Peak Water Demand Factor Calculator

Work out peak water demand factor instantly with clear inputs, formula shown and shareable results.

About Peak Water Demand Factor Calculator

Distribution networks are sized on peak hour flow, which is average demand multiplied by a maximum day factor of 1.3-1.8 and a peak hour factor of 1.5-3.0. Small communities show much sharper peaks; Harmon's formula, 1 + 14/(4 + sqrt(population in thousands)), captures that population-dependent damping.

Formulas

Peak flows

Maximum day = average x max day factor; peak hour = average x max day factor x peak hour factor

Harmon formula

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

Worked example

With the default values below, Peak Water Demand Factor Calculator returns Peak Hour Flow of 31,875.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Average Daily Demandminimum 0 m3/day: 8,500 m3/day
Population Servedminimum 1: 50,000
Maximum Day Factorminimum 1: 1.5
Peak Hour Factorminimum 1: 2.5

Results

Peak Hour Flow: 31,875.00
Maximum Day Flow: 12,750.00
Peak Hour Flow In Litres Per Second: 368.924
Harmon Peak Factor For This Population: 2.2646

Step by step

1Values used
2Peak flows
3Harmon formula
4Peak Hour Flow= 31,875.00
5Maximum Day Flow= 12,750.00
6Peak Hour Flow In Litres Per Second= 368.924
7Harmon Peak Factor For This Population= 2.2646

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.

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

Frequently Asked Questions

Which flow sizes which asset?

Source and treatment are sized on maximum day, distribution mains and service reservoirs on peak hour, and pumps often on maximum day with storage absorbing the hourly peak.

Why do small systems peak more sharply?

Fewer users means less statistical smoothing of individual demand. A village of 500 can see peak hour flows four or five times average, while a city of a million rarely exceeds twice.