Enter stock SH%, target mix%, square footage, and surface type — get exact gallons of SH, water, surfactant oz, and chemical cost. C1×V1=C2×V2 formula with substrate compatibility warnings.
Laminated-format 2-page field reference: SH dilution table (stock 6/10/12.5% × target 0.5/1/2/3%), surface coverage rates, surfactant dosing, batch size examples, mixing order, PPE checklist, and chemical safety warnings.
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The core formula for chemical dilution is C1×V1 = C2×V2 — where C1 is your stock concentration, V1 is the volume of stock SH needed, C2 is your target application concentration, and V2 is the total mixed volume. Every reputable soft-wash operator should be able to run this calculation in their head or on a reference card before any job.
Here is the practical version: to determine how many gallons of stock SH to add, divide your target percentage by your stock percentage, then multiply by your total batch size. The remainder is water. For example: to make 25 gallons at 1% using 12.5% stock, you need (1/12.5) × 25 = 2 gallons of SH stock and 23 gallons of water.
Sodium hypochlorite degrades over time — approximately 1% per month stored at ambient temperatures. The concentration on the label is the value at manufacture; what you have in the tank by week 3 may be 10–15% lower. This matters for applications where concentration is critical (roof soft-wash, heavy organic growth). Buy fresh SH, store it cool and dark, and use it within 30–60 days of delivery. The three concentrations this calculator uses reflect what's commercially available: 6% (retail bleach, not efficient for commercial use), 10% (pool shock, widely available), and 12.5% (bulk commercial supply, best cost per unit of active chlorine).
Industry practice and SDS data support the following general target ranges. These are at-application concentrations — what's hitting the substrate, not what's in your tank:
Roof soft-wash is handled differently — typical pre-diluted application is 1–2% at the nozzle for gloeocapsa magma (black streak algae). For heavy infestations, operators sometimes apply 3–6% neat. If you are targeting asphalt shingle roofs at concentrations above 3%, consult the dwell-time calculator to ensure safe exposure windows.
Coverage rate — how many gallons of mixed solution you use per square foot — drives total batch size. The rates in this calculator (house wash 0.04 gal/sqft, roof 0.06 gal/sqft, concrete 0.05 gal/sqft, fence 0.035 gal/sqft, commercial 0.045 gal/sqft) are typical for downstream injection soft-wash application. These are not exact — surface condition, application method, and equipment flow rate all affect actual consumption. Use these as planning estimates; track your actual consumption per job to dial in your numbers over time.
Surfactant is not optional on soft-wash applications where dwell time matters. A quality wetting-class surfactant (Elemonator, Apple Sauce, or equivalent) does two things: it reduces surface tension so the SH solution makes full contact with the substrate instead of beading off, and it slows evaporation so the active chemistry stays on the surface long enough to do its work. Without surfactant on a hot-day house wash, your SH can flash off a vinyl siding substrate in 2–3 minutes — before the chemistry has killed the organic growth.
Typical dosing rates are 0.8–1 oz of surfactant per gallon of mixed solution. Add surfactant after the SH has been diluted into water — not to the concentrated SH — to avoid foaming and chemical interaction during mixing.
The downloadable Wallet Card PDF compresses the SH dilution table into a laminated-format 2-page reference: oz of SH stock per gallon of final mix across stock concentrations of 6%, 10%, and 12.5% and target concentrations of 0.5%, 1%, 2%, and 3%. The back page covers batch size examples, mixing order, PPE checklist, and chemical safety warnings for the most common SH-related hazards.
For the full upstream question — how long to leave the SH mix on any given substrate — use the Dwell-Time Calculator. This calculator handles how to mix it; the dwell-time calculator handles how long to leave it on. Use both on every soft-wash job.
For operators running full OSHA-compliant chemical programs, the Chemical Safety Deep Dive ($24) provides full SDS binder templates, PPE log by chemical class, mixing hazard matrix, and NPDES chemical flagging for stormwater compliance. The wallet card is the field reference; the Deep Dive is the documentation behind it.