirm the water before you quote; price it with the fleet washing pricing calculator.

Buffer tank questions

Do I need a buffer tank if the spigot is strong?

Not for flow. If the bucket test shows the spigot giving at least what the data plate asks for, the pump is fed. A tank still covers the jobs where it is not: the weak supply, the long hose run, the site with no water at all.

What size buffer tank for an 8 GPM machine?

Run the numbers above with your own spigot. An 8 GPM machine on a spigot giving 5 GPM is 3 GPM short; for a 45-minute stretch with a 20% reserve that is a 162 gal tank. On a 3 GPM spigot the same stretch needs 270 gal.

Can the pump suck water up from a tank on the ground?

Do not plan on it unless your pump manual says it can. Pumptec’s guidance is to avoid negative head by placing the supply tank above the pump, and Alkota asks for 20–30 PSI at the inlet. A tank on the truck bed above the machine is the usual answer.

Should I charge for bringing water?

Yes. A no-water site means a tank, a fill and a haul. Put it on the quote as its own line, as the pricing guide sets out.

Water is the first line of the job checklist for a reason: spigot on site, or tank loaded, before the truck moves. For the rest of the starter rig, see how to start a pressure washing business, and for disc flow in detail, the surface cleaner guide.