Soft wash mix calculator

Every soft wash mix comes from one line of chemistry: the amount of chlorine you pour in gets spread through the whole tank. Chemists write it as C1 × V1 = C2 × V2. In soft wash words:

SH gallons = batch size × target % ÷ strength of your SH

Water gallons = batch size − SH gallons

So for a 50 gallon batch at 1% from 12.5% SH: 50 × 1 ÷ 12.5 = 4 gallons of SH, and 46 gallons of water, plus the surfactant.

The same line runs backwards. If you know what is already in the tank, the strength is stock % × SH ÷ (SH + water). Pressure Tek uses the example of 1 gallon of 12.5% SH in 4 gallons of water: 12.5 × 1 ÷ 5 = 2.5%.

A ratio is just another way to say it. Parts of water to 1 part SH = stock ÷ target − 1. For 1% from 12.5%, that is 12.5 ÷ 1 − 1 = 11.5 parts water to 1 part SH. Be careful with ratios you hear from other washers: "1 to 3" means 1 part SH and 3 parts water, 4 parts in all, which from 12.5% SH is 3.1%, not 3%.

There is no single right number. Suppliers publish starting points, and the dirt, the surface and the weather decide the rest. Start at the low end and go up only if it is not cleaning.

J. Racenstein recommends about 1% on most siding and no more than 3% on asphalt shingles from 12.5% commercial bleach. Pressure Tek gives 0.5% to 2% for house washes and sensitive surfaces, 2% to 4% for concrete and stone, and 4% to 6% for roofs with heavy growth. Both warn against going stronger than the job needs. J. Racenstein says a hotter mix usually does not clean better, wastes money and can discolor paint. Pressure Tek says it risks damaging surfaces and harming nearby plants.

For shingle roofs, the Asphalt Roofing Manufacturers Association says to apply a bleach and water solution gently, leave it 15 to 20 minutes, and rinse with a gentle spray. ARMA says never to use a power washer on asphalt shingles. Check the shingle maker's own care guide too, since a roof warranty can depend on it.

A 50 gallon house wash at 1% from 12.5% SH. 50 × 1 ÷ 12.5 = 4 gallons of SH. Water is 50 − 4 = 46 gallons. At 1 oz of surfactant a gallon of mix, add 50 oz. The ratio is 1 part SH to 11.5 parts water.

A 100 gallon roof tank at 3% from 12.5% SH. 100 × 3 ÷ 12.5 = 24 gallons of SH and 76 gallons of water. The ratio is 1 part SH to about 3.2 parts water.

A 5 gallon concrete mix at 3% from 10% pool chlorine. 5 × 3 ÷ 10 = 1.5 gallons of SH and 3.5 gallons of water. Weaker SH needs more of it: with 12.5% the same mix takes 1.2 gallons.

A 4 gallon pump sprayer at 1% from 12.5% SH. 4 × 1 ÷ 12.5 = 0.32 gallons, which is 41 oz of SH, topped up to 4 gallons with water.

Checking a shortcut. A common shortcut is to divide the batch by 10 and multiply by the percent you want. For 3% in 50 gallons that gives 15 gallons of SH. With 12.5% SH, the real strength is 12.5 × 15 ÷ 50 = 3.75%. Close, but a quarter stronger than meant. The exact figure for 3% is 12 gallons. The shortcut is exact only for 10% SH.

Old SH. If your 12.5% jug tests at 10%, the 3% roof mix in 50 gallons needs 15 gallons, not 12. Mixing by the label strength would give you 2.4%.

Gallons of SH for a 50 gallon batch, by the strength you want and the strength you bought. Water is the rest of the 50 gallons. For a different tank, scale in proportion: a 100 gallon tank takes twice as much, a 5 gallon bucket a tenth.

And the common ratios, read as parts of SH to parts of water, with 12.5% SH:

J. Racenstein points out that the old 50/50 roof advice was written for laundry bleach at 5.25% to 6%. A 1 to 1 mix of 6% bleach is 3%. The same 1 to 1 with 12.5% SH is 6.25%, twice as strong.

Surfactant helps the mix cling to siding and roofs so it dwells long enough to work. The rate comes from the surfactant's own label, so use that first. As a starting point:

• Pressure Tek gives 1 oz of Elemonator per gallon of finished mix, so 5 oz in a 5 gallon batch.
• Other surfactants are dosed at other rates, so the label on the one you buy decides.

The calculator multiplies your oz per gallon by the batch size. Use only a surfactant made to be mixed with SH. Ordinary soaps and detergents can react with chlorine, and the SH label warns against mixing it with detergents.

Sodium hypochlorite breaks down over time into salt and water, and heat and sunlight speed it up. The pool industry's trade group, PHTA, puts the half-life of 12.5% SH at about 180 days stored at 77°F, and only 48 days at 95°F. A jug that sat in a hot truck or shed for a summer can be well under its label strength.

That is why the same mix can clean well in spring and poorly in August. Ways to stay on top of it:

• Buy SH in amounts you will use within a few weeks, and store it cool, shaded and capped.
• Test it. A chlorine test kit or titration kit tells you the real strength, and the EPA-accepted label for 12.5% SH says outright that solutions degrade with age and to use a test kit.
• Put the tested strength into the calculator instead of the label strength.

One more wrinkle: labels sometimes give two figures. The label on one 12.5% product lists sodium hypochlorite at 12.5% and available chlorine at 15%. Soft wash mixes are figured on the sodium hypochlorite figure, the one this calculator uses.

• Protect yourself. The 12.5% label calls SH corrosive and says to wear safety glasses and rubber gloves. Add the respirator, boots and clothing the SDS calls for, especially when spraying overhead.
• Never mix it with acids, ammonia or other cleaners. The label warns that mixing SH with ammonia, acids, detergents or organic matter releases chlorine gas, which can be fatal. Keep separate buckets, hoses and tanks for acids such as rust or brightener products.
• Plants. Soak plants and grass with water before you spray and rinse them again after. Cover delicate plants.
• Runoff. The label says SH is toxic to fish and must not be discharged into lakes, streams or public waters without a permit. Keep runoff out of storm drains, and check your city's rules for wash water.

When you price the job, the pressure washing cost calculator works out the quote, and Koira's estimates send it from your phone. If you downstream instead of batch mixing, use the downstream injector calculator.