Plumbing Peak Demand Calculator (Fixture Units)
Probable peak flow from total Water Supply Fixture Units via a Hunter-curve fit.
Demand grows with the SQUARE ROOT-ish of fixtures — 2× the apartments needs only ~1.3× the riser. That's Hunter's 1940 insight, still in every code.
Formula
Note: Curve fit ±10% mid-range, flush-tank fixtures; flushometer systems demand more.
Plumbing Peak Demand Calculator (Fixture Units) is a free fixture units to gpm for pump engineers, plumbers and plant designers — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Plumbing Peak Demand Calculator (Fixture Units)
Probable peak flow from total Water Supply Fixture Units via a Hunter-curve fit. The calculation implements Hunter probability curve (flush-tank systems), power-law fit (IPC Appendix E / Hunter, BMS 65 (1940)). Demand grows with the SQUARE ROOT-ish of fixtures — 2× the apartments needs only ~1.3× the riser. That's Hunter's 1940 insight, still in every code.
How to use Plumbing Peak Demand Calculator (Fixture Units)
- 1Enter Total WSFU (WC flush-tank 2.2 · lavatory 0.7 · shower 1.4 · kitchen sink 1.4 (per IPC)).
- 2Read Probable peak demand instantly — no submit button needed.
- 3Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Plumbing Peak Demand Calculator (Fixture Units)?
- ✓Implements the standard formula — Hunter probability curve (flush-tank systems), power-law fit
- ✓Reference cited on-page: IPC Appendix E / Hunter, BMS 65 (1940)
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Plumbing Peak Demand Calculator (Fixture Units) use?+
It computes Hunter probability curve (flush-tank systems), power-law fit, per IPC Appendix E / Hunter, BMS 65 (1940). The formula is displayed under the result.
What should I keep in mind when using this calculator?+
Demand grows with the SQUARE ROOT-ish of fixtures — 2× the apartments needs only ~1.3× the riser. That's Hunter's 1940 insight, still in every code. Note: Curve fit ±10% mid-range, flush-tank fixtures; flushometer systems demand more.
Can I use this for pump selection?+
Use it to establish the duty (flow, head, NPSH, power) and then pick a pump whose curve passes through that point near best efficiency. The tool gives you the engineering numbers a supplier will ask for.
Is the Plumbing Peak Demand Calculator (Fixture Units) free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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