Fixture Units & Supply Size
Fixture units by fixture count, the demand curve that converts them to gallons per minute, and the supply pipe and meter size that carries it at 8 feet per second.
A 3 fixture unit fixture does not mean 3 gpm: a 10 fixture-unit house draws about 14 gpm at peak, and 25 fixture units draws 24 gpm, because not every faucet runs at once.
How the math works
- Fixture units (IPC Table 702.3, private/residential values): water closet flush tank 2.1, lavatory 1.1, kitchen sink 2.0, shower head 3.0, clothes washing machine 2.0, dishwasher 2.0, floor sink and hose bibb 2.0. Public fixtures run 3 to 6, and a service sink is 2.
- Demand curve (IPC Table 703.1, one-family values): 10 FU = 14 gpm, 15 = 18, 20 = 22, 25 = 26, 30 = 29, 40 = 36, 50 = 42, 60 = 47, 80 = 56, 100 = 65, 150 = 87, 200 = 110. Fixture units are a diversity model, not an addition: a 10 FU house is 14 gpm, not 10.
- Pipe size: size the main at 8 feet per second (velocity limit that keeps noise and erosion down in copper), so required area = gpm × 0.002228 ÷ 8 sq ft. A 24 gpm demand needs 0.677 in ID: 3/4 in copper (0.773 in).
- Fixture branches: 1/2 in for a water closet flush tank (1/2 in for flushometer with 1.6 gpf), 3/4 in for a shower (a 2.5 GPM head needs 3/4 to keep pressure at 8 psi residual), 1 in for a tub-shower combo, 3/4 for kitchen and laundry.
- Water service: the 2026 residential meter is 3/4 in for a 20 gpm peak, 1 in for 40 to 90, and the service line is sized from the main to the meter at the same velocity rule, with a 1 in service line standard in frost country.
How many fixture units is a one-bathroom house?
A bathroom group (water closet 2.1, lavatory 1.1, tub-shower 3.0) plus a kitchen sink 2.0 and a laundry 2.0 = 8.2 fixture units, which is about 12.5 gpm of peak demand and a 3/4 in supply line.
What size water service line for a house?
A typical 3 fixture bathroom + kitchen + laundry house lands at 24 to 30 fixture units and 26 to 29 gpm, which needs 1 in copper ID at 8 fps: a 1 in meter socket and a 1 in service line is the standard residential answer, and 3/4 in is the minimum in the code for a dwelling.
Does a shower need 1/2 inch pipe?
No. A 2.5 GPM shower head at 1/2 in pipe at 8 fps needs more than 1 psi to move, and fixture minimums are 8 psi: 3/4 in is the code answer for a shower, and a 2 gpm pressure-balance valve at 3/4 in is the reason showers in 1/2 in lines feel weak.