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 — fecal incident / heavy contamination)

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.