Water Demand Calculator: engineer's guide
Peak water demand converts fixture units to design flow via the empirical Hunter curve. It sets pipe size and tank draw rate.
Open the Water Demand calculator →
What this calculator does
The calculator sums fixture units by fixture type, applies the Hunter conversion, and reports probable peak demand in GPM or L/s.
Formulas used
- Total FU:
FUtotal = Σ Ni × FUi - Peak flow (Hunter):
Qpeak = f(FUtotal) from Hunter curve
Variables
- FUi = fixture unit value per IPC / UPC / BS 6700
- Ni = count of each fixture
Worked example
Scenario: 30-unit residential building: 30 WCs, 30 basins, 30 showers, 15 kitchen sinks, 15 washing machines.
- Total FU: 30×3 + 30×1 + 30×2 + 15×2 + 15×3 ≈ 255 FU
- From Hunter curve at 255 FU: peak ≈ 120 GPM
- Size main service pipe for 120 GPM at 5 fps → 3 in
- Storage tank at 3 hours of peak = 21,600 gallons
Common mistakes to avoid
- Adding fixture demands instead of using Hunter (over-sizes badly)
- Missing outdoor irrigation demand
- Wrong FU table for the applicable code
- Ignoring simultaneous non-fixture demand (cooling towers, boilers)
Frequently asked questions
IPC vs UPC vs BS?
IPC used in most GCC and US commercial, UPC used in western US and some GCC, BS 6700 in UK/Europe. Values differ slightly.
Domestic hot water separate?
Yes — run a separate FU total for hot demand, typically 60% of cold.
Standards and references
IPC, UPC, BS 6700, ASHRAE Applications Ch. 50
Related tools on this site
Open the calculator that pairs with this guide: Water Demand Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.