Soft-Wash Mix & Dilution Calculator

This uses the standard dilution mass-balance of active chlorine, the same identity the trade calls "stock % ÷ total parts = final %". It is not a proprietary recipe; here is the whole thing:

V_stock = V_batch × (target % / stock %)
V_water = V_batch − V_stock
surfactant = V_batch × dose rate
confirmed % = (stock % × V_stock) / V_batch

Worked example. Fill a 50 gal batch to 3% from 12.5% stock SH: 50 × 3/12.5 = 12 gal SH, 50 − 12 = 38 gal water, and at 0.5 oz/gal, 0.5 × 50 = 25 oz surfactant.

Stock % must exceed target %. You can only dilute down: if your on-hand strength is at or below the target, no recipe is possible. Stock sodium hypochlorite also degrades over time, so enter your drum's real on-hand % (often lower than the label) rather than a fixed value.

Downstream injector mode. A downstream injector draws mix from a container into the water stream after the pump, 1 part mix to N parts water, so what leaves the nozzle is the container strength diluted 1 + N times. Switch the mix method to the injector, enter your injector's ratio, and the batch box becomes the draw container and the target becomes the strength you want at the nozzle:

container % = nozzle % × (1 + N)
max nozzle % = stock % / (1 + N)        (straight stock in the container)
V_stock      = V_container × (container % / stock %)
V_water      = V_container − V_stock
nozzle surfactant rate = container rate / (1 + N)

Worked example, with a ratio picked for the arithmetic rather than as a rating for any injector: for 1% at the nozzle through 1 : 10, mix the container at 1 × 11 = 11%. From 12.5% stock, a 5 gal container takes 5 × 11/12.5 = 4.4 gal SH and 0.6 gal water, and the strongest that stock can deliver through 1 : 10 is 12.5 / 11 ≈ 1.14%. Ask for more than the ceiling and the tool says so instead of printing a recipe. Measure N on your own rig: over one timed run, divide the water the machine moved by the mix that left the container.

The chemical is a cost line as well as a recipe. What a house wash or a roof should be quoted at, surface by surface and with the mix included as materials, comes out of the pressure washing estimate calculator, while flatwork sold on its own is priced in the driveway cleaning price calculator.

Drums, pumps and the tank they sit in are part of what it costs to start, which the pressure washing startup cost calculator itemises. Calculators for the neighbouring exterior trades are collected on the cross-trade tools hub.

Surface presets are advisory. Typical targets: adjust to your surface and product. Not a prescription. They prefill the target field and stay fully editable, and your number always wins.