Water Demand Calculator
Estimate the daily and peak water requirements for a building or community with two methods: population-based (litres or gallons per person per day) and fixture-unit-based (IPC 2018 Hunter curve). Choose metric or imperial, add irrigation, a leakage allowance, and fire reserve, then size the storage tank. Results update as you type.
Formula
Worked example
A 50-person medium-standard residential building (170 L/p/d): domestic demand = 50 x 170 = 8,500 L/day. With 10% leakage: 8,500 x 1.10 = 9,350 L/day. Peak factor 2.5: peak flow = (8,500 / 1440) x 2.5 = 14.8 L/min. One day of storage plus a 200 L fire reserve: tank = 9,350 + 200 = 9,550 L.
What is water demand and why does it matter?
Water demand is the total volume of potable water that a building, development, or community requires over a given time period, usually expressed as litres per day (L/day) or gallons per day (gal/day). Accurate demand estimation drives every downstream decision in water supply design: pipe diameters, pump capacities, tank sizes, and service-connection pressure classes. Undersizing a system causes pressure drops and service interruptions; oversizing adds unnecessary capital cost and can increase stagnation risk in low-velocity mains. Engineers distinguish between average daily demand, which governs storage and source capacity, and peak demand, which governs the instantaneous supply infrastructure.
Population-based vs. fixture-unit method
Two main methods are used in practice. The population-based method multiplies the number of occupants by a per-capita consumption rate (LPCD in metric, or GPCD in imperial). It is fast, requires minimal data, and is well-suited to community-scale planning, early feasibility work, and situations where the final plumbing layout is not yet known. The fixture-unit method, standardised in the International Plumbing Code (IPC 2018) and originally developed by Dr. Roy B. Hunter in 1940, assigns each fixture a fixture-unit (FU) value that reflects its water demand weighted by its probability of simultaneous use. The total FU count is converted to a peak flow rate in GPM via the Hunter demand curve, which is a logarithmic relationship that accounts for the statistical unlikelihood that every fixture runs at the same time. This method is preferred for detailed plumbing design because it directly informs pipe sizing without the uncertainty of translating a daily volume into a momentary flow rate.
Peak demand, leakage, and system components
Peak demand is the highest flow rate the system must deliver, typically occurring in the morning or early evening. For the population method, a peak factor (commonly 2.0-3.0 for residential) is applied to the average hourly flow. For the fixture method, the Hunter curve inherently produces a peak flow. Leakage allowance accounts for water lost through minor pipe leaks, valve drips, and meter inaccuracies; the World Health Organization recommends budgeting 10-15% for well-maintained distribution systems and up to 25% for aging infrastructure. Irrigation demand is calculated separately as area times application rate, as landscape watering is a distinct demand profile that may not coincide with peak indoor use. Fire-fighting reserve is an emergency volume sized to the required flow rate and duration specified by the local fire authority, typically 150-300 GPM for 1-2 hours for residential occupancies.
Storage tank sizing and water conservation
Storage tank (cistern or reservoir) capacity is sized to buffer supply interruptions, cover fire reserve, and flatten pump cycling. Residential systems commonly hold one day of average demand plus any fire reserve. Hospitals, hotels, and critical facilities may require three to seven days. Switching to water-efficient (WaterSense or equivalent) fixtures - low-flow toilets, aerated faucets, and water-efficient showerheads - typically reduces indoor consumption by 20-30%. This calculator applies a 25% reduction to model an all-efficient scenario, helping you compare standard and efficient designs side by side. Peak demand is also reduced, which can allow smaller pipe diameters and pump selection, delivering capital savings as well as operational ones.
Standard water consumption rates by occupancy type
| Occupancy type | L/person/day | gal/person/day | Notes |
|---|---|---|---|
| Residential - basic | 135 | 36 | WHO minimum; developing world standard |
| Residential - medium | 170 | 45 | Typical suburban household (IPC/UPC) |
| Residential - high standard | 220 | 58 | High-use homes, multiple bathrooms |
| Office / commercial | 45 | 12 | Employees, daytime use only |
| School | 45 | 12 | Students; double for boarding schools |
| Hospital | 400 | 106 | Includes laundry, sterilization |
| Hotel | 250 | 66 | Per guest; includes all amenities |
| Restaurant | 70 | 18 | Per seat per meal service |
Typical per-capita daily consumption rates from international plumbing and water supply standards. Use these as a starting point; local authorities may require different values.
Frequently asked questions
What is the difference between average daily demand and peak demand?
Average daily demand is the total volume used over 24 hours divided by 24. Peak demand is the highest instantaneous or hourly flow rate that occurs during that day. Storage tanks, treatment plants, and source connections are sized on average daily demand, while pipes, pumps, pressure regulators, and service connections are sized on peak demand. The ratio between them is called the peak factor, typically 2.0-3.0 for residential buildings.
How many fixture units does a typical house have?
A typical three-bedroom single-family home might have two tank-type toilets (2 x 3 = 6 FU), two lavatories (2 x 1 = 2 FU), two showers (2 x 2 = 4 FU), a bathtub (2 FU), a kitchen sink (2 FU), a dishwasher (2 FU), and a washing machine (3 FU), for a total of about 21 FU. The Hunter curve maps 21 FU to approximately 16-17 GPM peak demand, well within a standard 3/4-inch service connection.
What is the Hunter curve?
The Hunter curve, developed by Dr. Roy B. Hunter for the National Bureau of Standards in 1940, is a demand curve that converts a building's total fixture units into a probable peak flow rate. It is logarithmic because the probability that all fixtures run simultaneously decreases as the total number of fixtures increases. A building with 10 FU is far more likely to have all fixtures running at once than a building with 500 FU. The IPC 2018 codified the curve with three linear segments: up to 10 FU, GPM = 0.75 x FU; 10-100 FU, GPM = 10 + 0.5 x (FU - 10); and above 100 FU, GPM = 55 + 0.25 x (FU - 100).
What leakage allowance should I use?
The World Health Organization recommends 10-15% for well-maintained systems with newer infrastructure. Older systems or those in regions with limited maintenance capacity may warrant 20-25%. Some local water authorities specify a required allowance in their design codes, so always check local requirements. A 10% default is conservative enough for most modern residential and commercial designs.
How large should my storage tank be?
For a single-family home or small apartment building, one day of average daily demand is the standard minimum. Critical facilities such as hospitals and data centers typically hold three to seven days. Add the fire reserve volume on top of the daily storage requirement. In areas with intermittent municipal supply (common in parts of Asia and Africa), two to three days of storage is the practical norm even for residential buildings.