Governing Formula
Water demand is built from the average daily consumption of a population and then scaled to the peak instantaneous rate that the supply, pumps, and storage must actually deliver. An optional fire demand is compared against the domestic peak; the governing (larger) value drives the design.
Q_avg = P × q / 1000 · Q_design = Q_avg × PF / 24 Where:
-
P= Population or number of service units [people] -
q= Per-capita water demand [L/capita/day] -
PF= Peak factor (ratio of peak to average) [—] -
Q_avg= Average daily demand [m³/day] -
Q_peak= Peak instantaneous demand [L/s] -
Q_design= Design flow for supply/storage sizing [m³/h] -
Q_fire= Fire flow demand [L/min] -
t_fire= Fire duration [h]
Derived Equations:
Q_avg = P × q / 1000 Q_peak = Q_avg × PF / 86.4 Q_design = Q_avg × PF / 24 V_fire = (Q_fire / 1000) × 60 × t_fire How the Calculation Works
The average daily demand is population × per-capita consumption / 1000 to convert litre-days into cubic metres per day. Dividing daily volume by 86,400 seconds gives the average flow in L/s; multiplying by the peak factor gives the peak instantaneous demand.
Q_avg (L/s) = Q_avg (m³/day) / 86.4
When fire flow is provided, the engine computes the fire demand as Q_fire / 60 (L/s) and the fire storage as (Q_fire/1000) × 60 × t_fire (m³). The governing flow is the larger of the domestic peak and the fire demand, which is the instantaneous basis for mains and pumping design while the fire storage term informs tank sizing.
Worked Engineering Example
Design Scenario: Community Supply, 1,000 People @ 150 L/cap/day, PF 1.5
- Average daily demand:
Q_avg = 1000 × 150 / 1000 = 150 m³/day - Average flow rate:
Q_avg = 150 / 86.4 = 1.74 L/s - Peak instantaneous flow:
Q_peak = 150 × 1.5 / 86.4 = 2.60 L/s - Design flow:
Q_design = 150 × 1.5 / 24 = 9.38 m³/h - Fire-flow check (2,500 L/min for 2 h):
Q_fire = 2500 / 60 = 41.67 L/s > 2.60 L/s → fire governs - Fire storage basis:
V_fire = (2500/1000) × 60 × 2 = 300 m³ of dedicated or combined storage
Engineering Notes & Design Benchmarks
| Application | Per-Capita Demand (L/cap/day) | Peak Factor |
|---|---|---|
| Basic rural / WHO basic access | 20 – 50 | 1.5 – 2.0 |
| Municipal residential (metered) | 100 – 180 | 1.3 – 1.8 |
| Commercial / institutional mix | 150 – 250 | 1.5 – 2.0 |
WHO guidance cites basic domestic needs around 20–50 L/cap/day while fully lapped and piped municipal service commonly reaches 100–250 L/cap/day. Fire-flow rules of thumb follow each jurisdiction’s code — where code fire flow exceeds the domestic peak, fire demand becomes the governing instantaneous sizing criterion.
Assumptions & Limitations
- Demand is treated as a steady annual average scaled by a single peak factor; diurnal and seasonal patterns vary by community.
- Per-capita rates are estimates — actual metered data, non-revenue water, and system leakage should override the default ranges.
- Fire-flow values must be taken from the applicable code (e.g., NFPA-based requirements for the occupancy and hazard class); the calculator never substitutes for a fire-flow study.
- Storage sizing from fire demand assumes the full fire duration is borne by storage/recharge capacity and ignores simultaneous domestic draw.
Frequently Asked Questions
Why does the governing flow switch to the fire demand?
Because instantaneous fire flow is usually far larger than the domestic peak. The engine reports the larger of the two as the governing flow so that mains and pump capacity are checked against the true worst-case instant, while domestic peak still drives normal operation.
How do I choose a peak factor?
Use metered or code guidance for the community type: 1.3–1.8 is common for metered municipal supply, rising to 2.0+ for unmetered or seasonal communities. The peak factor must be at least 1.0 (average consumption), which the engine enforces.
Is 150 L/cap/day realistic?
For fully served piped municipal systems yes — design allowances commonly sit between 100 and 250 L/cap/day depending on climate, metering, and unaccounted-for water. WHO basic-access guidance is far lower (20–50 L/cap/day), so match the figure to the actual service level.
Engineering Disclaimer
Engineering Note: This calculator provides demand estimates for preliminary planning and education. Final supply, pumping, and storage design must follow the applicable regional code, verified flow data, fire-flow requirements, and qualified engineering review.
Technical References
- World Health Organization, Guidelines for Drinking-water Quality, 4th ed., 2017 (domestic water quantity guidance).
- ISO 4064 (Water meters for cold potable water and hot water) — metered consumption basis.
- NFPA 1, Fire Code / NFPA 1142, Standard on Water Supplies for Suburban and Rural Fire Fighting.
- Mays, L.W., Water Resources Engineering, 2nd ed., Wiley (demand forecasting and municipal water systems).