Hunter Curve Method
Each plumbing fixture is given a water-supply fixture-unit (WSFU) weighting that reflects its flow and how often it is used. Because not all fixtures discharge simultaneously, the Hunter curve converts the total WSFU into a probable simultaneous peak demand that is far below the arithmetic sum of all fixture flows. The curve rises quickly for small systems and flattens for large ones. The resulting demand (GPM or L/s) is then used to size the supply main to a target velocity, typically 1.5 to 2.4 m/s to limit noise and erosion.
Frequently Asked Questions
Total the water-supply fixture units (WSFU) for all fixtures, then read the probable demand from the Hunter curve for your system type. For example 100 WSFU on a flush-tank system is about 36 GPM. The curve allows for the low probability that every fixture runs at once.
A weighting assigned to each fixture (tap, WC, shower) that represents its flow demand and usage frequency. Codes such as IPC, UPC and BS 8558 publish the values; you sum them for the system.
Because the chance of every fixture flowing at the same instant is tiny. The Hunter curve gives the realistic simultaneous peak, which is much lower and avoids massively oversized pipes.
Flushometer (valve) WCs draw a high instantaneous flow, so for the same fixture-unit count they give a higher peak demand than gravity flush-tank cisterns. Choose the curve that matches the predominant type.
Size the pipe to carry the peak flow at an acceptable velocity (about 1.5 to 2.4 m/s): diameter = square root of (4 x flow / (pi x velocity)). Round up to the next standard bore and check pressure/head loss.
Sizing Water Supply by Fixture Units
Domestic and commercial water mains are sized for the peak demand they will actually see, not the sum of every fixture flowing together. The fixture-unit and Hunter-curve method gives that realistic peak.
From fixtures to flow
Add the fixture units for all served outlets, then read the Hunter curve for the probable simultaneous demand. Flushometer systems sit on a higher curve than flush-tank systems.
From flow to pipe
Carry the demand at a sensible velocity to limit noise and erosion, giving the bore; round up to a standard size. Branch pipes are sized from their own fixture-unit subtotals. For the velocity/diameter step use the pipe sizing calculator.
Related: Pipe Sizing, Rainwater Downpipe, Pump Head.