Water, Drainage, Gas, and Storm Sizing
The master plumber exam is built on calculation, and this is the chapter where most of the arithmetic lives. You are expected to move from a fixture-unit count to a pipe size, hold the right slope over a run, balance a pressure budget from the meter to the most remote fixture, convert an appliance BTU rating into gas flow and then into pipe size, and size roof drainage from a rainfall rate. Each of these follows a repeatable procedure: total the load, convert the load to a flow, and read the flow against a sizing table at the governing length or slope. Master the four conversions in this chapter, WSFU to gpm, DFU to drain size, BTU to cfh, and roof area to storm gpm, and most of the sizing questions become mechanical.
Water Supply Sizing by Fixture Units
Water distribution is sized from water supply fixture units, or WSFU, a probability-weighted value assigned to each fixture so that dissimilar fixtures can be added on one scale. You total the WSFU carried by a section of pipe, then convert that total to a design flow in gpm using a demand curve, historically Hunter's curve, which accounts for the low likelihood that every fixture runs at once. A critical rule is that you add fixture units first and convert the combined total to gpm only once; converting each branch to gpm and adding the flows overstates demand badly. Flush-valve fixtures and flush-tank fixtures use different curves because a flushometer draws a heavy momentary flow. Once you have the design gpm, the pipe is chosen so velocity stays within limits, generally 8 ft/s for cold water and about 5 ft/s for continuously circulated hot water, using v = 0.408 x Q / d squared.
The Pressure Budget
Every water system must balance the pressure available at the source against everything that consumes it before the water reaches the most remote fixture. The consumers are elevation, at 0.433 psi per foot of rise; meter and device losses such as the water meter, softener, and backflow assembly; pipe friction, expressed as psi per 100 ft at the design flow; and the minimum flow pressure the fixture itself needs, often 8 to 25 psi. The design procedure subtracts elevation, meter, and fixture requirements from the static supply, and whatever remains is the pressure available for friction. Dividing that remaining pressure by the developed length and multiplying by 100 gives the allowable friction loss per 100 ft, the value you carry across the friction chart against the design flow to pick the pipe. If the friction loss at the chosen size exceeds the allowable rate, the pipe is enlarged or a booster is added.
Drainage and Vent Sizing by Fixture Units
Sanitary drainage is sized from drainage fixture units, or DFU, using tables split into three columns that must not be interchanged: the horizontal fixture branch, the stack, and the building drain and sewer at a stated slope. You total the DFU carried by a section and choose the smallest pipe whose column capacity meets or exceeds the load. Two override rules apply regardless of the count: a drain may never be smaller than the largest trap or trap arm it serves, and any drain receiving a water closet must be at least 3 in. Slope is the second half of drainage sizing, with a minimum of 1/4 in per foot for pipe 2-1/2 in and smaller and 1/8 in per foot for 3 in and larger; total fall is simply slope times run. Vents are sized at not less than half the diameter of the drain they serve, never smaller than 1-1/4 in, and are also limited by developed length.
Gas Pipe Sizing
Fuel-gas piping is sized by first converting each appliance BTU per hour rating into a volume flow in cubic feet per hour, cfh, by dividing by the heating value of the fuel: about 1,000 BTU per cubic foot for natural gas and about 2,500 BTU per cubic foot for propane. Because propane holds far more energy per cubic foot, the same BTU load produces a much smaller cfh and often a smaller pipe. The whole-house demand is the sum of all appliance inputs and it sizes the meter and the main. Individual sections are then sized by the longest-length method, in which every section of the system is sized using the single longest run from the meter to the most remote outlet, applied against the appropriate table. The common section upstream of any tee always carries the sum of all downstream appliance demands.
Storm Drainage Sizing
Roof and storm drainage is sized from the design rainfall rate for the locality, expressed in inches per hour, applied to the projected horizontal roof area. The rational method Q equals 0.0104 times area in square feet times rainfall rate in inches per hour converts these into a design flow in gpm, where the 0.0104 factor is the gpm produced by one inch per hour of rain over one square foot. That flow is read against the leader and horizontal storm-drain tables to size the conductors. Because published tables are often printed at a reference rate such as 4 in per hour, the allowable roof area for a given conductor scales inversely with the local rate: halving the rainfall rate doubles the area a conductor can drain. Secondary (overflow) drainage is required so a blocked primary drain cannot pond water and overload the roof structure.
Last updated: July 2026