What is a Water Supply Fixture Unit?

A WSFU is a dimensionless weighting that represents a fixture's probable water demand, accounting for both flow rate and how often it is used. IPC Table 604.3 assigns a WSFU value to each fixture so designers can add them up across a system.

What is Hunter's Curve?

Hunter's Curve is a statistical relationship, developed by Roy Hunter, that converts total fixture units into a probable peak simultaneous demand in gallons per minute. Because not all fixtures run at once, demand rises far more slowly than the raw fixture-unit total.

Why limit velocity to 8 feet per second?

The IPC limits water velocity to reduce noise, erosion of pipe walls, and water hammer. The tool sizes pipe so the calculated peak flow stays at or below the velocity you set, defaulting to the common 8 ft/s ceiling.

Does this replace a full hydraulic design?

It gives a sound minimum size by demand and velocity, but a complete design also checks pressure loss against available supply pressure over the run length, plus fixture minimum flow pressures. Use it as the demand-and-velocity step of a fuller calculation.

Are flush-tank and flushometer toilets different?

Yes. A flushometer valve closet draws far more instantaneous flow than a flush-tank closet, so it carries a much higher WSFU value. The tool lists both so you can pick the type actually installed.

What is the Water Supply Pipe Sizing Calculator (Hunter's Curve / WSFU)?

Converts plumbing fixtures to Water Supply Fixture Units per IPC Table 604.3, looks up peak demand from Hunter's Curve, and returns the minimum copper pipe diameter that keeps velocity at or below 8 ft/s. For plumbers and mechanical contractors. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Water Supply Pipe Sizing Calculator (Hunter's Curve / WSFU)

Name: Water Supply Pipe Sizing Calculator (Hunter's Curve / WSFU)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Sizing water supply piping comes down to estimating peak demand, then choosing a pipe big enough to carry it without exceeding a velocity limit. This tool follows the standard IPC method: fixtures to Water Supply Fixture Units, fixture units to gallons per minute via Hunter’s Curve, then GPM to a minimum pipe diameter.

How it works

Three steps turn a fixture count into a pipe size:

total WSFU = sum of (fixture count × fixture WSFU)   (IPC Table 604.3)
demand GPM = Hunter's Curve lookup(total WSFU)
pipe size  = smallest diameter where capacity at velocity limit ≥ demand GPM
capacity   = velocity (ft/s) × pipe area (ft²) × 448.831

Hunter’s Curve is the key step: because fixtures rarely all run at once, 100 fixture units does not mean 100 times one fixture’s flow. The curve flattens as fixtures multiply, giving a realistic simultaneous demand.

WSFU values for common fixtures (IPC Table 604.3)

Fixture	Cold WSFU	Hot WSFU	Total WSFU
Lavatory (private)	0.5	0.5	1.0
Bathtub	1.0	1.0	1.4
Shower (private)	1.0	1.0	1.4
Flush-tank closet	2.2	—	2.2
Flushometer valve closet	6.0	—	6.0
Kitchen sink (private)	0.5	0.5	1.0
Dishwasher (residential)	—	1.4	1.4
Washing machine (residential)	1.0	1.0	1.4

The flushometer toilet row is the one most often wrong in quick estimates — at 6 WSFU it dominates any system that uses flush-valve fixtures, such as commercial restrooms. Always confirm which type is actually being installed.

Why Hunter’s Curve matters

Roy Hunter observed in the 1940s that adding more fixtures to a system does not add demand linearly — it adds it at a decelerating rate, because the probability that all fixtures run simultaneously falls as the count grows. Hunter’s Curve captures this statistical relationship empirically. The result: a building with 50 WSFU does not need a pipe sized for 50 individual fixture flows; it needs one sized for the probable simultaneous peak, which Hunter’s Curve shows is significantly less.

This is why sizing by raw fixture count consistently oversizes piping and inflates material and installation cost.

Example and tips

A bathroom group of one flush-tank closet (2.2 WSFU), one lavatory (1.0 WSFU), and one shower (1.4 WSFU) totals about 4.6 WSFU, a peak demand near 8–9 GPM, which a 3/4 inch copper line carries within the 8 ft/s limit.

Adding three more identical bathroom groups brings the total to about 18 WSFU. Hunter’s Curve shows the simultaneous demand does not quadruple — it rises to roughly 18–20 GPM, which a 1.25 inch main handles comfortably.

Always pick the correct toilet type, since a flushometer valve closet alone is 6 WSFU. On long runs, follow up by checking pressure loss against the available supply pressure so fixtures at the far end still receive their minimum flow pressure — pipe sizing and pressure analysis are separate but linked steps.