Chlorine Residual Calculator

Work out chlorine residual instantly with clear inputs, formula shown and shareable results.

About Chlorine Residual Calculator

Chlorine decays approximately first order: C = C0 x e^(-kt). Bulk and wall decay coefficients depend on organic content, temperature and pipe condition, ranging from 0.02 per hour in clean cold water to over 0.5 per hour in warm water with tuberculated mains. Long travel times need booster chlorination rather than a higher initial dose.

Formulas

First-order decay

C = C0 x e^(-k t)

Source dose

Required source residual = minimum residual x e^(k t)

Worked example

With the default values below, Chlorine Residual Calculator returns Residual At The Point Of Interest of 0.1984. Change any input above and every figure here is recalculated in your browser.

Inputs used

Initial Chlorine Residualminimum 0 mg/L: 1.2 mg/L
First-Order Decay Coefficientminimum 0 per hour: 0.15 per hour
Water Travel Timeminimum 0 hours: 12 hours
Minimum Required Residualminimum 0 mg/L: 0.2 mg/L

Results

Residual At The Point Of Interest: 0.1984
Maximum Travel Time Before Failure: 11.945
Residual Needed At Source: 1.2099
Compliance Status: Below minimum - boost chlorination needed

Step by step

1Values used
2First-order decay
3Source dose
4Residual At The Point Of Interest= 0.1984
5Maximum Travel Time Before Failure= 11.945
6Residual Needed At Source= 1.2099
7Compliance Status= Below minimum - boost chlorination needed

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Maintaining a disinfectant residual is a regulatory requirement in most distribution systems. Verify residuals by field measurement at representative sampling points.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Maintaining a disinfectant residual is a regulatory requirement in most distribution systems. Verify residuals by field measurement at representative sampling points.

Frequently Asked Questions

Why not simply raise the source dose?

It creates high residuals and disinfection by-products near the works, causing taste and odour complaints and regulatory exceedances, while still failing at the extremities. Booster dosing is the correct answer.

How is the decay coefficient measured?

By bottle tests for bulk decay and by comparing modelled with measured residuals in the field to calibrate the wall decay term. It must be re-measured seasonally because temperature dominates.