Estimating water supply demand and the service pipe that carries it
Water supply fixture units converted to a probable peak demand, and the service size that follows.
The engineering question
How much flow does a building actually need at peak, when no fixture ever runs at full flow?
Worked example
The figures below were produced by running the Water Supply Fixture Unit Calculator with the same inputs — during the build, from its authoritative engine.
| Estimated peak demand | 31.646 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). |
- A bathroom group counts 3.6 units; other fixtures carry their own value.
- Add to a total, then interpolate the probable peak demand from the standard curve.
- That peak demand, not the sum of rated flows, is what sizes the service.
Sanity check
Peak demand grows far more slowly than the fixture count. A linear result means the fixture units have been multiplied by their rated flow.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- IPC 2018 Appendix E — supply fixture units and demand
- Hunter — Methods of Estimating Loads in Plumbing Systems
Related problems
Sizing drainage pipe by fixture units rather than by flow
Why drainage is sized by a loading unit, and reading the code table to get the building drain.
Sizing a sump pump for the storm that will actually arrive
Converting drainage area and rainfall intensity into inflow, then applying the margin a real storm will need.
Also: Environmental