Sewage Flow Estimate Calculator

Work out sewage flow estimate instantly with clear inputs, formula shown and shareable results.

About Sewage Flow Estimate Calculator

Sewage flow is estimated as water supply times a return factor of 70-85%, the balance being consumptive use and garden watering. Infiltration through joints and inflow from illicit connections adds 10-30%. Sewers and pumping stations are sized on peak flow using a population-dependent peak factor.

Formulas

Sewage flow

Dry weather flow = population x supply x return factor / 1000

Peak flow

Peak = average x (1 + 14 / (4 + sqrt(population/1000)))

Worked example

With the default values below, Sewage Flow Estimate Calculator returns Average Sewage Flow of 6,210.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Populationminimum 0: 50,000
Water Supply Per Capitaminimum 0 litres/person/day: 135 litres/person/day
Sewage Return Factoraccepts 0–100 %: 80 %
Infiltration And Inflowaccepts 0–100 %: 15 %

Results

Average Sewage Flow: 6,210.00
Dry Weather Flow: 5,400.00
Peak Flow: 14,062.90
Peak Flow In Litres Per Second: 162.765

Step by step

1Values used
2Sewage flow
3Peak flow
4Average Sewage Flow= 6,210.00
5Dry Weather Flow= 5,400.00
6Peak Flow= 14,062.90
7Peak Flow In Litres Per Second= 162.765

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 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

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

Frequently Asked Questions

Why does infiltration matter?

It dilutes the sewage, reducing treatment efficiency, and consumes hydraulic capacity that could serve real growth. In old networks it can double dry weather flow in wet periods.

Is the return factor the same everywhere?

No. Arid regions with heavy garden irrigation return as little as 60%, while dense urban areas with little outdoor use can exceed 90%. Local flow monitoring is far better than a generic factor.