Water Meter Sizing Calculator (Fixture Units)

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Imperial · sales tax

Total Water Supply Fixture Units (WSFU)

The sum of water supply fixture unit values for every fixture served by this meter.

Add up the WSFU value for each fixture (toilets, sinks, showers, etc.) from your local plumbing code's fixture unit table.

Demand Factor (Calibration Constant for Your Fixture Mix)

A calibration constant that scales the square-root approximation to your building's fixture mix.

This constant approximates the shape of the Hunter's curve for a typical fixture mix; adjust it if your building's fixtures (e.g. mostly flush valves vs. mostly tank-type) differ significantly from typical assumptions.

Estimated peak demand

22.6 GPM

Medium confidence

A meter is sized on peak demand and on the pressure it costs you, not on the size of the pipe it sits in. Fitting a meter one size up because the main is large is a common and expensive error — an oversized meter under-registers low flows.

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Simplified Hunter's curve approximation: peak demand (gpm) ≈ demand factor × √(total water supply fixture units, WSFU) — probability-based fixture unit methods (per IPC/UPC Appendix tables) estimate peak simultaneous demand from a building's total fixture count, since not all fixtures operate at once

Inputs used

Total Water Supply Fixture Units (WSFU): 50
Demand Factor (Calibration Constant for Your Fixture Mix): 3.2

Final result22.63 GPM

Confidence note: A meter is sized on peak demand and on the pressure it costs you, not on the size of the pipe it sits in. Fitting a meter one size up because the main is large is a common and expensive error — an oversized meter under-registers low flows.

What this calculation does not cover

Does not compute the meter's pressure loss at the design flow, which must be subtracted from the available pressure before sizing the distribution.
Excludes fire demand entirely. Where a supply serves both domestic use and a fire system, the sizing basis and often the meter type are different.
Meter accuracy classes have a minimum registrable flow. Continuous small leaks below that threshold pass unmetered, which matters for leak detection rather than for billing.

Add the equipment this sizes

This result is a specification — 22.6 GPM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

QtyItemPrice each ($)

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-08-26 · in the site-wide review of 2026-09-06 · v1.0.0

Regulatory standards & verification citations1

Simplified Hunter's curve approximation: peak demand (gpm) ≈ demand factor × √(total water supply fixture units, WSFU) — probability-based fixture unit methods (per IPC/UPC Appendix tables) estimate peak simultaneous demand from a building's total fixture count, since not all fixtures operate at once

Which documents these citations point at

International Plumbing Code (United States)Plumbing systems — fixtures, water supply, sanitary drainage, venting and storm drainage.
Uniform Plumbing Code (United States)Plumbing systems, where a jurisdiction adopts the IAPMO family rather than the ICC one. Its fixture-unit tables differ from the IPC's.

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Boosting a Weak Incoming Water Supplyuses this calculator

A shower that dies when a tap opens is a flow problem, not a pressure one, and the service pipe has to be measured before anyone prices a pump.

Sizing a Water Meter and Backflow Preventeruses this calculator

The meter size box on a utility application is a twenty-year charge, and the relief port under the assembly decides how big the drain beneath it must be.

Sizing and Plumbing a Water Softeneruses this calculator

A softener is sized twice over: the grains a household burns between regenerations, and the flow its valve passes with three fixtures open.

How to calculate water meter sizing (fixture units) in 3 steps

Total Water Supply Fixture Units (WSFU)The sum of water supply fixture unit values for every fixture served by this meter.

Demand Factor (Calibration Constant for Your Fixture Mix)A calibration constant that scales the square-root approximation to your building's fixture mix.

Estimated peak demandThe tool computes the estimated peak demand from those figures and shows the formula, its sources, and a confidence rating alongside it.

Total Water Supply Fixture Units (WSFU)	Estimated peak demand (GPM)
5	7.16
10	10.1
20	14.3
50	22.6
100	32
200	45.3
500	71.6

Total Water Supply Fixture Units (WSFU)

Estimated peak demand (GPM)

7.16

10.1

14.3

22.6

100

200

45.3

500

71.6

Frequently asked questions

Why not just match the meter to the pipe size?

Because a meter measures flow and a pipe carries it, and they are sized on different criteria. An oversized meter runs below its accurate range for most of its life and under-registers the low flows that make up ordinary domestic use. An undersized one loses excessive pressure at peak. The meter is sized on the demand profile, and the pipe on flow, pressure and velocity.

How much pressure does a meter cost me?

Whatever its published head-loss curve gives at your flow, which climbs steeply at the top of the meter's range and must be subtracted before anything downstream is sized. It is enough to matter: a system designed without deducting it can be several metres of head short at the highest fixture, which shows up as poor performance on the top floor.

What about a fire sprinkler supply?

Different problem entirely. Fire demand is a large, short-duration flow with its own regulatory basis, and it is not combined with domestic demand by simple addition. Many jurisdictions require a separate connection or a specific meter type for a combined supply, and the sizing is governed by the sprinkler design rather than by fixture units.

Will a meter detect a small leak?

Only above its minimum registrable flow. A weeping fitting or a slow WC leak can sit below the threshold of a mechanical meter and never appear on a bill. That is a limitation of the technology rather than a fault, and it is why leak detection uses a period of no intended use rather than a meter reading alone.