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Soft wash mix calculator

Soft washing is arithmetic before it is anything else. Stock strength, target strength, batch size — get those three right and the mix does the work with almost no pressure. Get them wrong in either direction and you have either wasted a trip or burned somebody's shrubs.

Soft wash mix

Mix for 1% at the surface

4.0 gal SH

+ 46.0 gal water · about 1:11.5 · 150 oz surfactant

0.5-1.5% is the working band for vinyl and fibre cement.

Add bleach to water, never water to bleach. Sodium hypochlorite loses strength on the shelf, faster in heat and light — a drum bought at 12.5% may be closer to 10% by mid-season, so set the stock field to what you actually have, not what the label says.
Photograph-style soiled mixing jug and drum on the weathered half of a timber bench, a clean jug and drum on the washed half
Mix by batch
A graduated jug and a plain drum on a bench: the mix is arithmetic before it is anything else.

PSI & nozzle quick reference

0 degRed
Spot jet. Etches most surfaces.4000 PSI
15 degYellow
Stripping bare concrete.4000 PSI
25 degGreen
General concrete and walkways.3500 PSI
40 degWhite
Siding, wood, vehicles.1500 PSI
~65 degBlack
Detergent draw, low pressure.500 PSI

Stock strength is not what the label says

Sodium hypochlorite degrades on the shelf, and it degrades faster warm and faster in light. A drum delivered at 12.5% can be several points weaker by the time it is half empty in August. That is why the calculator asks for stock strength rather than assuming it: set the field to what you actually have, test with strips if you buy in bulk, and buy in quantities you will use inside a few weeks.

Target strengths this site publishes

SurfaceAt the surfaceFrom 12.5% stock
Vinyl or fibre-cement siding0.5-1.5%about 1:8 to 1:12
Asphalt shingle roof~3.0%about 1:3
Concrete post-treat~0.75%about 1:16

Bands read live 2026-09-04 from trade sources (J. Racenstein, Wash Basics, PressureTek) plus the ARMA roof method. Some trade pages quote 3-4% for a house wash; this site publishes 0.5-1.5% for siding and says so plainly — the disagreement is real and is discussed on the sodium hypochlorite guide.

Surface damage risk

Sodium hypochlorite injures eyes and skin, kills planting, and produces chlorine gas if it meets an acid product. Never mix it with anything acidic, wear eye protection and gloves every time, and read the supplier's safety data sheet for the drum you actually bought.

Add bleach to water, not water to bleach

Pouring water into concentrate splashes concentrate. Fill the tank most of the way, then add the hypochlorite, then the surfactant last so it does not foam over.

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