Breakpoint Chlorination Calculator

Breakpoint chlorination is about destroying combined chlorine (chloramines), the source of the "chlorine smell," red eyes, and dull water, not just topping up free chlorine. Combined chlorine is what's left over:

combined chlorine (CC) = total chlorine (TC) − free chlorine (FC)
                         (free + combined = total available chlorine)

target free chlorine = multiplier × CC
  standard 10×   (the well-corroborated breakpoint rule)
  heavy    20×   (advisory, after a storm)
  extreme  30×   (advisory, heavy contamination; a plain multiplier)

FC to add (Δppm) = max(0, target FC − current FC)

dose = Δppm × (volume / 10,000) ÷ (ppm-per-unit factor for your product)
  cal-hypo 65%   7.7 ppm FC per lb per 10,000 gal   (cited)
  dichlor 56%    6.6 ppm FC per lb per 10,000 gal   (cited)
  6% bleach      6.2 ppm FC per gal per 10,000 gal  (cited)
  cal-hypo 73% 8.65 · liquid 12.5% 12.9 · sodium hypo 10% 10.3  (mass-balance derived)

Why the full dose matters. Under-dosing below breakpoint increases di- and trichloramine and makes the smell worse, so the tool targets the full breakpoint in one addition and warns against a partial dose.

10× is conservative, not exact. Bench chemists (Orenda / Richard Falk) note real chlorine demand is often lower than 10× CC (closer to ~0.5–3× CC). We ship the well-corroborated 10× default because it reliably clears the breakpoint; treat the number as a starting dose and re-test with your kit. Combined chlorine up to about 0.2 ppm is generally acceptable; operators typically act at 0.3–0.5 ppm.

Safety + stabilizer notes. Never mix chlorine products, and always add product to water, not water to product. Repeated dichlor shocking raises cyanuric acid (CYA): watch your stabilizer and check the FC/CYA ratio before you over-shock. Always defer to the bag or jug label.

Sources: PoolDial, Breakpoint Chlorination Calculator (the 10× target + product picker); In The Swim, "Breakpoint Chlorination" eGuide (the 10× rule, the under-dosing warning, and the per-product FC factors); Orenda Technologies, "Breakpoint Chlorination Explained" (the FC + CC = TAC identity, the conservatism caveat, and the ~0.2 ppm acceptable-CC maximum). The derived 73% cal-hypo / 12.5% / 10% liquid factors scale the cited same-family anchor by available-chlorine ratio. CDC, Fecal Incident Response Recommendations for Aquatic Staff (released June 22, 2018; retrieved 2026-10-17), is linked as the pointer for incident response only: it defines the CT inactivation value as free chlorine in ppm multiplied by time in minutes at a specific pH and temperature, and this calculator does not apply it.