HDPE Pipe Pressure Rating And Testing: How To Read SDR, PN And Wall Thickness Charts
Oct 12, 2026
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PN, SDR and wall thickness are three views of the same number. This guide shows how to read an HDPE pipe pressure rating chart, how temperature changes the answer, and how to run and document the hydrostatic test that proves a line or a joint is fit for service.
Three Numbers That Describe The Same Pipe
Almost every question a buyer sends us about pressure comes down to one confusion: PN, SDR and wall thickness look like three independent specifications, but they are three views of a single geometric fact. Fix any one of them and the other two follow. Once you see that, reading a pressure rating chart stops being guesswork.
PN - nominal pressure
PN is the allowable working pressure, expressed in bar, that the pipe is designed to withstand continuously for 50 years when it carries water at 20 °C. PN16 does not mean "bursts at 16 bar". It means the pipe can be operated at 16 bar, 24 hours a day, for half a century, at reference temperature. The actual burst pressure of a PE100 pipe is roughly three times its PN, which is where the safety margin comes from.
SDR - standard dimension ratio
SDR is the outside diameter divided by the wall thickness: SDR = dn / t. A lower SDR means a thicker wall and a higher pressure class. It is the number to quote when you order fittings, because an electrofusion fitting is moulded for one specific SDR - a coupling made for SDR11 will not fuse correctly onto SDR17 pipe of the same nominal diameter.
t - wall thickness
Wall thickness is what the mill actually produces and what you measure on site. It carries a tolerance, so the specification that matters for pressure is SDR, not a single micrometer reading. When a supplier quotes "PE100 dn110" without an SDR, the quotation is incomplete - dn110 exists from SDR7 up to SDR41, and the pressure class varies by a factor of five across that range.
From SDR To PN: One Formula
Every pressure class in a PE catalogue comes from the same short equation, taken from EN 12201, ISO 4427 and EN 1555:
Run the numbers for the classes you meet most often and the whole family falls into place:
PE100, SDR11
20 × 8.0 ÷ (11 − 1) = 16 bar → PN16. The high-pressure workhorse for industrial lines, mine slurry and above-ground mains.
PE100, SDR17
20 × 8.0 ÷ (17 − 1) = 10 bar → PN10. The default specification for municipal water distribution and gas networks in most markets.
PE100, SDR26
20 × 8.0 ÷ (26 − 1) = 6.4 bar → PN6.3 (the rounded catalogue value). Low-pressure gravity and outfall duty where flow capacity matters more than head.
PE100 Pressure Rating And Wall Thickness Chart
Wall thickness in millimetres, following the dimensional tables of EN 12201-2 and ISO 4427-2. Use it to sanity-check a quotation: if a supplier offers dn110 SDR11 with a wall thinner than 10.0 mm, the offer is not what the label says. Confirm the values against the edition your project specifies, and note that each SDR needs its own moulded fitting series.
| dn (mm) | SDR11 - PN16 | SDR17 - PN10 | SDR21 - PN8 | SDR26 - PN6.3 |
|---|---|---|---|---|
| 32 | 3.0 | 2.0 | – | – |
| 50 | 4.6 | 3.0 | 2.4 | – |
| 63 | 5.8 | 3.8 | 3.0 | 2.5 |
| 90 | 8.2 | 5.4 | 4.3 | 3.5 |
| 110 | 10.0 | 6.6 | 5.3 | 4.2 |
| 160 | 14.6 | 9.5 | 7.7 | 6.2 |
| 200 | 18.2 | 11.9 | 9.6 | 7.7 |
| 250 | 22.7 | 14.8 | 11.9 | 9.6 |
| 315 | 28.6 | 18.7 | 15.0 | 12.1 |
| 400 | 36.3 | 23.7 | 19.1 | 15.3 |
| 500 | 45.4 | 29.7 | 23.9 | 19.1 |
| 630 | 57.2 | 37.4 | 30.0 | 24.1 |
The full range up to dn1600, with every SDR class and the matching electrofusion, butt fusion and saddle fitting series, is published in our HDPE pipe fittings specifications library - 65 tables covering wall thickness, pressure rating and fitting dimensions by product family.
Why PN Drops Above 20 °C
Polyethylene softens as it warms, so the allowable stress falls. Every pressure class in the catalogue is referenced to 20 °C, and a line running hot water, industrial effluent or a buried main in a hot climate cannot be operated at the published PN. Apply the derating factor first, then subtract your surge allowance.
| Operating temperature | Derating factor (PE100, water) | SDR11 - PN16 becomes | SDR17 - PN10 becomes |
|---|---|---|---|
| 20 °C | 1.00 | 16.0 bar | 10.0 bar |
| 25 °C | 0.93 | 14.9 bar | 9.3 bar |
| 30 °C | 0.87 | 13.9 bar | 8.7 bar |
| 35 °C | 0.80 | 12.8 bar | 8.0 bar |
| 40 °C | 0.74 | 11.8 bar | 7.4 bar |
| 45 °C | 0.67 | 10.7 bar | 6.7 bar |
| 50 °C | 0.61 | 9.8 bar | 6.1 bar |
Typical factors for water service. Always confirm the values in the edition of EN 12201-2 or ISO 4427-2 that your project calls up, and check whether the client specification applies an additional design coefficient on top - many water utilities require a further 1.25 to 1.5 safety factor against surge.
Two Different Pressure Tests - Do Not Confuse Them
The single most common misunderstanding we see in procurement documents is a buyer asking the mill for a "pressure test report" when what the site actually needs is a field hydrostatic test, or the other way round. They prove different things and use different pressures.
Factory qualification test
This proves the compound and the pipe geometry. It is a type test run on pipe sections, not on a finished joint, and it is how PE100 is qualified at all. Under ISO 1167 a PE100 pipe is held at:
• 20 °C / 100 h at a hoop stress of 12.4 MPa
• 80 °C / 165 h at 5.5 MPa
• 80 °C / 1000 h at 5.0 MPa
Ask for the certificate for the compound batch, not just a photo of a burst test. The three points above are what separate a genuine PE100 resin from a down-graded or over-filled compound.
Site hydrostatic test
This proves the installed line and every joint in it. It is normally run at 1.5 times the nominal pressure and held for one hour, with the pressure recorded at intervals. The sequence that avoids false failures on PE is:
1. Soak. Fill slowly at a velocity below 0.5 m/s and leave the line to equalise for 12 to 24 hours so that dissolved air comes out and the pipe stops expanding.
2. Pre-test. Raise to about 0.5 × PN for ten minutes and re-tighten nothing. Simply confirm the line holds without a visible leak.
3. Test. Raise gradually to 1.5 × PN and hold for one hour, logging pressure every ten minutes.
4. Allow for creep. A short drop and recovery in the first minutes is normal viscoelastic behaviour of PE. Add water to restore the test pressure before starting the clock rather than declaring a failure.
5. Record. Log ambient temperature, water temperature, test pressure, duration, make-up water volume and the result at each joint.
Gas networks work differently
Gas lines are not hydrostatically tested - water in a gas main is a problem, not a proof. Gas networks under EN 1555 and ISO 4437 are commissioned with a pneumatic strength and tightness test at a pressure set by the network operator, often with air or nitrogen, and with a longer stabilisation period because gas compresses. If your project is a gas distribution network, ask for the pneumatic test procedure and the operator's maximum operating pressure (MOP) rather than a water test pressure.
What A Pressure Test Report Must Contain
A report that says "test passed" is worth nothing at handover. Check that yours carries every one of these items before the line is signed off.
| Item | Why it matters |
|---|---|
| Line identification | Pipe dn, SDR and material grade for each section, so the test can be tied to the delivery certificates |
| Test pressure and duration | Proof that 1.5 × PN (or the specified value) was actually reached and held for the full hour |
| Pressure log | Readings at fixed intervals, not a single start-and-finish pair |
| Make-up water volume | The real leak indicator on a PE line - a small steady top-up is normal, a rising one is not |
| Temperature record | Both ambient and water, because a 10 °C swing moves the pressure reading on a long line |
| Joint traceability | Fusion barcode or welding-machine record for each joint tested, so a failure can be traced back to the operator and the fitting batch |
| Instrument calibration | Gauge and logger certificates, valid on the test date |
| Result and signature | Named supervisor, date, and the acceptance criterion that was applied |
Five Mistakes That Fail A Pressure Test
1. No soak period
Filling and pressurising in the same shift leaves dissolved air in the line. The pressure then drifts down as the air dissolves and the operator reads it as a leak. Soak first.
2. Testing at the wrong pressure
Using 1.5 × the working pressure instead of 1.5 × PN is common and under-tests the line. On a derated hot-water main, the correct figure is 1.5 × the derated allowable pressure, not the catalogue PN.
3. Thrust and anchor blocks omitted
On a buried PE line the joints and bends move under test pressure unless the backfill is compacted and the anchor points are in place. A joint that opens under test was usually not restrained, and the line may be undamaged.
4. Reading the gauge at the wrong end
On a long rising main the static head adds to the reading at the low point. The test pressure must be referenced to the highest point of the section, with the head subtracted where the gauge sits below it.
5. Retesting a failed joint instead of cutting it out
A joint that leaked once has already been compromised. On PE, the correct repair is to cut out the joint and refit with a new fitting, or to add a repair saddle over a damaged section. Bringing the pressure back up on the same joint to "see if it holds" simply moves the failure to commissioning.
Frequently Asked Questions
The six questions buyers and site engineers ask us most often about pressure ratings.
Is PN the same as the maximum working pressure?
No. PN is the allowable pressure at 20 °C with a 50-year design life. The maximum working pressure in your system is PN multiplied by the temperature derating factor, and then reduced further for surge or water hammer. On a 40 °C line, PN16 pipe gives an allowable pressure of about 11.8 bar before you allow for surge.
Can a fitting with a lower rating be used on a higher-pressure line?
Never. The pressure rating of a fusion fitting, saddle or valve must be at least equal to that of the pipe it is joined to. A joint is only as strong as its weakest component, and short-term test pressure will not reveal a fitting that is simply not made for the class.
How long should a site pressure test be held?
One hour at 1.5 × PN is the usual specification for water service, following a soak and a low-pressure pre-test. Longer holds are sometimes required by the client, but the extra time does not usually reveal anything the first hour did not - it mainly tests the air content of the water.
Why does the pressure drop during the test even though there is no leak?
Polyethylene is viscoelastic. Under sustained pressure the pipe wall creeps slightly outward, the volume of the line increases and the pressure falls. A short drop in the first minutes followed by a stable plateau is normal. A continued fall with a rising make-up water volume is a leak.
Does a higher SDR class mean a worse pipe?
No, it means a different duty. SDR26 pipe at PN6.3 is a perfectly good product for a gravity sewer or a low-head irrigation main. It simply cannot be used where 10 bar is required. The mistake is choosing by diameter alone and assuming that a thicker wall is always better value - it costs more, reduces the bore and lowers the flow.
What documents should come with the pipe and the fittings?
For each batch: a material certificate naming the compound and its grade, a dimensional inspection record against the SDR ordered, and the ISO 1167 hydrostatic test result for the compound. For each fusion fitting: the traceability code, the welding parameters for the machine being used, and the fusion indicator if one is fitted. Together these are what make a pressure test result meaningful - they prove what was tested.
Related Reading
Need Fittings Rated For Your Line Pressure?
We mould electrofusion, butt fusion and saddle fittings for PE100 lines from dn20 to dn1200, with pressure ratings matched to your pipe SDR and the test documentation your project needs. Send us the dn, the SDR and the standard you are working to, and we will come back with a specification and a quotation.
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