Pipe diameters: how each system names the same size
Six pipe systems and how they differ
Before comparing numbers, work out which system they belong to. Each names its sizes its own way, joins its own way and holds its own temperature. Pick a type to see its size range and the one thing that sets it apart:
Five systems out of six are named by outer diameter — only steel by its bore. Nearly all the confusion on this page comes from that.
Quick match: which pipe goes with which thread
| PP-R (outer) | Thread | PEX / multilayer (outer) | Steel (DN) |
|---|---|---|---|
| 20 mm | 1/2″ | 16 mm | DN 15 |
| 25 mm | 3/4″ | 20 mm | DN 20 |
| 32 mm | 1″ | 25–26 mm | DN 25 |
| 40 mm | 1 1/4″ | 32 mm | DN 32 |
The match is by comparable water flow, not equal numbers: PP-R 25 has roughly the bore of a 3/4″ steel pipe.
Three different pipes all called “25”
“Give me a 25” means nothing until you name the system. Here are three pipes that all answer to it, drawn to scale in cross-section:
Blue marks the bore the water actually flows through. On the steel pipe it's half again as wide as on the PP-R with the same number in its name.
PP-R: outer vs inner diameter
PP-R is marked by outer diameter, but thick walls eat a lot of bore — a PN20 pipe loses about a third of its nominal size inside:
| Marking | Outer Ø, mm | Wall (PN20), mm | Inner Ø, mm |
|---|---|---|---|
| PP-R 20 | 20 | 3.4 | 13.2 |
| PP-R 25 | 25 | 4.2 | 16.6 |
| PP-R 32 | 32 | 5.4 | 21.2 |
| PP-R 40 | 40 | 6.7 | 26.6 |
PEX and multilayer
| Pipe | Outer Ø, mm | Typical wall, mm | Inner Ø, mm |
|---|---|---|---|
| Multilayer 16 | 16 | 2.0 | 12.0 |
| Multilayer 20 | 20 | 2.0 | 16.0 |
| Multilayer 26 | 26 | 3.0 | 20.0 |
| Multilayer 32 | 32 | 3.0 | 26.0 |
| PEX 16 | 16 | 2.2 | 11.6 |
| PEX 20 | 20 | 2.8 | 14.4 |
| PEX 25 | 25 | 3.5 | 18.0 |
| PEX 32 | 32 | 4.4 | 23.2 |
Walls vary by manufacturer and pressure class — check the print on the pipe itself.
Steel and DN
Steel pipe is named by DN — nominal bore, roughly the inner diameter in mm. The outer diameter is a separate, fixed number (the figures below are the GOST 3262 series used across Eastern Europe; the European EN 10255 differs by 0.1–0.3 mm):
| DN | Thread | Outer Ø, mm |
|---|---|---|
| DN 15 | 1/2″ | 21.3 |
| DN 20 | 3/4″ | 26.8 |
| DN 25 | 1″ | 33.5 |
| DN 32 | 1 1/4″ | 42.3 |
| DN 40 | 1 1/2″ | 48.0 |
| DN 50 | 2″ | 60.0 |
How to read the print on a pipe
Everything worth knowing about a pipe is printed along it. The line reads left to right and tells you more about the pipe than any measurement:
“25×4.2” is the outer diameter and the wall. The bore is mental arithmetic: 25 − 4.2 × 2 = 16.6 mm.
- PN is the working pressure in bar at 20 °C. PN10 is cold water, PN16 cold and warm, PN20 and PN25 hot water and heating (for hot water — reinforced, see below).
- SDR is the outer diameter divided by the wall: the smaller the number, the thicker the wall and the tougher the pipe.
- A stripe or the marks “GF”, “AL”, “Fiber” means glass-fibre or aluminium reinforcement. Without it a pipe on hot water expands three to five times as much.
- The date matters on site: polypropylene that has sat for years in an open yard — dull, chalky to the touch — welds noticeably worse.
Which diameter for what
A starting point for household water, by the number of fixtures on the run. Heating is sized by the load of the circuit, not by this table:
| Section | PP-R | PEX / multilayer | Thread |
|---|---|---|---|
| Connection to a tap or toilet | 20 | 16 | 1/2″ |
| Run serving one or two fixtures | 20 | 16–20 | 1/2″ |
| Distribution across a flat, 3–5 fixtures | 25 | 20 | 3/4″ |
| Riser, entry into a flat | 32 | 25–26 | 1″ |
| House inlet, line from a well | 32–40 | — | 1″–1 1/4″ |
How many fixtures a pipe carries
Diameter is chosen by flow, not by the number on the box: how many litres per minute must pass. Rough figures for household outlets — a basin tap takes 6–9 l/min, a shower 8–12, a toilet 5–6, washing and dishwashing machines up to 10–12 while filling, a bath 12–18. The bath is the outlet the main run gets sized around.
| Pipe | Bore, mm | Comfortable flow | Fixtures at once |
|---|---|---|---|
| PEX / multilayer 16 | 12 | up to 8 l/min | one outlet |
| PP-R 20 · multilayer 20 | 13–16 | 8–12 l/min | one or two |
| PP-R 25 · multilayer 26 | 16–20 | 16–20 l/min | two or three |
| PP-R 32 · steel DN25 | 21–27 | 28–35 l/min | a whole flat |
The limits come from keeping water below 1.5 m/s — above that the pipe starts to hiss and knock. Count the fixtures that genuinely run together, usually two or three, not all of them.
Why pressure drops on a long run
Every metre of pipe and every bend takes a share of the pressure as friction. On a short section you won't notice; on a long small-bore branch it adds up — which is why the far tap feels weaker than the near one at identical inlet pressure.
The practical rule: if a branch runs beyond 10–15 metres or takes more than five or six bends, go one size up from what the table suggests. Overpaying for pipe is cheaper than hunting for lost pressure afterwards.
- A bend costs roughly as much as a metre of straight pipe: ten corners ≈ another ten metres.
- A branch is rated by its narrowest point: one 16 mm section in a 25 mm run sets the flow for the whole branch.
- This is exactly where a manifold layout wins: each fixture has its own run and they don't steal pressure from each other.
Where people get the diameter wrong
Three slips that leave the pipe and the fittings refusing to meet:
Why the confusion exists
Each material inherited its own naming: steel from nominal bore (the useful number for flow), plastic from outer diameter (the useful number for fittings and dies), threads from a 19th-century inch standard where the inch once described the pipe bore, not the thread.
In practice you only need the first table on this page — and a builder that refuses to connect what doesn't match.
Field tips
Short rules that save you re-piping a run.
Common questions
A 25 mm pipe is how many inches?
PP-R 25 matches a 3/4″ thread, not a 1″ one: the match is by flow capacity, not by matching numbers. Alongside it: PP-R 20 to 1/2″, PP-R 32 to 1″, PP-R 40 to 1 1/4″.
Which pipe diameter should I use in a flat?
A drop to a single fixture is PP-R 20 or 16 mm PEX/multilayer, a run serving 3–5 fixtures is PP-R 25 or 20 mm PEX/multilayer, the inlet into the flat is PP-R 32. If a branch runs longer than 10–15 metres or has more than five or six turns, go one size up.
Why don't my 25 fittings fit my 25 pipe?
Because they are different systems. PP-R runs 20, 25, 32, 40; multilayer runs 16, 20, 26, 32. The number matches, the system does not: PP-R joins PP-R, multilayer joins multilayer, and you cross between families through a threaded fitting.