Chlorine Decay Calculator

Will your residual hold to the far end of the system? Enter your starting residual and decay constant to project the residual at any water age — and find out how long until it drops to your target.

Initial residual C₀ (mg/L)Residual leaving the plant or booster.

Decay constant k (1/hr)System-specific. Derive from field residuals — see notes.

Elapsed time t (hours)Water age at the point of interest.

Target residual (mg/L)Optional. Default 0.2 mg/L.

Projected Residual

Item	Value

How the calculation works

Formulas (first-order decay)
C(t) = C₀ · e^−k·t — C in mg/L, k in 1/hr, t in hr
Half-life t½ = ln(2) / k
Time to target = ln(C₀ / C_target) / k (only when 0 < C_target < C₀)

Notes & limitations

First-order chlorine decay is the standard distribution-system residual model (AWWA M20; distribution water quality literature). The decay constant k is system-specific — there is no universal textbook value.

Derive k from your own field data: measure residuals at two points with known water ages, then k = ln(C₁/C₂) / (t₂ − t₁). Re-derive seasonally — k rises in warm water.
First-order fits bulk decay well but ignores wall demand hotspots (unlined cast iron, dead ends). Field-verify before changing doses.
This projects disinfectant residual only. CT compliance at the plant is a separate calculation — see the CT calculator.

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