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.

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 — your number always wins.