Sodium Hypochlorite Mixing Calculator
for Pressure Washers

Why the SH Ratio Is the Most Important Variable in Soft Wash

Sodium hypochlorite is the active ingredient in every soft wash mix. Get the concentration right and the chemistry does the work. Get it wrong and you're either wasting product on surfaces that won't respond, or you're damaging substrates that can't be fixed without a full repaint or replant.

Most callbacks in pressure washing trace back to chemistry — not equipment, not technique. Either the mix was too weak and the biology came back in three weeks, or it was too strong and streaked the paint or burned the landscaping. The calculator above takes that variable off the table. You enter the numbers; it does the arithmetic.

The formula is simple: (Target % ÷ Stock %) × Batch Size = SH gallons needed. But on the job, with tanks to fill and customers waiting, even simple math gets rushed. That's what this tool is for.

Common Surface Percentages: What the Trade Actually Uses

The percentages below are at-the-surface concentrations — what's actually hitting the substrate after all dilution. If you're downstream injecting, account for your injector's dilution ratio (typically 10:1) and work backward from there.

• Concrete (driveways, patios, walkways): 2–3% SH. Pre-wet, apply, dwell 5–10 minutes, then run your surface cleaner. Hot water and a surface cleaner handle the heavy lifting; SH breaks down organic growth so the machine doesn't have to work as hard.
• Wood (decks, fences, pergolas): 0.5–1% SH. Wood is the surface most damaged by over-concentration. Exceeding 1% bleaches the fibers, raises the grain, and makes it nearly impossible to get a clean stain finish. Less is more here — let the dwell time compensate, not the concentration.
• Stucco / painted surfaces: 0.75–1.25% SH. Older latex paint is porous and absorbs SH faster than you expect. Always do a test patch on a concealed area, particularly on homes with worn or faded paint. Rinse before the mix dries to avoid tidemarks.
• Roof soft wash (asphalt shingles): 3–6% SH. Higher concentration is justified here because you're killing Gloeocapsa magma — a cyanobacterium that roots into shingle granules. You need to reach it at depth. Use the full 15–20 minute dwell and pre-wet all landscaping before you start. Roof runoff at 4–6% will destroy plants.

The Role of Surfactant in Your SH Mix

SH is a contact chemical — it needs dwell time on the surface to work. Without a surfactant, your mix sheets off vertical surfaces in seconds, especially on hot days or in direct sun. Surfactant is what makes the mix stick, spread, and penetrate biofilm.

Standard dosing is 1–2 oz per gallon of total batch volume — not per gallon of SH. For a 100-gallon tank, that's 100–200 oz (roughly 1–1.5 gallons) of surfactant. Common options: Elemonator, Simple Cherry, or F-18. Add it last, after SH is already in the water, to avoid excessive foaming.

For roof work, some operators use a thicker surfactant (Simple Cherry or Krud Kutter) to increase cling time on the shingle pitch. On flat concrete, a standard Elemonator dose is fine.

Dwell Time Fundamentals

Dwell time is how long the mix sits on the surface before rinsing. It's not a courtesy — it's how the chemistry actually works. SH disrupts cell membranes in algae, mold, and mildew through oxidation. That takes time. Rinsing at the 2-minute mark because the customer's watching from the window is how you get callbacks.

Standard dwell times: House wash 5–10 minutes. Roof 15–20 minutes (sometimes two applications on heavily stained roofs). Concrete 5–10 minutes. Wood 3–5 minutes — shorter because higher dwell at even moderate concentrations can gray the wood. Stucco 5–8 minutes with a rinse before dry.

Temperature and direct sunlight accelerate evaporation and reduce effective dwell. In summer, apply in the shade of the building first, then the sun-exposed faces. If you're losing mix visibly in under 2 minutes on a hot south-facing wall, either increase surfactant or apply in the early morning.

Safety and PPE: What's Non-Negotiable

SH at working concentrations (1–6%) is corrosive to eyes and mucous membranes. At the concentrations used for roof work, it'll bleach clothing on contact and cause skin irritation with extended exposure. Minimum PPE: chemical-splash safety glasses, nitrile gloves, and clothing you don't mind ruining.

Don't mix SH with ammonia-based cleaners or acids — ever. The reaction produces chloramine gas (toxic) or chlorine gas depending on the acid. If you're running an oxalic acid wash on the same job (for wood brightening), rinse the oxalic completely before any SH touches that surface.

Why SH Degrades and What to Do About It

Sodium hypochlorite is unstable. It loses active chlorine (measured as available chlorine percentage) over time, especially when exposed to heat, sunlight, or metal contamination. SH stored in a black tank in a trailer on a 95°F summer day can lose 20–30% potency in weeks.

Symptoms of degraded SH: the mix smells weaker, results are inconsistent, and stains that should respond in 15 minutes aren't responding in 30. If you suspect degradation, test with a pool chlorine test kit before you load the trailer. Using degraded SH means longer dwell times, more product, and explanations to customers.

Best practice: rotate stock every 60–90 days, store in white or opaque containers away from direct sun, and source from a supplier who provides dated product.

From Mix Ratios to Job Pricing

Once you know your chemical volumes per job, you can calculate actual cost per square foot — which is the number that makes or breaks your margin. A 100-gallon batch at 12% SH costs you around $8–12 in chemical depending on your supplier rate. Add surfactant and you're at $15–20 per batch. If that batch covers a 2,500 sq ft house, your chemical cost is well under $0.01/sq ft. It shouldn't be eating your margin — but it will if you're over-mixing or over-applying because you're not measuring.

The SurfaceOps free estimator calculates job pricing by surface type and square footage. Pair it with this calculator and you have the full picture: what you're spending and what you should be charging. Read the complete chemical mixing guide for a deeper dive into surface-by-surface ratios.