When Dana started detailing cars on the far side of a storage lot, she quickly hit a wall that has nothing to do with soap or nozzles: there was no water. The nearest spigot was a hundred yards away, and dragging a garden hose across the pavement was both slow and unsafe. Her solution was a pressure washer tank — a 100-gallon reservoir strapped to her trailer that feeds the machine wherever she parks. Suddenly the whole property became a wash bay. If you have ever worked where a hose cannot reach, or your machine starves for water the moment you add a second operator, a tank is the missing piece.
A pressure washer tank stores water so your pump always has a steady, air-free supply. It can hold fresh water for remote jobs or detergent for a soft-wash setup. This guide explains the types of tanks, how to size one, how they feed a pressure washer, and how to avoid the cavitation and air-lock problems that kill pumps.
What a Pressure Washer Tank Really Is
At its simplest, a pressure washer tank is a container that holds water and delivers it to the pump inlet. That sounds trivial until you understand what a pressure washer pump actually needs. Positive-displacement pumps cannot pull water well; they depend on a flooded inlet, meaning gravity or pressure must push water into the pump, not the other way around. A tank sitting a few inches above the pump, with a large feed line, does exactly that.
Tanks come in two broad jobs. A water tank supplies clean water for washing. A chemical or detergent tank holds soap, degreaser, or bleach and injects it into the line at low pressure. Many mobile rigs carry both.
Why Owners Add a Pressure Washer Tank
Freedom from a fixed water source. Mobile detailers, fleet washers, farmers, and contractors work where no spigot exists. A tank turns any parking lot, field, or jobsite into a work zone.
Buffer against weak supply. Even with a spigot, some homes and buildings deliver too little flow to feed a 4–8 GPM machine. A tank absorbs the mismatch: the tap refills the tank slowly while the pump draws fast.
Steady, air-free feed. A flooded inlet prevents the pump from sucking air, which is the number one cause of cavitation damage.
Onboard detergent. A dedicated tank lets you apply soap precisely without carrying a separate sprayer.
Quick Note: Never let a pump run dry, even for a few seconds. Running a pressure washer tank empty lets the pump pull air, overheat the seals, and fail. Install a low-water shutoff or keep a close eye on the level.
Types of Pressure Washer Tanks
Tanks are grouped by where they sit and how water moves out of them. Matching the type to your workflow saves both money and headaches.
Onboard / Buffer Tanks
Mounted directly on the machine frame or cart, these small tanks (often 5–20 gallons) smooth out supply and let the pump draw comfortably. They are the lightest way to add reserve capacity.
Cart Tanks
A wheeled tank with a built-in or bolt-on holder for the washer and gun. Perfect for driveways, patios, and anywhere you move in small loops. Our pressure washer cart guide covers how to pair a tank with a rolling frame.
Skid and Trailer Tanks
Large reservoirs (50–200+ gallons) bolted to a skid or trailer, usually feeding a belt- or gear-driven pump. This is the standard for commercial washing where output runs 5 GPM and up.
Chemical / Detergent Tanks
Smaller tanks, often 3–15 gallons, plumbed to a downstream injector or a dedicated pump. Learn how they inject soap in our guide to pressure washer soap tanks.
| Tank Type | Typical Size | Feed Method | Best For |
|---|---|---|---|
| Onboard buffer | 5–20 gal | Gravity | Smoothing weak supply |
| Cart tank | 20–50 gal | Gravity | Driveways, patios |
| Skid / trailer | 50–200+ gal | Gravity or pump | Commercial, mobile work |
| Detergent tank | 3–15 gal | Injector or pump | Soap and soft wash |
How to Size a Pressure Washer Tank
The math is simple: tank size divided by flow rate equals run time. Multiply your machine’s GPM by the minutes you want to run between refills, and add a safety margin of about 20 percent so you never draw the tank dry.
For example, a 4 GPM machine running 20 minutes needs roughly 80 gallons, so a 100-gallon tank gives comfortable headroom. A 2 GPM machine can run a full hour on a 100-gallon tank. Commercial 8 GPM rigs often tower tank capacity into the 200–300 gallon range to keep a crew working without constant refills.
Also remember weight. Water weighs about 8.34 pounds per gallon, so 100 gallons is roughly 834 pounds before you add the tank, pump, and hose. Confirm your trailer, axles, and tow vehicle are rated for the total. A full 200-gallon tank tips the scale at well over 1,600 pounds of water alone.
| Machine Flow | 50-Gal Tank | 100-Gal Tank | 200-Gal Tank |
|---|---|---|---|
| 2 GPM | ~25 min | ~50 min | ~100 min |
| 4 GPM | ~12 min | ~25 min | ~50 min |
| 8 GPM | ~6 min | ~12 min | ~25 min |
Note: These figures assume no refill. If a spigot or float valve tops the tank off while you work, your effective run time is limited only by the refill rate, not the tank size.
How Water Feeds From the Tank to the Pump
There are three common feed methods, and choosing the wrong one creates air problems that feel like pump failure.
Gravity feed. The tank sits above the pump inlet and water falls through a large-diameter hose. This is the most reliable and simplest method. Keep the feed line short, at least the same diameter as the pump inlet (often 3/4-inch), and avoid high spots where air can collect.
Direct / suction feed. The pump draws water from a tank at or below its level. This only works with pumps designed to self-prime and short, oversized lines. It is common on light-duty electric units but risky on high-flow pumps.
Booster pump feed. A small transfer pump pushes water from the tank to the washer inlet. This solves long runs and low-mounted tanks but adds cost and another part to maintain.
Whatever the method, air is the enemy. Any leak on the suction side lets air in and causes the pump to surge and lose pressure. Use thread sealant on suction fittings, keep the line tight, and fit an inlet filter to catch debris before it reaches the pump. Clean, unrestricted feed also protects the hose and reel downstream.
Materials, Fittings, and Features to Look For
Most water tanks are molded from food-grade polyethylene, which resists UV, cracking, and many chemicals. Look for UV-stabilized, one-piece construction with molded-in baffles, which reduce water slosh while driving. A sealed lid keeps debris and rain out, and a bottom drain makes winterizing easy.
Fittings should be brass or stainless, not potted plastic, so you can service them. A float valve lets the tank refill automatically from a spigot. A tank gauge or sight glass saves you from guessing the level. For corrosive chemicals, use a chemically compatible tank rather than a general-purpose water tank.
Setting Up Your Pressure Washer Tank
1. Position the tank so the outlet is above the pump inlet. A few inches of drop is enough for gravity to do the work.
2. Plumb a short, large feed line straight down to the pump, with as few fittings as possible.
3. Fit an inlet strainer to stop debris before it reaches the valves.
3. Vent the tank so it can breathe; a sealed tank will starve the pump as vacuum builds.
4. Secure the tank with straps or a frame so it cannot shift when full.
5. Test with the gun open. Watch for steady flow, no air bubbles, and stable pressure before you start the real work.
Note: A quick-connect on the suction line is convenient but risky — if it is not rated for suction, it will leak air. Prefer barbed fittings or full-flow couplers on the inlet side.
Common Mistakes to Avoid
Undersized feed lines. A garden hose on the suction side chokes flow and starves the pump. Go bigger, not smaller.
Running the tank dry. Cavitation damages seals, valves, and the pump itself. Watch the level or fit a shutoff.
Mounting the tank too low. Gravity can only work if the tank outlet is above the pump inlet.
Ignoring weight limits. Overloading a trailer or truck bed is a safety hazard. Do the math first.
Mixing chemicals in your drinking-water tank. Keep potable water and detergent in separate tanks and label them clearly.
Placement, Plumbing, and Weight Balance
Where you put the tank affects both performance and safety. Keep the outlet as close to the pump inlet as possible so the feed line runs short and downhill. Every extra foot of suction hose adds friction and another chance for an air leak, so resist the urge to route plumbing around the frame for looks. If the run must be long, switch to a booster pump rather than fighting physics.
Weight distribution matters just as much. Water is heavy and it sloshes, so center the tank over the trailer axles or between the truck rails rather than hanging it off the tail. Baffled tanks reduce the surge that can shove a trailer around on turns, and properly rated straps or a welded frame keep the load from shifting on rough ground. Check your trailer’s gross vehicle weight rating before filling a large tank, and never tow with an unsecured load.
For stationary or shop-based setups, plumbing a float valve from a wall spigot lets the tank refill automatically and turns it into a buffer tank that smooths supply for a high-flow machine. That arrangement is common in car washes and fleet bays, where the demand of a 6–8 GPM pump would otherwise outrun the building’s water line.
Soft Wash and Chemical Compatibility
If you plan to soft wash or handle bleach, sodium hypochlorite, and other harsh chemicals, choose the tank and fittings with that in mind. Standard polyethylene handles many chemicals well, but the metal parts matter: aluminum and mild steel corrode quickly around bleach, so specify stainless or chemically rated components. Never store concentrated chemicals in a tank rated only for potable water, and always label each tank so cross-contamination never happens. Rinse and neutralize thoroughly when switching a tank from one chemical to another.
Maintenance Tips
Rinse the tank seasonally to prevent algae and sediment buildup. Drain it completely before freezing weather, since ice expands and can crack even rugged polyethylene. Inspect hoses and clamps for cracks, and replace swollen or hardened feed lines. Clean or replace the inlet filter regularly so the pump never has to work harder than it should.
Frequently Asked Questions
Can I connect a pressure washer directly to a tank? Yes, provided the pump has a flooded, gravity-fed inlet or a booster pump. Most pumps cannot pull water uphill.
How big a tank do I need? Divide your desired run time by flow rate, then add about 20 percent. For a 4 GPM machine and a 20-minute job, plan on about 100 gallons.
Are gravity-fed tanks enough for high GPM machines? Often yes, if the feed line is large and short. High-flow pumps simply need more water than a small line can deliver, so upsize the plumbing.
Can I use the same tank for water and soap? Only if you thoroughly rinse it and never need the water for sensitive work. Dedicated tanks avoid contamination.
Final Thoughts
A pressure washer tank is less about extra capacity and more about control. It removes your dependence on a fixed spigot, prevents the air starvation that destroys pumps, and lets you carry soap on the same rig. Size it to your flow rate and expected run time, feed it by gravity or a booster pump, and keep the suction side airtight.
Get that right, and you can wash anywhere the job takes you — no hose, no guesswork, no downtime. If your next step is a bigger machine to match that freedom, compare flow rates in our guide to 5 GPM pressure washers and when a 15 GPM unit makes sense.