Pipe Sizing Calculator
Count the fixtures, read the demand. This tool runs the peak water demand method from UPC Appendix M (2018–2024 Uniform Plumbing Code), the modern alternative to Hunter’s curve, entirely in your browser. Free, no signup, built for a phone at the supply-house counter.
Fixture counts below are the totals across all the apartments in this calculation, not per apartment.
Apartments in this calculation can’t exceed the building total.
Bathroom
Kitchen
Laundry
Bar
Other fixtures
Indoor fixtures the list doesn’t cover: a pot filler, a dog wash, a hot-water dispenser. Give each a probability of use (0.1% to 6%) and its flow rate. No hose bibbs or irrigation here; add continuous flows to the result by hand.
Add fixtures and the demand appears here, recalculated on every tap.
99th percentile demand
0.0GPM
Convolution · the exact calculation for small fixture groups
Fixtures (n) 0
Hunter number (Σ n p) 0.00
Stagnation probability (no flow) 100%
An independent tool from BeepBack. Not affiliated with or endorsed by IAPMO. Results are design estimates; your local authority may require calculations from the code-referenced software for permit submittals.
The link carries your counts, flows, and settings, so the counter guy or your apprentice opens exactly what you see.
What this computes
The number is the 99th percentile of instantaneous water demand while the building is actually using water: the flow your supply pipe should carry so that busy-hour demand exceeds it only one time in a hundred. Each fixture type carries a measured probability of running at any given moment during the peak period (shown as the small p next to each fixture), taken from a large study of real households. The tool combines your counts and those probabilities three different ways depending on the size of the system: an exact convolution of the probabilities for small fixture groups, and Wistort’s normal approximation, adjusted or plain, as the group gets large. It picks the right one automatically and tells you which it used.
Hunter’s curve, the fixture-unit method most of us learned, dates to 1940. It assumed congested use, one person in line behind another, and fixtures that flowed hard. Both assumptions are false in a home today, which is why fixture-unit sizing overshoots and why the 2018 Uniform Plumbing Code added Appendix M. Smaller, honest numbers are the point: right-sized pipe costs less to install, gets hot water to the tap sooner, and holds less stagnant water.
See the research: The Oversize Audit measures the gap across eight typical homes and buildings, both methods side by side, every figure reproducible.
Sizing branches, not just the main
Size each pipe section with the subset of fixtures it actually serves. The whole house at the meter is every fixture. The hot branch at the water heater is every fixture that draws hot water, so the water closet drops out. A cold branch keeps the water closet and drops the dishwasher, which is hot-only. Run the calculator once per section, changing only the counts. When a branch is down to one fixture, the answer is simply that fixture’s flow rate, and the calculator says so.
Outdoor taps and continuous flows
Hose bibbs, irrigation, and anything else that runs continuously for minutes at a time sits outside the method, which models short, intermittent indoor use. Size the indoor demand here, then add continuous flows to the result by hand: a house that computes to 9.0 GPM with a 2.0 GPM hose bibb designs at 11.0 GPM.
Questions plumbers ask
Is this the same math as UPC Appendix M?
Yes. It uses the published probabilities of use and flow rates for the eleven residential fixture types, the multi-family scaling, and the three published algorithms: exact binomial convolution for small fixture groups, the adjusted modified Wistort method for the middle range, and Wistort’s method for large ones, all at the 99th percentile. The method comes from UPC Appendix M (2018–2024 Uniform Plumbing Code) and the research behind it is the Peak Water Demand Study (IAPMO Study 4-2024, first published 2017). Results are design estimates; your local authority may require calculations from the code-referenced software for permit submittals.
How do I turn the flow number into a pipe size?
The calculator gives you the design flow. Pipe size then comes from your plumbing code’s sizing appendix: you pick a pipe material, work out friction loss for your run length and pressure, respect the velocity limits, and read the size off the charts. That step is jurisdiction and material dependent, which is why this tool stops at the flow number instead of guessing at it.
Why is the result lower than the fixture-unit method?
Hunter’s curve dates to 1940 and assumes congested use and high-flow fixtures, so it overshoots badly in homes. The peak water demand method replaces those assumptions with measured probabilities of use from a large study of real households and today’s water-conserving flow rates. Lower simultaneous demand is the honest answer, and it is the reason the method exists: right-sized pipe costs less, delivers hot water faster, and holds less stagnant water.
Does it work for commercial buildings?
No. The probabilities behind the method were measured in single-family and multi-family homes, and UPC Appendix M scopes it to residential buildings. For commercial work, stay with the sizing method your code requires.
What about hose bibbs and irrigation?
Outdoor and continuous flows are outside the method, because a hose or an irrigation zone runs for a long time instead of in short bursts. Size the indoor demand here, then add the continuous flows to the result by hand: a 9.0 GPM house with a 2.0 GPM hose bibb designs at 11.0 GPM.
Where does my project data go?
Nowhere. The whole calculation runs in your browser and nothing you enter is uploaded. The copy-link button encodes only the fixture numbers and settings in the web address so you can share a calculation, and the project name is used only in the printed page, never transmitted.
Method: the peak water demand method published in UPC Appendix M (2018–2024 Uniform Plumbing Code). Research: Buchberger, Omaghomi, Wolfe, Hewitt, and Cole, Peak Water Demand Study, IAPMO Study 4-2024, first published 2017. Parameter values are the published facts from those documents.
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Pipe sizing calculation
| Fixture | Count | p | Flow |
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Peak water demand method, UPC Appendix M (2018–2024 Uniform Plumbing Code). An independent tool from BeepBack (getbeepback.com/pipe-sizing-calculator/). Not affiliated with or endorsed by IAPMO. Results are design estimates; your local authority may require calculations from the code-referenced software for permit submittals.