Outside diameter is fixed, bore is not
A 110 mm HDPE pipe is 110 mm across the outside whatever its pressure rating. What changes with the rating is wall thickness — and because the wall grows inwards, a higher pressure class leaves a narrower bore. A DN 110 PN 6 pipe carries noticeably more water than a DN 110 PN 16 pipe of the same nominal size.
This matters when you are sizing for flow. Specify by outside diameter and pressure class together, never by outside diameter alone.
How to read the table
- DN is the nominal outside diameter in millimetres.
- PN is the pressure class in bar at 20 °C for water.
- Wall is the permitted minimum and maximum wall thickness.
- Approx. bore is the internal diameter at minimum wall.
| Size | PN 6 | PN 10 | PN 12.5 | PN 16 | PN 20 |
|---|---|---|---|---|---|
| DN 20 | — | — | — | 2.3 / 15.4 | 2.7 / 14.6 |
| DN 25 | — | 1.9 / 21.2 | 2.3 / 20.4 | 2.8 / 19.4 | 3.4 / 18.2 |
| DN 32 | — | 2.4 / 27.2 | 2.9 / 26.2 | 3.6 / 24.8 | 4.4 / 23.2 |
| DN 40 | — | 3.0 / 34.0 | 3.7 / 32.6 | 4.5 / 31.0 | 5.4 / 29.2 |
| DN 50 | 2.4 / 45.2 | 3.7 / 42.6 | 4.6 / 40.8 | 5.6 / 38.8 | 6.8 / 36.4 |
| DN 63 | 3.0 / 57.0 | 4.7 / 53.6 | 5.8 / 51.4 | 7.0 / 49.0 | 8.6 / 45.8 |
| DN 75 | 3.6 / 67.8 | 5.6 / 63.8 | 6.9 / 61.2 | 8.4 / 58.2 | 10.2 / 54.6 |
| DN 90 | 4.3 / 81.4 | 6.7 / 76.6 | 8.2 / 73.6 | 10.0 / 70.0 | 12.2 / 65.6 |
| DN 110 | 5.3 / 99.4 | 8.1 / 93.8 | 10.0 / 90.0 | 12.3 / 85.4 | 4.7 / 53.6 |
| DN 125 | 6.0 / 113.0 | 9.2 / 106.6 | 11.4 / 102.2 | 13.9 / 97.2 | — |
| DN 140 | 6.7 / 126.6 | 10.3 / 119.4 | — | — | — |
| DN 160 | 7.7 / 144.6 | 11.8 / 136.4 | — | 17.8 / 124.4 | — |
| DN 180 | 8.6 / 162.8 | — | — | — | — |
| DN 200 | 9.6 / 180.8 | 14.7 / 170.6 | 18.2 / 163.6 | 22.3 / 155.4 | — |
Choosing a pressure class
Pressure class must cover the working pressure plus any surge. As a rule of thumb for gravity schemes, take the static head in metres and divide by ten to get bar, then choose the next class up. A 60 m head is 6 bar static, so PN 10 gives sensible margin once surge is allowed for.
For pumped mains, size on the pump shut-off head rather than the duty point, and consider surge protection on longer runs.
Dimensions are verified, not assumed
Every figure above comes from testing to NS 40:2079. Our laboratory checks mean outside diameter, ovality and wall thickness on production samples, alongside internal pressure creep rupture, longitudinal reversion, carbon black content and dispersion, melt flow rate, oxidation induction time, density, elongation at break and yield strength.
Written by the technical team at Maruti Pipes Pvt. Ltd., Nepalgunj, Nepal.