Last Updated on September 23, 2026 by Umar Farooq
A pressure washing water tank is a buffer between a tap that cannot keep up and a pump that will not wait. Size it to your machine’s GPM and how long you pull the trigger, sit it so the water level is above the pump inlet, and plumb it with an inlet line one size larger than the pump fitting. The tank is the easy part. The part that kills pumps is assuming the pump can suck.
It cannot. Most of them cannot. That is the whole article in two sentences, and the rest of it is the arithmetic that proves it.
When a Tank Beats the Tap
There are three honest reasons to put a tank between the water and the machine, and one bad one.
Your tap does not deliver what the machine drinks. Simpson publishes its supply requirement plainly: a cold water supply at “a minimum of five gallons per minute (GPM) and twenty pounds per square inch (PSI)” (Simpson Cleaning FAQ). Go and time a five-gallon bucket off your outside tap. If it takes ninety seconds, you have 3.3 GPM. A 4 GPM machine on that tap is running a deficit of 0.7 gallons every minute it is open, and the pump is the thing that absorbs the shortfall. Our full method for measuring and matching supply is in what GPM pressure washer do I need.
There is no tap. Parking lots, cemeteries, new-build sites, boat yards, anywhere the nearest hose bib belongs to somebody who has already said no. A tank on a trailer is the only supply you have.
You are on a well. Well systems deliver in bursts. The pressure tank empties, the pump kicks in, and pressure sags in between. Simpson also notes that on a well, the supply hose should be no more than thirty feet to limit pressure loss. A buffer tank turns a stuttering well into a steady column of water, which is exactly what a positive displacement pump wants.
The bad reason is “more water in the tank means more pressure at the gun.” It does not. A tank changes nothing about what the pump produces. It only changes whether the pump gets fed.
Sizing a Pressure Washing Water Tank to GPM and Runtime
Tank volume is a multiplication, and the number people get wrong is not the GPM, it is the runtime. You do not run the trigger for the whole job. Between moving the ladder, mixing detergent, letting it dwell and walking around the house deciding you have made a mistake, actual trigger time lands somewhere near half of total time on site. So size on trigger minutes, not clock minutes.
| Machine GPM | 30 min on the trigger | 60 min on the trigger |
| 1.4 (electric) | 42 gallons | 84 gallons |
| 2.0 | 60 gallons | 120 gallons |
| 2.5 | 75 gallons | 150 gallons |
| 3.0 | 90 gallons | 180 gallons |
| 4.0 | 120 gallons | 240 gallons |
That table is for a tank running with no refill — a job with no tap at all. If you have a slow tap feeding the tank while you work, the sum changes and gets much friendlier. What matters then is the deficit, not the total:
buffer minutes = tank gallons ÷ (machine GPM − tap GPM)
A 4 GPM machine on a 2.5 GPM tap runs a 1.5 GPM deficit. A 65-gallon tank gives you 65 ÷ 1.5 = about 43 minutes of continuous trigger, and every pause you take is the tank clawing volume back at 2.5 gallons a minute. A 65-gallon drum will carry a homeowner machine through a whole driveway on a tap that could never have fed it directly.
One thing nobody mentions until you try to move it: water weighs 8.34 pounds per gallon. A 100-gallon tank holds 834 pounds. Fill it where it is going to sit.
Your Pump Almost Certainly Cannot Lift Water
This is the part that gets skipped, and it is why buffer tank threads are full of people asking why a new pump sounds like a coffee grinder.
Your pressure washer pump is a positive displacement pump. It moves a fixed slug of water per stroke, and it has no mechanism for pulling a column of water up out of a container. Industrial pumps that are rated for suction publish a figure for it. Cat Pumps, for example, states that its piston pumps handle “not more than 8.5 psi suction” and its plunger pumps not more than 5 psi on some models, with flooded inlet required on others (Cat Pumps, Inlet Design). Those are real numbers on purpose-built pumps with properly sized suction plumbing.
I went looking for the equivalent figure on consumer machines and found none. What they publish instead is a minimum *inlet pressure*, usually 20 PSI. That absence is the answer: a machine specifying a minimum inlet pressure expects water to arrive already pushed, not to go and fetch it.
Flooded suction means the water level in the tank sits above the pump inlet, so water falls into the pump under its own weight and the inlet is never empty. Get that wrong and you get cavitation. If the pump cannot get enough water in, it starts pulling air instead. Those bubbles collapse inside the pump with surprising violence, thousands of tiny impacts a second all landing on the same valve seats, and the pump that was healthy last weekend quietly wears itself out. A rattling, surging machine that loses pressure at random is often a supply problem wearing a pump problem’s clothes — the same failure we trace in pressure washer low pressure.
Gravity Feed Versus Letting the Pump Draw
Now the arithmetic that settles the argument. Head pressure converts at a fixed rate: 1 PSI = 2.31 feet of water. Flip it and every foot of elevation buys you 0.433 PSI.
| Water level above pump inlet | Pressure delivered |
| 2 feet | 0.87 PSI |
| 4 feet | 1.73 PSI |
| 8 feet | 3.46 PSI |
| 46 feet | 20 PSI |
A tank sitting on a pallet four feet off the ground is handing your pump 1.73 PSI. The manufacturer asked for 20. To meet that spec by gravity alone you would need the water surface forty-six feet in the air. (At which point you do not have a buffer tank. You have a water tower, and probably a conversation with the council.)
So does gravity feed work at all? Sometimes, and here is the honest version. With a short, fat, unrestricted inlet line and the tank outlet above the pump, plenty of belt-drive and triplex machines will happily run on a couple of PSI of head, because the inlet check valves are light and the pump is turning slowly. Direct-drive axial pumps — the sealed wobble-plate units in nearly every homeowner machine — spin at engine or motor speed and are far less forgiving. They are also the pumps that are not designed to be rebuilt, so when they go, the machine goes.
This is my one firm position on the topic: if you are feeding a direct-drive consumer machine from a tank, budget for a transfer pump, not a taller stand. A small 12-volt or 120-volt transfer pump delivering 20 to 40 PSI at more GPM than your machine needs costs a fraction of a pump head and removes the entire question. Raising the tank another two feet buys you 0.87 PSI. That is not a fix, it is a rounding error with a ladder involved.
Why a Garden Hose From a Tank Starves the Pump
This is the single most common way a correctly sized tank still kills a pump.
A garden hose is 5/8 inch internal on a good day, often less at the couplings, and it is engineered to be *pushed* — 40 to 60 PSI of mains pressure shoving water through it. On the suction side of a tank you have somewhere between one and three PSI doing the shoving. Ask a garden hose to pass full flow on that and it will not. Worse, an unreinforced hose under even mild suction collapses. It folds flat, the pump starves, and you have built a cavitation machine with a spray gun attached.
Cat Pumps is specific about it: inlet plumbing should be “a minimum of one size larger than the pump inlet fitting,” giving the example of a 3/4-inch inlet fitting needing 1-inch flexible reinforced hose (Cat Pumps, Inlet Design). They also single out restrictive plumbing — small diameters and elbows — as a direct cause of cavitation.
What that means at the tank:
- Use reinforced suction hose, 1 inch or larger. Wire or spiral reinforced, so it holds its shape. Not garden hose. Not clear vinyl tube.
- The bulkhead fitting is usually the real restriction. A drum with a 3/4-inch outlet stub is a 3/4-inch tank no matter what hose you hang off it. Fit a 1-inch or 1-1/4-inch bulkhead.
- Full-port ball valve only. A gate valve or a garden-hose shutoff drops more pressure than you have to spend.
- Keep it short and keep it straight. Every elbow is a fitting you are paying for out of two PSI.
- Strain at the tank, not at the pump. A coarse screen on the tank outlet stops the debris; the machine’s own inlet filter catches what gets past.
The rule of thumb that survives all of this: if your inlet line is the same size as your garden hose, it is too small. If the machine is already drawing nothing at all, start with the checks in pressure washer no water coming out.
Float Valve or a Person Watching the Hose?
A float valve is what turns a barrel into a buffer tank. Tap feeds the tank continuously, float shuts it off at the top, pump draws from the bottom, and you stop thinking about it.
Two things determine whether it works.
Flow capacity. The float valve has to pass your tap’s full GPM, or it becomes the new bottleneck and you have moved the restriction rather than removed it. A toilet-cistern float valve passes well under a gallon a minute and will quietly undo the entire point of the tank. What you want is a full-flow or horizontal float valve rated at or above your tap’s measured output, on a 3/4-inch or 1-inch inlet.
Air gap. Mount the valve so its outlet discharges *above* the water line, not submerged. A submerged fill line in a tank of dirty water connected to a hose bib is a cross-connection, and a physical air gap is the oldest and most reliable backflow protection there is. Add an overflow port near the rim while you are at it, because float valves stick, and a stuck float valve above an air gap is a puddle rather than a pumped-out garage.
If you are running without a float valve, you are the float valve. That works right up until the moment you are round the other side of the house with the trigger held down.
What Grows in a Tank Left Full
Nobody warns you about this one, and it is the topic all three of the top-ranking buffer tank guides skip entirely.
Standing water plus sunlight plus warmth equals algae, and a translucent white poly tank is essentially a greenhouse. Within a fortnight of warm weather you get a green film on the walls; within a month you get a slime layer that sheds into the water and straight into your inlet screen. Then the machine surges, you blame the unloader, and the actual fault is a tablespoon of biofilm sitting on a 50-mesh filter.
Three things fix it, in order of how much effort they cost you:
1. Drain it. Empty tank, empty suction line, lid off to dry. Nothing grows in a dry drum. This is free and it is what I would do between jobs.
2. Use an opaque tank. Black or dark green blocks the light that algae needs. The cheap translucent drums are the ones that go green.
3. Chlorinate it if it has to sit. A small dose of plain unscented household bleach holds a stored tank for a week or two. Your tank is not drinking water and should never be treated as if it is, but the same chemistry applies to what you are trying to kill.
Worth knowing why this matters beyond a clogged filter. A 69-year-old man was using a commercial high-pressure washer with well water to strip rust from pipework when the jet caught his right forearm — three five-centimetre lacerations with air driven into the soft tissue. He got to hospital the same day, surgeons left the wounds open, and he was fully healed by day 29 (case report, 2024). Water-based injection injuries carry roughly a 6% amputation rate; contaminated and solvent-based ones run 50 to 80%. The water in your tank is part of your safety equipment, whether you think of it that way or not.
A Full Tank Is a Frozen Tank in January
Water expands by roughly 9% when it freezes, and a poly tank has more give in it than the hardware bolted through its wall. So the tank usually survives and the fittings do not. The bulkhead fitting splits, the float valve cracks, the suction hose stiffens and tears at the barb. You find out in April.
Then there is the machine itself. A pump left wet through a freeze cracks the head, and a cracked head generally costs more than the machine is worth. Storage kills more pressure washers than work does, and a tank setup gives freezing three more places to work. Drain, in this order:
1. The suction hose — disconnect both ends and hang it in a loop so it clears.
2. The tank — open the bulkhead valve fully and leave it open. A partly drained tank still has a slug of water sitting on the outlet.
3. The float valve and its feed line.
4. The pump — full winterizing routine, as in how to winterize a pressure washer.
And if you leave a 100-gallon tank full in an unheated garage, you are not storing water. You are growing an 834-pound ice block that you will not be moving until spring.
FAQ
Can I run a pressure washer from a bucket?
Briefly and badly. A five-gallon bucket holds about 90 seconds of water for a 3 GPM machine, which is not long enough to do anything useful and is more than long enough to run the pump dry. If you genuinely have no supply, the minimum practical setup is a drum of 30 gallons or more, with its water level above the pump inlet and a proper reinforced suction line.
What size buffer tank do I need for pressure washing?
Multiply your machine’s GPM by the minutes you actually hold the trigger. A 2.5 GPM machine and 30 trigger minutes needs 75 gallons with no refill. If a tap is topping the tank up while you work, divide the tank volume by the difference between machine GPM and tap GPM to get your working buffer — a 65-gallon tank on a 1.5 GPM deficit gives roughly 43 minutes.
Do I need a pump between the tank and the pressure washer?
If the machine is a direct-drive axial unit, yes, assume so. Gravity from a tank on a pallet supplies under 2 PSI and most manuals ask for 20. Belt-drive and triplex machines are far more tolerant and often run on gravity alone with a short, wide, unrestricted inlet line. If in doubt, a transfer pump is cheaper than a pump head.
How high does the tank need to be for gravity feed?
Higher than the pump inlet is the requirement; higher than that is a bonus you can calculate. Every foot of water above the inlet gives 0.433 PSI, so four feet gives 1.73 PSI. There is no realistic height that reaches a 20 PSI spec, which is why elevation is a starting condition and not a solution.
Can I use a rain barrel to feed a pressure washer?
Mechanically yes, if you fit a large enough outlet and keep the water level above the pump. Rainwater carries grit, leaf debris and whatever was on the roof, so it needs a coarse screen at the tank outlet and a clean inlet filter at the machine. Grit in the inlet is the fastest way to score a valve seat.
Will a bigger tank increase my pressure?
No. Pressure comes from the pump and the nozzle orifice. A tank only determines whether the pump has enough water to do its job. If your pressure is low with a full tank, the fault is in the inlet line, the filter, the nozzle or the unloader — not the tank volume.
Keep the Water Falling Into the Pump
Everything above collapses into one instruction. Water should arrive at the pump inlet by falling, never by being pulled, and the path it falls through should be wider than you think it needs to be. Get that right and the tank size is just arithmetic, the float valve is a convenience, and the algae is a chore.
Get it wrong and the machine will tell you — that surging, rattling note, the pressure that comes and goes for no reason you can see. That sound is not the pump complaining about the work. It is the pump complaining about the drink.
