Soft Wash Mix Calculator
Every other mix calculator starts at the tank. This one starts at the surface — because the question that actually strands you on a roof is not the ratio, it's how many gallons the job takes.
Enter the square footage and a target strength. You get the gallons of mixed solution the job needs, the sodium hypochlorite–to–water split for a batch tank, the applied percentage your downstream injector can actually reach at its draw ratio, and what the chemical costs you on that job. The dilution itself is a mass balance: SH gallons = mix gallons × (target % ÷ stock %), with water making up the rest.
Soft wash mix calculator
1 · Size the job
Typical for asphalt shingle roof: 75–150 sq ft per gallon of mixed solution. Heavier growth, rougher texture, and a windy day all push it toward the low end — track your own on a few jobs and replace this number.
2 · Set the strength
Common working range for asphalt shingle roof: 2–4%. Always test a small area first, and remember SH loses strength on the shelf — a jug that sat through a hot month is not the number on the label.
3 · How you're applying it
Dosing varies enormously by product — anywhere from about 3 to 16 oz per 100 gallons. Use your label's rate, not this one. Set it to 0 if you're not adding any.
Mixed solution needed
20 gal
2,000 sq ft ÷ 100 sq ft per gal = 20 gal at 3%
What goes in the tank
- Sodium hypochlorite (12.5%)
- 4.8 gal
- Water
- 15.2 gal
- Surfactant
- 1.6 oz
Water first, then SH, then surfactant last — adding hypochlorite to a dry or nearly empty tank splashes it everywhere it doesn't belong.
- Chemical cost, this job
- $19.20
- Per 1,000 sq ft
- $9.60
Stock SH only — surfactant, fuel, and labor sit on top. Downstreaming doesn't change this figure: the same strength over the same area burns the same hypochlorite either way. It buys you mixing time, not chemical.
Every number above is downstream of one input: the square footage. Guess it and you under-mix on the roof or drive home with 40 gallons you paid for — measure the surface from satellite imagery before you load the trailer.
Defaults are industry-typical starting points, not prescriptions. Test a small area, track your own coverage over a few jobs, and replace these numbers with yours.
Three Lines of Arithmetic, One Missing Input
The dilution is not the hard part. It's the same mass balance every chemistry class opens with: to make a weaker solution from a stronger one, you take the ratio of the two strengths and that's your fraction.
What makes soft wash mixing feel harder than it is, is that the volume gets decided by feel. Contractors mix "a hundred gallons, that should do it," then either run short with a strip of roof left or drive home with chemical they paid for. Both are the same error: the batch size is a function of the area, and the area was never measured.
Coverage rate is the bridge between the two. It varies with surface, texture, organic load, and how heavy-handed you are on the gun — so treat the published range as a starting point and replace it with your own after a few jobs. Once you have it, the whole mix falls out of the square footage.
- Roof surface area
- 2,000 sq ft
- Coverage rate
- 100 sq ft / gal
- Mixed solution needed
- 20 gal
- Target strength
- 3%
- Stock SH
- 12.5%
- Sodium hypochlorite
- 20 × (3 ÷ 12.5) = 4.8 gal
- Water
- 15.2 gal
- Chemical cost at $4/gal
- ≈ $19
That's roof surface area, not building footprint. A standard-pitch roof is about 12% larger than the shape you see from above — the roof calculator applies the pitch factor for you.
Applied Strength & Coverage by Surface
These are the ranges the trade converges on, expressed as applied percentage at the surface — not as a ratio, and not as what's in the jug. Percentages are the only unit that survives a change of supplier, a change of stock strength, or a drum that's been sitting since June.
| Surface | Applied % | Sq ft / gal | Notes |
|---|---|---|---|
| Asphalt shingle roof | 2–4% | 75–150 | ARMA endorses low-pressure chemical application only. Pre-wet and rinse landscaping. |
| House siding — vinyl, stucco | 1–3% | 200–300 | Smooth siding drinks far less than a roof. Start low; oxidized vinyl streaks if you overdo it. |
| Concrete flatwork | 2–3% | 150–250 | Usually a pre-treat or post-treat around surface cleaning, not the whole job. |
| Wood deck or fence | 0.5–1% | 150–200 | Lowest strength of anything you'll wash. Wood lightens fast and the damage is permanent. |
| Painted or coated surfaces | Test first | — | No safe default exists. Find an inconspicuous patch, dwell it, rinse, and wait before committing. |
Ranges reflect common industry practice and are a starting point for your own testing — not manufacturer guidance for any specific surface, coating, or product. Follow the roofing or siding manufacturer's direction where one exists, read your chemical supplier's label, and test an inconspicuous area before committing to a whole property.
Batch Tank vs. Downstream — the Real Trade
This gets argued as a preference. It isn't — it's a ceiling. A fixed-rate downstream injector pulls one part chemical to roughly ten or more parts water, so the strongest thing it can put on a surface is your stock divided by eleven. Straight 12.5% through a 1:10 injector applies about 1.14%. That is a perfectly good house wash and nowhere near roof strength, which is the entire reason roof work gets batched.
Downstream injector
Chemical is pulled into the line after the pump, so nothing corrosive passes through it. You carry stock, not pre-mixed solution, and there's no mixing step at the truck.
- Applied % = jug % ÷ (ratio + 1)
- Fast, simple, nothing to dump at the end of the day
- Hard strength ceiling — usually around 1% from straight stock
- Real ratio drifts with hose length, nozzle wear, and elevation
Best for house washes, fences, and light concrete work.
Batch tank
You mix to a known percentage in the tank and apply it with a 12V pump or proportioner. What you mixed is what lands on the surface, so the strength is exact rather than inferred.
- SH gallons = mix gal × (target ÷ stock)
- Any strength you want, up to your stock
- Repeatable — the same batch gives the same result next week
- Costs a mixing step, and leftover mix degrades
Best for roofs and anything where the strength has to be defensible. It needs its own tank and its own lines — rig setup covers keeping chemical away from your water plumbing.
One thing that surprises people: downstreaming does not use less chemical. The same applied strength over the same area consumes the same hypochlorite either way — it's the same chemistry on the same surface. What downstreaming buys is mixing time and the convenience of carrying stock. What it costs is the ceiling. Choose it on those terms, not on a cost saving that isn't there.
Run a Draw Test Before You Trust Any Ratio
The ratio on the injector's packaging describes the injector on a bench. Yours is at the end of 200 feet of hose, behind a gun and a specific nozzle, possibly uphill from the jug. Every one of those moves the real draw, and contractors routinely spend a season blaming their chemical for an injector that's pulling half of what they assumed. The test takes four minutes.
- Fill a jug you can measureOne gallon of water, marked, with the chemical line sitting in it. Water is fine — you're measuring the draw, not washing anything.
- Put the soap tip inThe injector only engages when pressure drops. A large-orifice black soap nozzle is what opens the venturi — with a high-pressure tip in, your draw ratio is zero.
- Pull the trigger for exactly one minuteSpray to waste, at your normal working hose length. Hose length matters: the same injector draws differently through 100 feet than through 300.
- Measure what disappearedOunces drawn from the jug is your answer. Ratio = (machine GPM × 128) ÷ ounces drawn. A 5 GPM machine that pulled 55 oz in a minute is running about 1:11.6.
The formula
Draw ratio = (machine GPM × 128) ÷ ounces drawn in one minute
A 5 GPM machine puts out 640 ounces a minute. If 55 of those came from the chemical jug, you're running about 1:11.6 — not the 1:10 on the box. Feed the real number into the calculator above and the applied percentage stops being a guess. Which tip you're running matters too: the injector only opens when pressure drops, and the nozzle chart covers the orifice sizing behind that.
The Chemical Is the Cheapest Thing on the Job
Run the numbers and the conclusion is uncomfortable for anyone who prices by feel. A 2,000 square foot roof at 3% burns under five gallons of stock SH — call it twenty dollars. Under ten dollars per thousand square feet.
Which means chemical cost should almost never move your price. What moves your price is hours: setup, ladder work, dwell time, rinsing, and protecting the landscaping. A soft wash job is priced from measured area and a production rate, exactly like flatwork — the mix is a line item you barely notice.
The trap is the opposite direction. Because the chemical is cheap, it's easy to feel like a big job "didn't cost much," and to discount it. Knowing what the mix actually costs is what stops that instinct — it's a number, and it's small, and the rest of the price is labor you should be charging for.
Build the rate itself from your own costs in the pricing guide, and hold a floor on the small ones with a job minimum.
Six Ways a Mix Goes Wrong
Mixing to a ratio instead of a percentage
"Three to one" means nothing until you know what's in the jug. Three-to-one from 12.5% stock is 3.1% applied; the same ratio from a tired jug that's drifted to 8% is 2%. Percentages travel between suppliers and seasons — ratios don't.
Trusting the label on old stock
Sodium hypochlorite degrades on the shelf, faster in heat and light. A drum that sat in the sun through August is not 12.5% anymore. If your results changed and nothing else did, that's usually why.
Sizing the batch by eye
Running out at the last 400 square feet of a roof means a second mix, a second climb, and a visible line where the first batch stopped. Mixing 40 extra gallons means you paid for chemical you then have to store or dump.
Assuming the injector's rated ratio
Hose length, nozzle wear, elevation, and the injector itself all move the real draw. Contractors chase disappointing results for a season before discovering the injector they thought was 1:10 has been running 1:18.
Quoting from the tank instead of the area
Chemical is the cheapest input on the job. Pricing a roof off gallons used rather than measured square footage and a production rate is how a full day's work ends up priced like a half day's.
Skipping the pre-wet and the rinse
Plants are the most common damage claim in soft washing and the easiest to prevent. Wet everything down before you start, keep it wet, and rinse when you're finished — every time, not just on the jobs that look risky.
Soft Wash Mixing: What Operators Ask
Ratios, roof strength, downstream ceilings, and what the chemical actually costs.
It's a straight dilution: gallons of sodium hypochlorite = total mix gallons × (target strength ÷ stock strength), and the rest is water. For a 100-gallon batch at 3% from 12.5% stock SH, that's 100 × (3 ÷ 12.5) = 24 gallons of SH and 76 gallons of water. The part most contractors skip is the step before it — how many gallons of mix the job actually needs. That comes from the square footage divided by your coverage rate, which is why this calculator starts there.
Work it from area, not guesswork. A shingle roof absorbs roughly one gallon of mixed solution per 100 square feet, so a 2,000 sq ft roof takes about 20 gallons of mix. At a 3% target from 12.5% stock, that's about 4.8 gallons of SH and 15.2 gallons of water. Steeper, rougher, or heavily colonized roofs run toward 75 sq ft per gallon, so measure the roof surface — not the building footprint — before you load the trailer.
Industry-typical applied strengths are roughly 2–4% for asphalt shingle roofs, 1–3% for house siding, 2–3% for concrete flatwork, and 0.5–1% for wood decks and fences. ARMA's guidance for algae on asphalt shingles is a 50/50 mix of chlorine bleach and water applied at low pressure, which lands around 3% when you start from standard household bleach. Always test a small area first — organic load, surface porosity, dwell time, and surfactant all shift what strength you actually need.
Most fixed-rate downstream injectors land somewhere between 1:10 and 1:20 water-to-chemical in real use, and the figure printed on the box is rarely what your specific hose length, gun, and nozzle produce. The only reliable number is a draw test: with the soap tip in, run the gun for exactly one minute into a bucket, then measure how much came out of the chemical jug. Machine GPM × 128 ÷ ounces drawn gives your actual ratio.
Because the injector sets a hard ceiling. At a 1:10 draw ratio, one gallon of output is one part chemical to ten parts water, so the applied strength is your jug strength ÷ 11. Straight 12.5% stock through a 1:10 injector applies about 1.14% — fine for a house wash, well short of roof strength. To apply 3% downstream you'd need 33% in the jug, which doesn't exist. That ceiling is the whole reason roof work gets batched and applied with a 12V pump instead.
No. For a given applied strength over a given area, you burn the same hypochlorite either way — it's the same chemistry hitting the same surface. Downstreaming saves you the mixing step and lets you carry stock instead of pre-mixed solution; it doesn't save chemical. What it does cost you is the strength ceiling, which is the trade you're actually making when you choose it.
Less than most contractors assume, which is exactly why it's the wrong thing to price against. A 2,000 sq ft roof at 3% from 12.5% stock uses roughly 4.8 gallons of SH — at $4 a gallon that's about $19, or under $10 per 1,000 square feet. Surfactant, fuel, and labor sit on top. The chemical is a rounding error next to the hours, which is why your price should be built from measured square footage and a production rate rather than from what's in the tank.
Everything the Mix Depends On
The area it covers, the gear that applies it, and the price it feeds.
Mix for the Job You Measured
Trace the roof or the elevation from satellite imagery, get real square footage in under a minute, and load the trailer for the job in front of you instead of the job you guessed at. 1 free measurement, no credit card.