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.

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

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.

Example and tips

A bathroom group of one flush-tank closet, one lavatory, and one shower totals about 4.3 WSFU, a peak demand near 8 GPM, which a 3/4 inch line carries within 8 ft/s. 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 get their minimum flow.