What is SDR in Poly Pipe?

What is SDR in Poly Pipe?

Ultramax Systems on 16th Oct 2026

Short answer: SDR (standard dimension ratio) is a poly pipe's outside diameter divided by its wall thickness, so a lower SDR means a thicker wall. Together with the PE grade it sets the pressure rating, and on compressed air that rating is lower than the PN figure printed for water.

Why it matters

A poly air line is only as safe as the rating it was chosen on. The same length of poly pipe can carry very different pressures depending on its SDR, its material class and the air temperature. If you pick the pipe by the PN marking alone, you can end up running a compressed air main above what the pipe is meant to carry. SDR is the number that connects the pipe's dimensions to its strength.

How SDR is worked out

SDR is the ratio of the nominal outside diameter to the nominal wall thickness, so a higher SDR means a thinner wall at any given diameter (PE100+ Association n.d.-a). A 63 mm pipe at SDR11 has a wall of about 63 ÷ 11 = 5.7 mm. At SDR17 the wall is about 3.7 mm, and at SDR7.4 it is about 8.5 mm. The published dimension tables in AS/NZS 4130:2018 set the exact minimum wall for each size and SDR, so use those, not the rounded ratio, when you check pipe on site.

Because SDR is a ratio, every size of pipe in one SDR and grade carries the same pressure rating. A 25 mm SDR11 pipe and a 110 mm SDR11 pipe in PE100 share a rating. That is why suppliers quote pipe as "PE100 SDR11" rather than listing a wall thickness for every size.

Three 63 mm pipe cross-sections drawn to scale. SDR17 has a wall of about 3.7 mm and a compressed air rating of 625 kPa, SDR11 has about 5.7 mm and 1000 kPa, and SDR7.4 has about 8.5 mm and 1563 kPa, all at 20 °C.
Same outside diameter, three SDRs: a lower SDR means a thicker wall and a higher rating.

How SDR sets the pressure rating

Pipe designers start from the material's minimum required strength (MRS), which is 10 MPa for PE100. The PE100+ Association (n.d.-b) gives the maximum operating pressure in bar as 20 × MRS ÷ (C × (SDR − 1)), where C is a design coefficient. For water, C is at least 1.25. That gives 16 bar (1600 kPa) for PE100 SDR11, which is the familiar PN16 class.

Compressed air is treated differently. PIPA (2026) derives compressed air ratings with a design factor of 2, because of the energy stored in compressed air and the potential for pressure spikes. The same PE100 SDR11 pipe is therefore rated at 1000 kPa (10 bar) for air at 20 °C. SDR17 drops to 625 kPa, while SDR9 reaches 1250 kPa and SDR7.4 about 1563 kPa.

Bar chart comparing PE100 water PN ratings with compressed air ratings at 20 °C. SDR17 is 1000 versus 625 kPa, SDR11 is 1600 versus 1000 kPa, SDR9 is 2000 versus 1250 kPa and SDR7.4 is 2500 versus 1563 kPa.
PE100 water PN ratings and compressed air ratings at 20 °C, calculated with design factors of 1.25 and 2.

Temperature changes the answer

These ratings apply at 20 °C. POP002 applies a temperature factor above that. At 40 °C, for example, PE100 SDR11 falls to about 833 kPa (PIPA 2026). Air leaving a compressor, pipe in a hot roof space and runs in direct sun can all sit well above 20 °C. Check the factor for your highest likely temperature, and ask the pipe maker about anything hotter.

Common mistakes

  • Reading PN16 as 1600 kPa on air. It is a water rating.
  • Assuming a bigger SDR number means a stronger pipe. It means a thinner wall.
  • Sizing to normal working pressure only. Compare the rating with the highest pressure the system can reach, such as the relief valve set pressure.
  • Using PE80 figures for PE100 pipe, or the reverse. Grade and SDR always go together.
  • Mixing SDRs in one line without planning the joints. Fusion methods handle wall differences in different ways.

Related terms

References

This article is general information only. Check manufacturer documentation and applicable Australian Standards for your installation.