# Water Demand Calculator

Peak day and peak hour demand, with pipe and tank sizing.

- Page: https://jianzhujisuan.com/en/water-demand
- Category: M&E (https://jianzhujisuan.com/en/c/me)
- Chinese version: https://jianzhujisuan.com/zh/water-demand.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Occupancy 100 persons, Use per person 50 L/person·day, Hourly peaking factor 2.5, Operating hours 8 h, Design velocity 1.2 m/s): **Peak day demand 5.00 m³/d**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Occupancy | number | 100 | persons |  |
| Use per person | number | 50 | L/person·day | Dwellings 130-200, offices 30-50, dormitories 80-100 |
| Hourly peaking factor | number | 2.5 | — | Dwellings 2.0-2.5, offices 1.5-2.0 |
| Operating hours | number | 8 | h |  |
| Design velocity | number | 1.2 | m/s |  |

## Formula

- **Peak day**: occupancy × per-capita ÷ 1000
- **Average hourly**: peak day ÷ operating hours
- **Peak hourly**: average hourly × Kh
- **Peak flow**: peak hourly × 1000 ÷ 3600
- **Pipe bore**: √(4q ÷ (π × v))
- **Tank volume**: peak day × 20-30%

## Worked example

100 people at 50 L each, Kh 2.5 over 8 hours: 5.00 m³/d, 0.625 m³/h average, 1.563 m³/h peak, 0.434 L/s, a 21.5 mm bore so DN25, and a tank of about 1.50 m³.

## Questions

### Why do per-capita figures vary so much by building type?

Because the behaviour differs: dwellings include bathing, laundry and cooking at 130-200 L per person, offices only drinking and toilets at 30-50, dormitories with shared showers reach 80-100, and restaurants take 15-20 L per meal. A wrong figure under-sizes the tank and starves the pressure. Check local codes and use the upper end in hot humid regions.

### How is the hourly peaking factor set?

By population and use: 2.0-2.5 for dwellings, 1.5-2.0 for offices, 2.0-2.5 for hotels, 2.5-3.0 for hospitals. Fewer people means more concentrated use and a higher factor — for one person it is effectively infinite, so small buildings are sized on instantaneous flow rather than average. That is why a villa often needs fatter pipe than an apartment.

### How is tank volume determined?

Domestic tanks take 20-30% of the peak day, or two to three times the peak hour, whichever is greater. Too small means constant pump cycling and pressure swings; too large means long stagnation, chlorine decay and poorer water quality. Any tank holding water beyond 48 hours needs disinfection or UV treatment — stagnation is the main source of secondary contamination.

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