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Water Meter Sizing Calculator (Fixture Units)

Estimate peak water demand from total water supply fixture units, using a Hunter's curve style approximation, to help select a water meter size.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Estimated peak demand

22.63 GPM

Check your inputs

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.

At the values currently entered, the estimated peak demand works out to 22.6 GPM. Note that total water supply fixture units (wsfu) sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 22.627 GPM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 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

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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?
Enough to matter, and it must be subtracted before anything downstream is sized. Every meter has a published head-loss curve against flow, and at the top of its range the loss climbs steeply. 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.