There is no single correct ratio, because the right mix depends on your soap, your cannon and — most of all — your machine’s flow rate. What there is, is a sensible starting point and a way to tune from it.
A starting point
For a standard one-liter cannon bottle: 2 to 3 ounces of concentrate, topped with warm water. That is roughly a 1:12 to 1:16 mix, which sits comfortably inside the range most snow foams specify.
Check your soap’s label and stay inside its range. The figures above are a starting point, not a recommendation that overrides the product you bought.
Tune from there, in this order
1. Set the cannon dial rich and see what you get. Most cannons have an adjustment knob controlling how much solution is drawn.
2. If the foam is thin, add soap half an ounce at a time. Expect to land higher than the label’s middle if your machine is low-flow.
3. If the foam is thick but runs off instantly, you likely have plenty of soap and a thin mix from too much water — or a soap that is not a dedicated snow foam. Soap choice matters more than ratio here.
4. Once it is right, write it down. Scratch the measure on the bottle. You will not remember next month.
Four things that improve foam for free
Warm water. The biggest free improvement. Cold water leaves concentrate as a gel at the bottom of the bottle; warm water dissolves it and warm solution foams more.
Soap first, then water. Reduces foaming in the bottle while you fill, so you actually get a full liter of solution rather than half a liter of foam.
Clean the mesh filter. A partially clogged mesh is the most common cause of foam getting worse over a season. Thirty-second rinse.
Pre-wet the car. Foam clings to a damp panel better than a dry dusty one.
Why your ratio may need to be richer than the label
Soap labels assume a reasonably capable machine. If yours is 1.2 GPM you are moving less water through the mesh, producing less foam, and the honest compensation is more soap per liter.
That costs a little more per wash and it is cheaper than a new cannon, which would not fix it anyway — the low-flow case in full.
How much solution per car
One liter of mixed solution typically covers a mid-size car with a reasonable layer. A large SUV or a pickup may want a second fill, particularly if you are doing a proper pre-wash layer rather than a light coat.
If a liter is leaving you short on a normal car, the mix is probably too lean — the cannon is discharging solution faster than it needs to in order to make thin foam.
What not to put in
Dish soap (strips wax, alkaline). House wash or any outdoor cleaner (far too aggressive for clear coat). Wheel cleaner (acidic, and should not be spread over paint). Anything with grit or solids, which blocks the mesh.
Then run the wash in order: how to pressure wash a car.
