PN vs SDR vs Schedule: What to Specify on a Pipe PO
Convert PN, SDR and Schedule using ISO 4065 pipe series S. See why Schedule 40 drops from 810 to 120 psi across sizes — and what to write on the PO.
Three quotes land for the same job. One offers PN 16. One offers SDR 11. One offers Schedule 40. A buyer who assumes these are three ways of saying the same thing is about to discover they are not — and that only one of the three tells you anything about pressure until you supply more information.
Key Takeaways
- ISO 4065:2018 defines SDR as approximately equal to diameter divided by wall thickness — a designation, not an exact ratio. An SDR 11 pipe's wall is not literally OD ÷ 11.
- ISO 161-1:2018 defines PN as "a numerical designation used for reference purposes" drawn from a preferred-number series. PN 16 is a label, not a licence to run 16 bar.
- Pipe series S is the bridge that makes the systems comparable: S = (SDR − 1) ÷ 2, and S = design stress ÷ PN.
- Schedule is not a pressure class. Schedule 40 PVC runs 810 psi at 1/8" and 120 psi at 24" — one designation, a 6.75× spread. SDR 21 holds 200 psi at every size.
- PVC has two separate ASTM standards for this: D2241 for the SDR series, D1785 for Schedules 40/80/120. That is why one material arrives quoted in two languages.
- ISO 15874-2 designates PP-R by pipe series S against a service class and carries no PN designation at all — the "PN 20" on PP-R is commercial convention.
- A pressure designation binds only when the material grade, the standard and the reference temperature travel with it on the same PO line.
Three Systems, One Wall: Why the Numbers Do Not Line Up
All three systems are describing one physical thing: how thick the wall is relative to how wide the pipe is. That ratio is what resists internal pressure. Where they differ is in what they choose to put on the label — and each made a different choice, for reasons that are historical rather than technical.
SDR states the geometry directly and says nothing about pressure. PN states a pressure and says nothing about geometry. Schedule states neither: it names a wall-thickness table inherited from steel pipe, where the wall grows with diameter on a schedule that was never designed to hold pressure constant. Put three quotes side by side and you are comparing a ratio, a rating and a table entry.
| System | What the number actually is | Does it state a pressure? | Constant across diameters? |
|---|---|---|---|
| SDR | A designation for a pipe series, approximately diameter ÷ wall | No — not until you supply a material grade | Yes, the geometry is the same at every size |
| PN | A reference designation in bar, selected from a preferred-number series | It names one — for water at 20 °C over 50 years | Yes, for a stated grade and temperature |
| Schedule | A wall-thickness table entry inherited from steel pipe practice | No — the pressure falls out of the size | No — this is the trap |
For PVC the split is written into the standards themselves. ASTM D2241 is the specification for pressure-rated pipe in the SDR series; ASTM D1785 is a separate specification covering Schedules 40, 80 and 120. Same material, two standards, two vocabularies — which is precisely why a single enquiry can come back quoted both ways without anyone having made a mistake.
One number shows how far apart the systems can drift. A nominal 4" pipe in Schedule 40 PVC is rated 220 psi, which is 15.2 bar — close enough to PN 16 that the two look interchangeable on a quote. Move to a nominal 12" pipe and Schedule 40 drops to 130 psi, or 8.96 bar, while PN 16 still means 16 bar. The same two designations that appeared equivalent at 4" are now a factor of 1.8 apart, and nothing in either document changed.
SDR: A Designation That Only Looks Like a Ratio
Almost every explanation of SDR says it is outside diameter divided by wall thickness. Try the arithmetic on a real pipe and it does not come out clean, which leaves buyers assuming they have found a tolerance problem. They have not. The standard never claimed the division would be exact.
"standard dimension ratio, SDR — numerical designation of a pipe series, which is a convenient round number, approximately equal to the ratio of the nominal outside diameter dn of a pipe to its nominal wall thickness en" — ISO 4065:2018, clause 3.5
"Convenient round number" and "approximately equal" are the standard's own words. SDR is a label for a series, chosen to be memorable, and the tabulated wall is then rounded up to a manufacturable thickness. The rounding always goes the same way — walls are rounded up, never down — so real pipe is marginally stronger than the nominal ratio suggests, not weaker.
What the rounding looks like on real pipe
Take DN 20 PP-R and run the standard's own wall formula, en = dn ÷ (2S + 1), against the wall thicknesses actually tabulated in ISO 15874-2. Every tabulated value sits at or above the calculated one:
| Pipe series S | Equivalent SDR | Calculated wall at DN 20 | Wall tabulated in ISO 15874-2 |
|---|---|---|---|
| S 5 | SDR 11 | 1.82 mm | 1.9 mm |
| S 4 | SDR 9 | 2.22 mm | 2.3 mm |
| S 3.2 | SDR 7.4 | 2.70 mm | 2.8 mm |
| S 2.5 | SDR 6 | 3.33 mm | 3.4 mm |
| S 2 | SDR 5 | 4.00 mm | 4.1 mm |
The gap is a tenth of a millimetre and it is deliberate. What matters commercially is the direction of the check: if a supplier's wall measures below the tabulated figure, the pipe is out of specification regardless of what the SDR on the print line says. The designation is the claim; the tabulated wall is the requirement.
Lower SDR means a thicker wall and more pressure capacity, because the wall is dividing a smaller number into the diameter. That direction confuses buyers who expect bigger numbers to mean stronger pipe. SDR 41 is thin-walled; SDR 7.4 is heavy-walled.
The series that are not for pressure at all
The 2018 edition of ISO 4065 left three series — S 10.5, S 13.3 and S 16.7 — available for non-pressure purposes only. A quotation that offers one of these against a pressure duty is not offering a cheaper pressure pipe; it is offering a pipe the standard does not recognise for the job. The same edition deleted the S-values and design stresses for 6.0 bar, so an older document quoting that basis is working from a superseded table.
PN: A Reference Number, Not a Working Pressure
PN looks like the friendliest of the three because it appears to answer the question directly: PN 16 means 16 bar. The standard that defines PN is markedly more careful than that.
"nominal pressure, PN — numerical designation used for reference purposes related to the mechanical characteristics of the components of a piping system. Note 1 to entry: It is a convenient number selected from the R 10 series as defined in ISO 3." — ISO 161-1:2018, clause 3.3
A reference designation selected from a preferred-number series. That is what PN is. The R 10 series is a list of rounded values used across engineering to keep catalogue numbers tidy — it is the reason pressure classes step 6.3, 8, 10, 12.5, 16, 20, 25 rather than in even increments. The number is chosen for the catalogue first and describes the pipe second.
Where the number comes from
Behind PN sits a chain of three defined quantities, each of which a buyer can ask a supplier to state:
- MRS — the minimum required strength, defined as the 97.5% lower confidence limit of the material's long-term hydrostatic strength at 20 °C and 50 years, rounded down to a preferred-number series. This is where every "at 20 degrees" caveat in the trade originates.
- C — the design coefficient, a safety factor greater than 1 covering service conditions the strength test does not represent.
- σs — the design stress, defined as MRS ÷ C, rounded down again.
Two pipes can carry the same PN and rest on different design coefficients. That is legitimate under the standards and it is invisible on the print line. It is also the reason a PN figure alone cannot be audited: without the grade and the coefficient behind it, there is nothing to check the number against.
One edition change worth knowing
ISO 161-1:2018 changed PN 6: it must now be designed using 6.3 bar, and the designation based on 6.0 bar was deleted outright. A PN 6 pipe engineered against the older 6.0 bar basis is a slightly different pipe from one engineered against 6.3 bar. For most building services the difference is academic — for a tender written against a specific edition it is not, and it is exactly the kind of detail a supplier should be able to confirm without hesitating.
The Bridge Nobody Publishes: Pipe Series S
There is a way to put SDR and PN on the same axis, and it has been sitting in a short ISO standard since 1996. Pipe series S is a dimensionless number that connects the geometry to the stress. Two formulae are all a buyer needs.
The first converts geometry to series:
S = (SDR − 1) ÷ 2 — ISO 4065:2018, clause 3.6
The second converts series to pressure — and it is the one that shows why SDR alone can never give you a pressure:
S = σs ÷ PN, where σs is the design stress — ISO 4065:2018, clause 4.2, formula (5)
Read the second formula backwards and the whole problem becomes visible. PN = σs ÷ S. The series comes from the geometry, but the pressure needs the design stress, and the design stress is a property of the resin and the safety factor — not of the pipe's shape. Geometry alone cannot produce a pressure. That is the entire reason these three systems refuse to convert into one another cleanly.
The standard's own worked grid
ISO 4065:2018 publishes the arithmetic as a table. Taking the 10 MPa design-stress row — a common value for pressure-grade material — the series required for each nominal pressure runs as follows, quoted with the standard's own decimal formatting:
| PN (bar) | S-value at σs = 10 MPa | Corresponding SDR (2S + 1) |
|---|---|---|
| 25 | 4,0000 | 9 |
| 20 | 5,0000 | 11 |
| 16 | 6,2500 | 13,5 |
| 12,5 | 8,0000 | 17 |
| 10 | 10,000 | 21 |
| 8 | 12,500 | 26 |
| 6,3 | 15,873 | 32,7 |
| 5 | 20,000 | 41 |
| 4 | 25,000 | 51 |
Notice the untidy entries. At PN 6.3 the series is 15,873 and the SDR lands on 32,7 — not a round number, which is why the catalogue SDR nearby is 32.5. This is the rounding described earlier, visible in the standard's own arithmetic.
Using it on two real quotes
Supplier A quotes SDR 11. Supplier B quotes PN 16. Are these the same wall? Convert both to series. SDR 11 gives S = (11 − 1) ÷ 2 = 5. For supplier B, S = σs ÷ PN, so at a 10 MPa design stress S = 100 bar ÷ 16 bar = 6.25 — a thinner wall than supplier A is offering. But change supplier B's design stress to 8 MPa and the same PN 16 now demands S = 5, identical to supplier A.
The comparison flipped without either pipe changing. That is the practical lesson: you cannot compare a PN quote against an SDR quote until the design stress is on the table. Asking for it is a one-line email, and a supplier who cannot answer has told you something useful.
Schedule: The One That Changes Pressure as the Pipe Gets Bigger
Schedule is the system most often mistaken for a pressure class, and it is the one where that mistake costs the most. Schedule 40 is a wall-thickness series carried over from steel pipe. The wall does grow as the pipe gets larger — just not fast enough to keep the pressure rating level.
Published manufacturer engineering data makes the consequence unambiguous. Across one Schedule 40 PVC table, the maximum working pressure at 73 °F falls as the pipe grows:
| Nominal size | Schedule 40 (psi) | Schedule 80 (psi) | SDR 21 / Class 200 (psi) |
|---|---|---|---|
| 1/2" | 600 | 850 | 200 |
| 1" | 450 | 630 | 200 |
| 2" | 280 | 400 | 200 |
| 4" | 220 | 320 | 200 |
| 8" | 160 | 250 | 200 |
| 12" | 130 | 230 | 200 |
| 24" | 120 | 210 | 200 |
The Schedule 40 column falls from 600 psi to 120 psi across this range, and taking the full table from 1/8" (810 psi) to 24" (120 psi) the spread is 6.75×. One designation, seven very different pipes. The SDR column does not move at all — the class tables are headed "all sizes", because holding the diameter-to-wall ratio constant is precisely what holds the pressure constant.
The failure this causes. A specification written as "Schedule 40 throughout" on a system with a 150 psi working pressure is satisfied at 4" (220 psi) and violated at 12" (130 psi) — by the same designation, from the same supplier, on the same purchase order. Nothing in the paperwork flags it. The large-diameter runs are usually the mains.
Two more conditions travel with those numbers and are easy to lose. They are stated at 73 °F, and they already include a long-term 2:1 safety factor — so the figures are not a burst pressure to be divided down again. Published data for the same pipe puts it at 88% of its rating at 80 °F and 62% at 100 °F, which matters wherever pipe runs above ground in a hot climate. Temperature derating is a subject in its own right; our guide to PP-R pressure and temperature derating works through it by service class.
When Schedule is the right answer
None of this makes Schedule wrong. For small-bore industrial and irrigation work in North American practice it is the established language, fittings are stocked against it, and at 2" and below Schedule 40 comfortably out-rates most metric classes. The error is not using Schedule — it is reading it as a pressure class and assuming it holds across a size range.
PP-R Is the Awkward Case: Sold as PN, Standardised as S
PP-R is sold worldwide as PN 20 and PN 25. Open ISO 15874-2, the product standard for PP-R piping systems, and there is no PN designation for the pipe anywhere in it. The dimension tables are indexed by pipe series — S 8, S 6.3, S 5, S 4, S 3.2, S 2.5, S 2 — and the permissible series is set against a design pressure and a class of service conditions.
This is not a technicality. The standard's approach recognises something the PN label cannot express: the same wall is adequate at different pressures depending on how hot the system runs and for how long. Its table of maximum calculated series values for PP-R shows the same design pressure demanding a different wall by application class:
| Design pressure pD | Class 1 | Class 2 | Class 4 | Class 5 |
|---|---|---|---|---|
| 4 bar | 6,9 | 5,3 | 6,9 | 4,7 |
| 6 bar | 5,0 | 3,5 | 5,5 | 3,2 |
| 8 bar | 3,8 | 2,6 | 4,1 | 2,4 |
| 10 bar | 3,0 | 2,1 | 3,3 | 1,9 |
Read the 10 bar row across. Class 2 needs S 2,1 while Class 4 permits S 3,3 — a materially thinner wall at the same pressure, because the service conditions differ. A "PN 10" label collapses that distinction to a single number and discards the class information entirely.
There is also a floor that overrides the arithmetic. ISO 15874-2 requires a minimum wall of 2.0 mm for pipes intended to be joined by fusion, whatever the series calculation returns. That is why the thin series in small diameters all sit at 1.8 to 2.0 mm instead of thinning proportionally — a jointing requirement beats a geometry rule, because a socket-fusion joint needs material to melt.
The position, stated plainly. PN marking on PP-R is commercial convention rather than the product standard's designation — and it is universal, including on our own pipe. The practical response is not to fight the convention but to require both: the PN class the market speaks, and the series or wall thickness plus the service class that the standard actually binds. One line on the PO carries all three.
The PN ladder the PP-R market actually uses runs PN 10 at S 5 (SDR 11), PN 16 at S 3.2 (SDR 7.4), PN 20 at S 2.5 (SDR 6) and PN 25 at S 2 (SDR 5). Those pairings are consistent across catalogues and they are arithmetically sound: PN 20 at S 2.5 puts 3.4 mm of wall on a DN 20 pipe, which is exactly what the ISO table gives two sections above. What they are not is a designation the product standard makes. The ladder rests on a design stress that ISO 15874-2 never prints beside a PN number, which is why the same PN can land on a different series once the material grade or the class of service changes — and why a PN on its own still does not tell you what wall you are buying. Put the series and the wall in millimetres on the line next to the PN, and the ladder stops being load-bearing.
Before the Diameter: IPS and DN Are Different Pipe
Every comparison above assumes both quotes describe the same diameter. Often they do not, and the mismatch is easy to miss because both documents will say "4 inch" or something close to it.
Nominal sizes are not outside diameters. Published dimension data for a single nominal 4" pipe gives four different actual outside diameters depending on the sizing family it belongs to:
| Sizing family | Actual OD at nominal 4" | Typically used for |
|---|---|---|
| IPS (Iron Pipe Size) | 4.500 in | Schedule and SDR pressure pipe |
| PIP (Plastic Irrigation Pipe) | 4.130 in | Agricultural irrigation |
| PSM sewer | 4.215 in | Gravity sewer mains |
| AWWA C900 (cast-iron OD) | 4.800 in | Municipal water |
Manufacturer guidance is blunt about the consequence: these classifications "are not interchangeable, but can often be connected with specialty adapter fittings". Adapters exist; equivalence does not.
Metric pipe adds a further gap. The ISO nominal outside diameter series is a fixed list — 20, 25, 32, 40, 50, 63, 75, 90, 110, 125, 160 mm and upward — so DN 110 means an outside diameter of 110 mm, which is 4.331 in. Against IPS 4" at 4.500 in that is a difference of nearly 4.3 mm on the outside diameter. Neither fittings nor gaskets will bridge it, and no pressure comparison between the two quotes means anything until the OD family is settled.
What to Write on the Purchase Order
A pressure designation on its own — any of the three — leaves room for a compliant delivery that is not the pipe you costed. Four elements together close that room, and they fit on one line.
| Element | Why the order is ambiguous without it |
|---|---|
| The standard and its edition | D2241 and D1785 are different pipe; ISO editions changed the PN 6 basis and retired the 6.0 bar designation |
| The material grade | Design stress comes from the resin, so the same geometry gives a different pressure on a lower grade |
| The wall: series S, SDR or a stated thickness | This is the part that is actually manufactured and actually measurable on arrival |
| The service class or design temperature | Every pressure figure quoted is tied to a reference temperature — 20 °C in ISO practice, 73 °F in ASTM |
Written out for a PP-R order, that becomes: PP-R pipe to EN ISO 15874-2, DN 32, series S 2.5 (PN 20), Class 2 service, 100% virgin PP-R 100 material. The PN stays because it is the language the market speaks and the warehouse will look for it. The series is what a caliper can check against the standard's table.
Normalising two quotes that arrive in different systems
- Settle the OD family first. IPS, metric DN, PIP and C900 are different pipe at the same nominal size. Everything downstream is void until this matches.
- Convert both offers to series S. From SDR use S = (SDR − 1) ÷ 2. From PN you need the design stress, so ask for it.
- For a Schedule offer, ignore the designation and read the psi at your actual size — then check that figure at your working temperature, not at 73 °F.
- Compare the wall thicknesses in millimetres. Once both are in series or in millimetres, the cheaper quote either has the same wall or it does not, and the reason for the price gap becomes visible.
What a supplier should be able to tell you
Some of what a first enquiry needs is not published data and has to be asked for. Our own PP-R range runs DN20 to DN160 in PN12.5 to PN25, produced to DIN 8077/8078 and ISO 15874, in 100% virgin PP-R 100 with batch certificates issued per shipment; CE and SGS are held, with SASO, SONCAP and NOM available on request for the markets that require them. The minimum order is one container and mixed sizes are accepted, so a pressure-class trial does not require a dedicated shipment. Two things we quote rather than publish, and the reason for each: pricing moves with resin cost and the size mix, so it is quoted per specification rather than per item; lead time is production plus shipping against your actual size mix and destination port. Send a size mix and a destination and both come back as numbers, not ranges. Samples are available before an order is placed.
Once the designation is settled, the remaining question is dimensional. Our PP-R pipe size chart gives wall thickness and weight by PN class across DN20 to DN160, and the HDPE pipe size and SDR chart carries the equivalent for PE100 including the SDR-to-PN mapping for that material. For the PVC side, our guide to Schedule 40 PVC covers where the schedule system genuinely fits.
The trade-off, honestly
None of the three systems is better than the others; they answer different questions and were built for different eras. SDR describes geometry that can be verified with a caliper but tells you nothing about pressure alone. PN gives a number a buyer can act on but hides the grade and the safety factor that produced it. Schedule is deeply embedded in North American practice and stocked accordingly, at the cost of a pressure rating that moves with every size step. Specify all three where they are all in play, and the pipe that arrives will be the pipe that was priced.
Frequently Asked Questions
Is SDR the same as PN?
No. SDR is geometry — the diameter-to-wall designation. PN is a reference pressure designation. They connect only through pipe series S and a design stress, so the same SDR gives a different PN on a different material grade.
How do I convert SDR to pipe series S?
S = (SDR − 1) ÷ 2, per ISO 4065:2018 clause 3.6. SDR 11 gives S 5, SDR 9 gives S 4, and SDR 7.4 gives S 3.2. Reverse it with SDR = 2S + 1.
Can I convert Schedule 40 to an SDR or a PN?
Not as a fixed equivalence. Schedule is a wall table whose pressure rating changes with diameter, so any conversion holds only at one specific size. Read the psi for your actual size, then compare.
Does a lower SDR always mean a higher pressure rating?
For one material at one temperature, yes — lower SDR is a thicker wall. Across materials it fails: the same SDR on a lower-grade resin carries a lower pressure, because design stress comes from the material.
Why does my PP-R pipe say PN 20 when the standard uses series S?
ISO 15874-2 designates PP-R by pipe series against a service class and carries no PN designation. PN marking is commercial convention. Ask for the series or wall thickness alongside it.
Is DN 110 the same as 4-inch pipe?
No. DN 110 has a 110 mm outside diameter, or 4.331 in. IPS 4" is 4.500 in. The two are different pipe and their fittings do not interchange without an adapter.
Do published pressure ratings already include a safety factor?
Yes. ASTM pressure ratings carry a long-term 2:1 safety factor and are stated at 73 °F. ISO nominal pressures rest on a design coefficient applied to the material's minimum required strength.
