Pipe sizing basics for houses

Entry in Sizes, codes and marks · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

Pipe sizes for house plumbing are chosen so every fixture gets enough flow at enough pressure when several run at once, without pipe so large that water sits still. Sizing starts at the water service and works out through mains and branches to each fixture.

Sizes here are nominal; what a "3/4 inch" pipe actually measures depends on the sizing system (see nominal pipe size).

1 Code minimums

Model codes set floors rather than finished designs. The 2024 International Plumbing Code (IPC), Section 603.1, requires the water service to be sized for the quantities and pressures the code requires and to be not less than 3/4 inch in diameter^[1]. Section 604.3 of the same code requires the distribution system to deliver at least the flow and pressure in its Table 604.3 at each fixture outlet during peak demand^[1].

For manifold (home-run) systems, the 2024 International Residential Code (IRC), Section P2903.9.2, allows individual lines as small as 3/8 inch where the line is 60 feet or shorter and the pressure at the meter is at least 40 psi; some fixtures, such as one-piece toilets and whirlpool tubs, need a larger line where the maker says so^[1]. Manifolds themselves are sized from a code table by the total gallons per minute of the outlets they serve^[1].

In the 2024 IRC these rules sit in Chapter 29, Water Supply and Distribution^[3]. These are national model code figures. States and cities adopt and amend them, so the local building department's code governs; see plumbing codes.

2 Flow, velocity and pressure

The same flow moves faster in a smaller pipe. The plastics pipe industry's PEX design guide sets maximum recommended velocities of 10 ft/s for cold water up to 80°F and 8 ft/s for water above 80°F up to 140°F^[2]. Its velocity table shows how quickly a small line reaches those limits:

Undersized pipe brings high velocity, erosion of metal parts, water hammer from fast-closing valves and too little pressure at the far fixtures^[2]. Oversized pipe has costs too: hot water takes longer to arrive, more water and energy are wasted, and slow, stagnant water lets disinfectant fade and can favor biofilm and organisms such as Legionella^[2]. Every fitting and every foot of pipe also takes some pressure, which is why long runs are sized up.

3 Layout changes the sizes

In a trunk-and-branch layout, a larger main feeds smaller branches. In a zoned layout, trunk lines run to a remote manifold or multi-port tee near a kitchen or bathroom group, and branch lines, typically 3/8 or 1/2 inch, run from there to each fixture^[2]. In a home-run layout, each fixture gets its own line from a central manifold, and 3/8 inch tubing holds about 54% of the water in 1/2 inch tubing, so hot water arrives sooner^[2].

The guide also notes that some code sizing tables rest on older assumptions about fixture demand, and points to newer methods such as IAPMO's Water Demand Calculator for certain buildings^[2]. Where a pressure-reducing valve is installed, it recommends an expansion tank or other thermal expansion relief, subject to local code^[2].

4 Who does the sizing

Real sizing weighs fixture demand, street or well pressure, elevation, pipe length, fittings and material, and it is checked against the adopted code. That is work for a designer or licensed plumber, and new work or a repipe usually needs a permit. For material choices see which pipe for which job and PEX pipe.