What this calculator does
The supply side of plumbing uses the mirror image of the drainage method: instead of counting drainage units you count water supply fixture units, and the total converts to a peak demand in gallons per minute. That figure sets the service pipe size.
How to use it
- Enter the number of each fixture type; a bathroom group (WC, basin and bath) counts as 3.6 units.
- The total converts to a peak flow by interpolating the standard Hunter curve.
- Use the resulting demand to check the service pipe size against the available pressure.
- The curve assumes flush tanks; quarter-turn and pressure-assisted valves need the code appendix instead.
Governing formula
Σ WSFU = Σ (n_i × WSFU_i) ; Q = f(Σ WSFU) Where:
-
WSFU_i= Water supply fixture unit value [–] -
Q= Probable peak demand [GPM]
Worked example — Eight bathroom groups plus kitchen and laundry fixtures
The figures below were produced by running this page's own engine with the example inputs (Bathroom groups, Kitchen sinks, Dishwashers, Clothes washers, Laundry trays, Hose bibbs / yard taps). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Estimated peak demand | 31.65 GPM |
|---|---|
| Total supply load | 61.2 WSFU |
| Typical service size | 1.5 in |
| Assessment | Estimated peak demand ≈ 31.6 GPM for 61.2 WSFU (Hunter flush-tank curve) → starting service size ≈ 1.5" (verify against municipal pressure and the required fixture pressure). |
- Eight bathroom groups at 3.6 each is 28.8 units before the other fixtures are added.
- The kitchen, dishwasher, washer, tray and bibb fixtures add a further 19 units.
- The total converts to a peak demand that a 1½ inch service would normally carry.
Assumptions & limitations
Engineering calculation. Water supply sizing indices (Hunter method) converted to an estimated peak flow by interpolating the flush-tank demand curve.
- Interpolation of the Hunter curve is an approximation; the code appendix gives the exact tabular value.
- No allowance for irrigation, fire fighting or commercial kitchen equipment unless entered as fixture units.
- Assumes flush-tank fittings throughout.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- IPC 2018 Appendix E — water supply fixture units and demand curves.
- Hunter, Methods of Estimating Loads in Plumbing Systems (10th edition).
- UPC 2021 Table 604.4 — water supply fixture unit values.