½ inch CPVC pressure loss at 8 GPM
| Pressure loss per 100 ft | 79.69psi |
|---|---|
| Actual inside diameternominal ½″ — the nominal size is a name, not a measurement | 0.469″ |
| Same size in Type L copperbore 0.545″ | 43.6psi/100 ft |
| Velocitypast 8 ft/s — noisy, and erosive on copper over time | 14.9ft/s |
| Flow | 8GPM |
| Materialcheap and rigid; bore is close to PEX, and it gets brittle with age | CPVC (SDR-11) |
Notes
- Nominal size is a name, not a measurement, and PEX is the narrowest of them. ½″ PEX has a 0.475″ bore against 0.545″ for the same size in Type L copper. Friction rises with the fifth power of diameter, so that 15% difference in bore becomes 1.72× the pressure loss.
- This is why a repipe in the same size can feel like a downgrade. Copper out, PEX in, same nominal sizes, and the shower at the far end is noticeably weaker. Nothing was done wrong — the pipe is simply narrower inside. On any run over about 40 feet, or on a branch feeding two fixtures, go up one size when switching to PEX.
- PEX wins on the things that are not flow. Continuous runs from a manifold mean almost no joints inside walls, and a joint you never made is a joint that never leaks. It survives a freeze that would split copper, it installs in a fraction of the time, and it does not tempt anyone to steal it out of a vacant house.
- 14.9 ft/s is too fast. Past 8 ft/s the pipe becomes audible — that hiss and the water-hammer bang on a quick-closing valve — and on copper the velocity erodes the inside of elbows until they pinhole. Go up a size rather than accepting the noise.
- Copper still has two arguments in its favour. It is rated for higher temperatures, which matters on a recirculation loop; and it is unaffected by UV, while PEX degrades in sunlight and must not be stored or run where daylight reaches it. Against that, copper needs a competent solderer and priced-in time — most of a copper repipe is labour.
- CPVC is the cheapest of the three and the one to think hardest about. The bore is close to PEX, so the flow story is the same, and it becomes brittle with age: a twenty-year-old CPVC line can crack when a fitting nearby is disturbed. It also will not tolerate many common thread sealants and lubricants, which is a surprisingly common cause of early failure.
The bore is 0.469″, and the label says ½ inch
Nominal pipe size is a name. What decides flow is the actual inside diameter, and at ½ inch nominal the four common materials are not close to each other. Friction rises with roughly the fifth power of diameter, so a bore difference that sounds trivial is not.
At 8 GPM this pipe loses 79.69 psi per 100 ft, running at 14.9 ft/s.
Why a repipe in the same size can feel worse
Copper out, PEX in, every size matched — and the far shower is weaker. Nothing was installed incorrectly. PEX is simply narrower inside than copper of the same name. On runs past about 40 feet, or on any branch feeding two fixtures at once, going up one size when switching to PEX restores what was lost.
What each material is actually good at
PEX runs continuously from a manifold, so there are almost no joints buried in walls — and a joint never made is a joint that never leaks. It survives freezing that splits copper. Copper handles higher temperatures, which matters on a recirculation loop, and it is unaffected by sunlight, while PEX degrades in UV and must never be run or stored where daylight reaches it.
CPVC is the cheapest and the one to think hardest about: the bore is close to PEX, and it grows brittle with age until a twenty-year-old line cracks when something near it is disturbed.
Nearby sizes
Read more
- Connecting PEX to Copper — Four Methods and the One Real RiskMixing the two is normal and code-compliant. The failures come from heat, from the wrong fitting for the tubing, and from sunlight — not from the metals touching.
- Whole-House Repipe Cost by State — 12 Fixtures, All 51Repipe is 70% labour, so the spread between states is wider here than on almost any other trade job. The table shows all 51 and where each number comes from.
- Drain Gurgles When the Washing Machine Empties — A Venting ProblemThe gurgle is the sound of a trap seal being emptied. Get it wrong for long enough and the smell that follows has an obvious source.
- Low Water Pressure at One Faucet Only — Five Causes, Cheapest FirstOne weak tap is never a pressure problem. Pressure is shared by the whole house — this is a restriction, and it is usually within a foot of the spout.