Water Demand Calculator
Peak day and peak hour demand, with pipe and tank sizing.
Inputs
Result
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% |
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.
Reference water use by building type
| Building type | Unit | Allowance | Hourly factor |
|---|---|---|---|
| Dwelling with shower | L/person·day | 130-200 | 2.0-2.5 |
| Dwelling without shower | L/person·day | 80-120 | 2.0-2.5 |
| Office | L/person·shift | 30-50 | 1.5-2.0 |
| Dormitory | L/person·day | 80-100 | 2.5-3.0 |
| Hotel room | L/bed·day | 250-400 | 2.0-2.5 |
| Restaurant | L/person·meal | 15-20 | 1.5-2.0 |
| Teaching building | L/person·day | 30-50 | 1.5-2.0 |
| Hospital ward | L/bed·day | 400-600 | 2.5-3.0 |
FAQ
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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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.