Short definition
Friction loss is the pressure drop along a pipe due to viscous drag at the wall and turbulence in the flow. It increases with pipe length, flow velocity, and pipe roughness; decreases with larger inside diameter. New copper or PEX has very low friction loss; old tuberculated galvanized pipe can lose dramatic pressure across the same run.
What it is
The Hazen-Williams “C” factor captures pipe roughness for water-flow calculations. New copper or PVC has a C factor around 140; 40-year-old cast iron drops to 64–83. That’s why a sprawling 1960s rambler with original galvanized supply piping can deliver good pressure at the meter and a trickle at the far-end shower — most of the pressure is being burned off as friction loss in the long old pipes.
The practical hierarchy:
- Pipe length — longer runs lose more.
- Pipe diameter — small ID loses dramatically more (friction loss scales with the inverse of diameter to the fourth or fifth power, depending on flow regime).
- Pipe roughness — old corroded pipe is rougher; new pipe is smooth.
- Flow velocity — turbulent flow at high velocity loses more than laminar low-velocity flow.
Why it matters to a homeowner
Three common scenarios where friction loss is the answer:
- Sprawling rambler with low pressure at far-end fixtures — long original galvanized run, friction loss eats the pressure.
- Repipe to PEX at larger diameter — going from a 1/2-inch galvanized main to a 3/4-inch PEX trunk doesn’t just upgrade the material; it dramatically reduces friction loss, often resolving multi-fixture pressure complaints.
- Sprinkler system not delivering rated GPM — friction loss in the manifold and laterals reduces what reaches the heads.