Soft-Wash Dilution Calculator
Mix a tank to an exact strength, or solve what your downstream injector really puts on the surface. No login, no paywall, and the math never leaves your browser.
Fresh 12.5% SH degrades. Test old stock or treat it as weaker.
To fill your tank, load
- Mix ratio
- 1 : 5.3 (SH : water)
- On surface
- 2% SH
- Surfactant
- 300 oz total
Presets are directional starting points, not a standard. SH percentages that suit one surface will strip or burn another, and the general- liability “your work” exclusion means a damaged roof or killed landscaping is on you. Confirm strengths against your own results and your chemical labels before you spray.
How the two modes work
Mixing a batch
When you fill a tank and apply it neat, the strength in the tank is the strength on the surface. To hit a target percent, the SH you add equals tank size times target divided by stock strength. The calculator returns the gallons of SH and water, the mix ratio, and any surfactant.
Downstreaming
A downstream injector pulls chemical into the water flow after the pump, so the surface strength is a fraction of the tank strength. How small a fraction depends on the injector draw rate, which changes with GPM, hose length, and nozzle. The only honest way to know it is a draw test.
The draw test, step by step
- Fill a marked container with water and note the level. Put the chemical hose and filter in it.
- Set up as you would to soft-wash: downstream nozzle on, soap valve open, spraying to waste.
- Run for a timed interval, 60 seconds is easy math, then read how many ounces the injector pulled.
- Draw ratio equals GPM times 128, divided by ounces per minute. A 4 GPM machine pulling 40 oz a minute draws at 512 divided by 40, about 12.8 to 1.
- Your on-surface strength is the tank strength divided by that ratio plus one. Enter your numbers above and the calculator does the division and tells you the tank strength to load.
This is the step most calculators skip. Downstream ratio is not a fixed number you can look up. It is a property of your rig, and it drifts as the injector wears. Re-test when your results change.
Questions operators ask
How do I calculate a soft-wash mix?
Work from three numbers: your sodium hypochlorite (SH) stock strength, the strength you want on the surface, and your tank size. The SH you need equals tank size times target percent divided by stock percent. The rest is water. For a 50-gallon tank at 2% from 12.5% stock, that is 8 gallons of SH and 42 gallons of water.
What is a downstream draw test and why does it matter?
Downstream injectors dilute your chemical with the machine's water flow, so the mix that lands on the surface is far weaker than what is in your tank. A draw test measures how much chemical the injector actually pulls. Run the machine drawing from a measuring jug through the injector, time it, and read the ounces. Draw ratio equals your GPM times 128, divided by the ounces drawn per minute. Without this number you are guessing at your real on-surface strength.
What SH percentage should I use on each surface?
Common working ranges are roughly 1 to 2 percent for house siding, 2 to 4 percent for concrete, 3 to 6 percent for asphalt-shingle roofs, and under 1 percent for wood. These are directional starting points, not a standard. Sources disagree, surfaces and stains vary, and too strong will strip paint, burn plants, or void a roof warranty. Confirm against your own results.
Is this calculator free?
Yes. No login, no paywall. Some field-service apps charge a monthly fee for the same math. This runs entirely in your browser and nothing you enter is sent anywhere.
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