Water Demand Calculator
Peak day and peak hour demand, with pipe and tank sizing.
Quick answer
With the default inputs: Occupancy 100 persons · Use per person 50 L/person·day · Hourly peaking factor 2.5 etc.Peak day demand 5.00 m³/d
Edit the inputs below to recalculate.Inputs
Result
Peak day demand
5.00m³/d
| Average hourly | 0.625m³/h |
| Peak hourly | 1.563m³/h |
| Peak flow | 0.434L/s |
| Calculated bore | 21.5mm |
| Suggested DN | 25mm |
| Actual velocity | 0.88m/s |
| Tank at 30% of peak day | 1.50m³ |
| Annual demand | 1825m³ |
| Per person per day | 50L |
The hourly factor is peak over average hourly flow, and the smaller the population the larger it gets — up to 4-5 below ten people, because use concentrates. This covers domestic use only, excluding fire, irrigation and unaccounted water, usually another 10-15%. Potable tanks are sized at 20-30% of the peak day; fire storage is calculated separately to code.
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Enter a target result; the other inputs stay fixed and this one is worked out.
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 |
How to work it out
- Set the demand figure (GB 50015-2019): 130-300 L per person per day for dwellings with central hot water, 100-200 without; 30-50 L per person per shift for offices; 100-150 for dormitories; 20-25 per meal for canteens. Base it on actual occupancy, not on beds or desks.
- Peak-day volume Qd = occupancy × demand ÷ 1000, in m³/day.
- Average hourly Qh = Qd ÷ operating hours; peak hourly Qh,max = Qh × hourly peaking factor Kh (2.0-2.5 for dwellings, 1.5-2.0 for offices).
- Peak flow q = Qh,max × 1000 ÷ 3600, in L/s. Two unit conversions happen here, which is where orders of magnitude go wrong.
- Bore d = √(4q ÷ (π × v)) with v at 1.0-1.5 m/s for domestic supply, at the low end for small bores.
- Tank volume V = Qd × 20-30%. Where the pumps cannot run continuously or mains pressure swings widely, size on the peak shift instead of defaulting to 20%.
Where it goes wrong
- Sizing the pipe on average hourly flow. At the morning peak every tap runs at once, and the instantaneous flow is 2.5 times the average — a pipe sized on the average collapses in pressure.
- Basing occupancy on beds or workstations. Offices typically run at 80% of desk count, while outpatient clinics and schools run over. Judge by how the building is actually used.
- Making the tank hold a full peak day. The tank then takes half the roof, and water resident for more than 48 hours loses chlorine residual and degrades in quality.
- Sharing one tank between domestic supply and fire reserve. Fire pumps draw the domestic reserve down with them; the codes require the two to be effectively separated.
Material and cost
Stainless domestic tanks cost 1000-1800 CNY/m³ including insulation and base. Variable-speed booster sets run 30,000-80,000 CNY for 10-30 m³/h. Upsizing the pipe one step adds 30-50% to PPR material, which is trivial next to the cost of complaints and rework over poor pressure.
On site
- The service pipe into a dwelling is the most overlooked item. A 100 m² three-bedroom flat peaks around 0.2 L/s, so DN20 is nominally enough — but with two showers and a washing machine filling, DN20 drops noticeably. Specify DN25.
- Do not confuse the hourly peaking factor Kh with the daily factor Kd. Kd governs source and treatment works; Kh governs the network and pipe sizing.
- Roof tanks need level control and an overflow pipe discharging indirectly through an air gap. Connecting the overflow straight to a drain is a contamination risk.
- When an old estate has poor pressure, check the service pipe size, meter bore and whether valves are half shut before adding a pump. Local blockage explains a surprising number of cases.
Basis
- GB 50015-2019 Standard for design of building water supply and drainage, water supply and demand figures
- GB 50555-2010 Standard for water conservation design of civil buildings
- GB 5749-2022 Standards for drinking water quality
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Water Demand Calculator. 2026-10-25. https://jianzhujisuan.com/en/water-demand
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.