Sodium Hypochlorite Dilution and Strength Reference

Why this matters

Every dilution error runs in one of two directions, and both cost money. Too weak and the algae comes back in weeks, the customer calls, and the shop either eats a redo or loses the account. Too strong and the mix damages what it touched, stripped wax, yellowed shrubs, cooked shingle granules on a roof the shop just warrantied. The math itself is simple. What actually goes wrong is a ratio remembered from a different job, a different supplier's concentrate, or a different target percentage, carried over without being recalculated.

Reading source strength off the product, not off memory

Available-chlorine percentage varies by manufacturer, product line, and lot, and it must be read off the label or SDS in hand every time, not assumed from the last jug. Professional-grade sodium hypochlorite (SH) for softwash work commonly runs in the range of 10 to 12.5 percent available chlorine, while retail bleach runs lower, typically 5.25 to 8.25 percent. Two containers both labeled "12.5 percent" from different manufacturers are close enough for field work, but a 10 percent product used with a ratio built for 12.5 percent will land meaningfully under target, not close enough to ignore.

Two reading traps are worth knowing on top of that. First, the label's percentage is almost always percent by weight, not by volume; field mixing in this reference treats it as a volume ratio for practical measuring purposes, the accepted trade convention and close enough for softwash work, though it is not a laboratory-grade equivalence. Second, a ratio written as "1:5.25" below means one part concentrate to 5.25 parts water, not one part concentrate to 5.25 parts of the finished total; confusing the two understates the concentrate needed by roughly one part in the total, a real difference at the strengths this trade works with.

The dilution formula

The relationship is C1V1 = C2V2: source concentration times the volume of concentrate used equals target concentration times the total volume of finished mix. Rearranged to solve for what you actually need to measure, concentrate volume:

V1 (concentrate) = (target percent / source percent) x total volume desired

Everything else in this reference is that one formula applied to specific numbers.

Target ranges by job type

Roof soft-wash mixes run weaker than siding mixes, commonly in the neighborhood of 1 percent available chlorine at the surface, because shingle granules and roof hardware tolerate less than vinyl or stucco does. Siding and general house-wash mixes start well below the roof figure, not above it: the shop standard for mild to moderate growth, the condition most jobs actually present, is 0.6 to 0.8 percent, the same starting ratio the soft wash house procedure standard runs on an ordinary visit. Reserve the 1.5 to 2 percent range for genuinely heavy soil load or growth that has gone multiple seasons without service, and treat it as the exception, not the routine target. Heavier soil load still on a hard, non-porous surface can run higher yet, approaching but never exceeding the product label's stated maximum. Every one of these figures is a starting point, not a ceiling: the binding number is always the specific product's label and, for roofing, the shingle manufacturer's soft-wash allowance, both of which vary by manufacturer and edition. Confirm the actual ceiling before mixing above the low end of these ranges.

Strength and dwell time trade off against each other. A stronger mix works faster and leaves less margin for a dwell-time mistake; a weaker mix needs more time on the surface but tolerates a longer dwell without doing damage. This is part of why a roof mix runs both weaker and typically dwells longer than even a standard siding mix: a 1 percent mix left on shingles for its full recommended dwell does the job without pushing toward the granule-damage range, while the same shingles hit with a heavy-growth 2 percent siding mix would need a much shorter, harder-to-judge dwell to stay safe. When a job seems to be moving slowly, the correct fix is almost always more dwell time or a repeat application at the same target percentage, not a jump to a stronger mix to save time. A stronger mix bought to save ten minutes of dwell is the direction that shows up as roof or plant damage on the next visit.

Dilution ratio table (12.5 percent source)

This table is only valid for a 12.5 percent source. A 10 percent source needs its own ratio: the same 2 percent target from a 10 percent source works out to a 1:4 ratio, 25.6 ounces per gallon, noticeably more concentrate per gallon than the 12.5 percent table shows. Recompute from the formula whenever the source strength changes; do not interpolate between this table's rows for a different source percentage.

Concentration decay during storage

Available chlorine degrades over time, faster with heat and light exposure, so a jug that's been sitting since last season is weaker than its label states even though the label hasn't changed. The dilution math in this reference assumes fresh, full-strength concentrate; see the chemical storage, shelf life, and transport reference for the decay rate and how to account for an aged container rather than trusting the printed percentage on an old jug.

Adjusting for a different source strength mid-season

A supplier switch is the most common real-world trigger for a dilution error, because the ounce-per-gallon number that worked all season stops applying the moment the source percentage changes, even if the target percentage on the job doesn't. If a shop moves from a 12.5 percent supplier to a 10 percent supplier for the same 2 percent siding target, the ounce-per-gallon figure moves from 20.48 to 25.6, a real difference, not a rounding error. This is exactly the scenario the SDS review and product substitution standard exists to catch before it reaches a mixing tank: any new source product gets its own dilution math recalculated and written down, never carried over from the prior supplier's numbers.

Worked example: 50 gallons of a 2 percent siding mix from a 12.5 percent source

This example deliberately uses the heavy-growth end of the range from the table above, 2 percent, rather than the 0.6 to 0.8 percent standard, because the larger numbers make the underlying arithmetic easier to follow; the formula and method apply identically at any target percentage.

The job calls for 50 gallons of finished 2 percent mix. Using V1 = (target / source) x total volume:

V1 = (2 / 12.5) x 50 = 0.16 x 50 = 8 gallons of concentrate

Water needed = 50 total gallons minus 8 gallons concentrate = 42 gallons.

Reasoning: the 0.16 factor is the fraction of the finished mix that must be concentrate to hit 2 percent from a 12.5 percent source, the same ratio the table above expresses as 1:5.25. Multiplying that fraction by the total volume desired gives the actual gallons to measure, rather than working from a memorized ounces-per-gallon figure that assumes a specific batch size.

What changes the answer: a lower source strength requires more concentrate volume for the same target and total volume, as the mid-season example above shows. A higher target percentage scales the concentrate volume up proportionally, for example a 3 percent target on the same 50-gallon batch would need 12 gallons of concentrate rather than 8. A smaller or larger total volume scales linearly from the same 0.16 fraction.

The failure mode: a tech who remembers "about a quart per five gallons" from a prior job, without knowing which source strength or target percentage that ratio was built for, applies it to a different combination of both and gets a mix that is off in a direction nobody checks until the wash either fails to hold or damages something.

A second pass: catching a wrong source strength before it reaches the roof. A roof job calls for 20 gallons of a 1 percent mix. Using the formula, V1 = (1 / 12.5) x 20 = 1.6 gallons of concentrate, with 18.4 gallons of water. The tech measures 1.6 gallons from the jug on the truck and mixes as planned.

The test strip reads well above the expected band for 1 percent, closer to what a 2 percent mix would show. Rather than trusting math that checked out on paper, the tech re-reads the jug's label and finds it isn't the usual 12.5 percent container: it was topped off earlier that week from a 20 percent industrial-strength drum kept for a commercial account, and the jug's own label never got updated. The arithmetic was correct for the wrong source percentage. The batch is discarded and remixed from a confirmed 12.5 percent container, and the 20 percent drum gets moved to a clearly separate, distinctly labeled storage spot so it stops looking like the everyday jug; see the chemical storage, shelf life, and transport reference for keeping distinct-strength containers from being mistaken for each other.

How to verify you got this right

Confirm a chlorine test strip reads within the expected band for the target percentage once the batch is mixed, not just that the measured volumes matched the math. Confirm the total volume mixed matches the batch ticket's stated total, concentrate plus water, since a mismatch there usually means a measuring error upstream of the math being fine on paper. And re-read the source percentage off the actual container used, not the number written on the batch ticket from memory, in case a second container from a different lot got grabbed partway through mixing.

References

• Product SDS and label, Section 3 (composition) or the printed available-chlorine percentage, for the specific concentrate and lot in hand
• ARMA (Asphalt Roofing Manufacturers Association) soft-wash guidance and the shingle manufacturer's own documentation, edition-specific, for roof dilution ceilings
• See related: sodium hypochlorite mix and PPE standard, chemical storage shelf life and transport, SDS review and product substitution standard