Which Soap Goes in a Foam Cannon and How Much to Use

The short version: what goes in the bottle

A foam cannon is a bottle with a nozzle that attaches to your pressure washer. You pour soap and water into the bottle. The washer pushes water through the cannon, which pulls soap from the bottle and mixes it with air. Out comes foam.

So yes, you mostly just put soap in the cannon. The details are which soap, how much, and what you fill the rest of the bottle with. Get those right and you get foam that clings to the paint. Get them wrong and you get thin, runny suds that slide off in seconds.

Which soap works best

Any soap that foams will make some bubbles. That does not mean it belongs on your car. The best choice is a car shampoo made for foam cannons. These are formulated to make dense foam from a small amount of liquid, and they are gentle on wax, sealant and ceramic coatings.

A regular pH-neutral car wash soap also works in most cannons. It may foam a little less than a dedicated foam soap, so you might need slightly more in the bottle. Wash-and-wax formulas work too, though some make looser foam.

Soap quality matters more than most people expect. Even one maker of a cannon on our list says foam density depends heavily on the shampoo you choose. A good cannon with weak soap still gives weak foam.

Soaps to skip

Dish soap is the most common mistake. It is built to cut grease, so it can strip wax and sealant from your paint and leave the finish unprotected. Laundry detergent, degreasers and all-purpose household cleaners are worse choices, since they can dry out rubber and plastic trim.

Waterless wash and quick detailer sprays are not meant for a cannon either. They are made to be sprayed and wiped, not diluted and foamed.

Here is a quick comparison of common options:

How much soap to use

Start small. For a typical 33.8 fl oz (1 liter) bottle, like the one listed for the Tool Daily cannon, begin with roughly 1 to 2 fl oz of soap and fill the rest with warm water. Then spray and look at the foam.

More soap does not always mean thicker foam. Past a certain point you waste product and can make the foam heavy and slow to rinse. Your soap's label is the best guide, because concentrations vary by brand. If the label gives a ratio for foam cannons, follow it.

Your cannon's own controls change the result too. A foam knob sets how much soap mixes into the water stream. On the Tool Daily listing, turning the knob toward the minus sign gives the thickest mixture. Try adjusting the knob before you add more soap to the bottle.

How to fill a foam cannon, step by step

Filling takes a minute. Order matters, because pouring soap in last or shaking hard makes a mess of bubbles in the bottle.

• Turn off and disconnect the pressure washer, or at least release the trigger so nothing is spraying.
• Unscrew the bottle from the cannon head.
• Pour in your measured soap first.
• Add warm water slowly, tilting the bottle a little, until you reach the fill line or just below the top. Warm water helps the soap mix.
• Screw the bottle back on firmly and check that the pickup tube sits in the liquid.
• Connect the cannon to your wand or gun, set the foam knob and spray pattern, and test on a low panel.

Check your gear before you pour

Foam depends on more than soap. Most cannons on our list use a 1/4 in quick connect and need a pressure washer. Several listings say they will not fit a garden hose or a faucet. If you only have a hose, you need a different type of foam gun.

Pressure and orifice size also affect foam. A swappable orifice lets you match the cannon to your washer. The MJJC Pro V3.0, for example, lists 1.1mm, 1.25mm and 1.35mm orifices that cover 1305 to 4061 PSI. If your foam looks thin even with good soap, a mismatched orifice may be the cause. Our best foam cannon page lists the pressure ranges makers publish, so you can compare them with your washer.

Bottle size matters for refills. Capacity among the models that list it runs from 32 fl oz to 44 fl oz, and a wide mouth makes pouring and mixing easier. If you wash a truck or SUV, a bigger bottle means fewer stops. You can see the options side by side in our foam cannon comparison.

Safety and care

Pressure washers move water at high force. Wear safety glasses, closed-toe shoes and gloves, and never point the nozzle at people, pets or your own hands. If your washer is electric, keep the plug and any extension cord off wet ground and connected to a GFCI outlet.

Keep soap out of your eyes and off bare skin if you can, and store it away from children. When you finish, run clean water through the cannon for a few seconds, then empty the bottle. Dried soap can clog the orifice and the internal mesh, which leads to weak foam next time.

Frequently asked questions

Can I use dish soap in a foam cannon if I run out of car shampoo?

It will foam, but it is not a good idea for regular use. Dish soap can strip wax and sealant from paint. Keep a bottle of proper car soap on hand instead.

Do I need to dilute soap before pouring it in?

No. Pour the soap into the bottle and add water there. The water you add in the bottle is the dilution. Do not pour it in neat and expect the cannon to dilute it for you.

Why is my foam thin even with plenty of soap?

Check the orifice size against your washer's pressure, the foam knob setting and the condition of the internal mesh. Also try warm water and a soap made for foam cannons.

Should I leave soap in the bottle between washes?

It is better to empty it. Rinse the bottle and run clean water through the cannon so dried soap does not clog the orifice.

Can I use a foam cannon with a garden hose?

Not the models on our list. Several state they will not fit a garden hose or faucet and need a pressure washer with a 1/4 in quick connect.