PPR Pipe Size Chart: mm to Inch, Wall Thickness & BSP/NPT Threads

A drawing arrives marked 1 inch. The pipe on the shelf is marked 32. Somebody reaches for a calculator, multiplies 1 by 25.4, gets 25.4 mm, and orders 25 mm pipe. Two weeks later a container of the wrong size is sitting in a yard, and the argument about who made the mistake starts.

This page is a conversion and mating reference. It exists to answer two questions and no others: which millimetre size does an inch label mean, and which thread is on the fitting that connects it to metal. Choosing a pressure class is a different job with a different answer, and it lives in the PPR pipe sizes, SDR and PN ratings guide. This page hands you the numbers; that page decides which column you should be reading.

Why a 1 inch PPR pipe is 32 mm, not 25.4 mm

One inch is exactly 25.4 millimetres. That is a definition, not an approximation, and ISO 7-1 states it in the note under its own dimension table. So if the inch label on PP-R were a conversion, a 1 inch pipe would measure 25.4 mm across the outside. It measures 32 mm. The gap is not rounding error — it is 26% — which tells you the label is not describing the same thing the millimetre figure describes.

The inch name is inherited. It comes from iron pipe, where the trade size described the rough internal bore and the outside diameter was whatever the wall thickness made it. A 1 inch iron pipe had something in the region of a 1 inch hole through it. When plastic systems arrived, installers and merchants kept talking in the sizes they already knew, and the names stuck to the nearest equivalent plastic pipe regardless of what either dimension actually measured.

PP-R does not work that way. ISO 15874-2:2013 specifies the pipe on its outside diameter, because the outside is what a fusion socket grips. The bore is a consequence of whichever wall thickness you specify, and it changes with pressure class while the outside diameter stays put. That is the whole logic of the system: one socket size accepts every pressure class of that diameter.

Look for the inch column in the standard and you will not find one. ISO 15874-2 Table 5 lists nominal size, nominal outside diameter, mean outside diameter limits and wall thickness per pipe series. There is no inch anywhere in it. The inch label lives in the trade, alongside the standard, never inside it — which is exactly why two suppliers can disagree about what “3/4 inch” means while both being entirely correct about their millimetre figures.

• Use the inch label to talk. It is how merchants, installers and older drawings communicate, and refusing to speak it just makes you hard to deal with.
• Use the millimetre outside diameter to order. It is the only figure that is defined by a standard and measurable with a caliper on arrival.
• Never convert arithmetically between the two. The relationship is a lookup table, not a calculation, and every attempt to compute it produces a size that does not exist.

PPR pipe size conversion chart: mm, inch trade size and DN

This is the table to keep. Every millimetre figure below comes from ISO 15874-2:2013 Table 5, dimension class A — the metric series used across the 20 to 110 mm range that covers ordinary building services work. The inch column is the trade label that customarily attaches to each size; it is nomenclature, not a specification, and it is the one column here you should never put on a purchase order by itself.

The mean outside diameter columns are worth more attention than they usually get. They are the numbers you gauge an incoming delivery against, and they are not symmetrical.

Millimetre values from ISO 15874-2:2013 Table 5, dimension class A. Inch column is trade nomenclature and appears in no standard. The DN column follows the iron-pipe trade convention, where DN15 means the 20 mm OD pipe; ISO 15874-2 labels that same row DN/OD 20, because its DN number is the outside diameter. Both are in daily use, so cross-reference the standard on the millimetre OD, never on the DN number.

Notice the shape of the tolerance. Every band opens at the nominal figure and runs upward: 20.0 to 20.3, never 19.8 to 20.2. The standard permits a pipe to be slightly oversize and never permits it to be undersize, because an undersize pipe would sit loose in a fusion socket and the joint would depend on melt alone to close a gap that should not exist. Note also that the permitted band widens with diameter — 0.3 mm at 20 mm, but a full 1.0 mm at 110 mm — so a large pipe measuring 110.8 mm is conforming, not defective, and rejecting it costs you a container for no reason.

These values are live text, not a picture. Select the rows you need and paste them straight into an enquiry, a bill of quantities or a receiving checklist. If you want the whole thing as a file, use your browser’s print dialogue and choose Save as PDF — the table is styled to survive it, and a printed copy in the goods-in office beats a chart nobody can find.

Wall thickness by pressure class: what each SDR takes off the bore

Outside diameter alone does not tell you what will flow through the pipe. The same 32 mm socket accepts three quite different pipes, and the difference is the wall. This is where a conversion chart stops being a curiosity and starts affecting hydraulics: at 32 mm, moving from the thinnest common wall to the thickest takes the bore from roughly 26 mm down to roughly 21 mm.

Two vocabularies describe the same thing and you will meet both. ISO 15874 uses the pipe series S; the trade mostly says SDR. They convert exactly: SDR = 2S + 1. So S5 is SDR 11, S3.2 is SDR 7.4, and S2.5 is SDR 6. From there the wall follows from a single division: wall equals outside diameter divided by SDR. Check it on the largest row — 110 divided by 11 is 10.0, which is precisely the tabulated figure.

Minimum wall thickness (e_min) in millimetres, from ISO 15874-2:2013 Table 5, dimension class A.

One number in that table is not produced by the division, and it is worth knowing why. At 20 mm in the thinnest series the arithmetic gives 1.82 mm, but the standard tabulates 1.9 mm and adds a separate rule in clause 6.2.2: pipes intended to be joined by fusion shall have a minimum wall thickness of 2.0 mm. A fusion joint needs material to melt. Below roughly 2 mm there is not enough wall to form a reliable bead, so the standard overrides its own pressure calculation. That is why you rarely see genuinely thin-walled small-bore PP-R, and why a quotation offering an unusually thin 20 mm pipe deserves a question rather than an order.

Which of these three columns your project needs is a separate decision, driven by operating temperature, design pressure and service class rather than by the size table. That decision is set out in full in the SDR and PN ratings guide, and the service-class background sits in the EN ISO 15874 service classes explainer. Read one of those before you commit a column; this page will still be here to convert whatever you land on.

The tolerance class question, answered honestly

Buyers ask for “the tolerance class” expecting a single grade letter. The real answer has two halves that behave differently, and knowing which is which is what lets you argue a rejection properly.

The outside diameter is controlled by the mean OD limits in the first table — a plain band per size. The wall thickness is controlled separately, by ISO 15874-2 Table 9, and that tolerance is plus only. The footnote is explicit: the tolerance is expressed in the form +x mm. There is no minus figure at all. A wall may be thicker than specified and may never be thinner, which follows directly from the fact that the tabulated wall is e_min — a minimum, not a target.

The permitted plus figure steps with the wall band:

Selected bands from ISO 15874-2:2013 Table 9. The full table runs to a 33.0 mm wall.

If a datasheet answers your tolerance question with a grade instead of a number, the grade you are most likely to see is Grade V. ISO 15874-2’s Table 9 footnote states that the level of these tolerances conforms to Grade V in ISO 11922-1, which is the general dimensions-and-tolerances standard for thermoplastics pipes. So “Grade V” and “the Table 9 figures” are the same statement in different words, and a supplier citing one is not saying anything different from a supplier citing the other.

Quality control on arrival: what to check against the chart

A conversion chart is only useful if you can test a delivery against it. ISO 15874-2 Table 12 sets out the minimum marking that must be printed on conforming pipe, and it is a better receiving tool than most inspection checklists because it is on every length:

• ISO 15874 — the standard number itself.
• Manufacturer name or trade mark.
• Nominal outside diameter and nominal wall thickness, in the standard’s own example format 16 × 2,2 — this is the line that must agree with your order.
• Pipe dimension class, for example A.
• Material, for example PP-R.
• Application class with operating pressure, for example Class 1/10 bar.
• Production period and site code, so a batch can be traced.

Read the marking first, then reach for the caliper. Measure the outside diameter at several points around the circumference rather than once, because ovality from coiling or stacking shows up as a diameter that passes on one axis and fails on the other.

The useful question to put to any supplier is not do you test — everyone says yes — but which named body tested which specific item. Scopes are written per product, and the gaps between them are where a specification quietly fails. Hitze’s read as follows, and work as a template for what to demand from anyone:

• SKZ (Würzburg, Germany) testing covers PP-R pressure pipe and PP-R fittings as separate scopes — plus PE-Xb heating pipe. Ask for both pipe and fitting scopes by name: a certificate covering only the pipe leaves the fusion fitting, which is where joints actually fail, untested by that body.
• DVGW type examination for PP-R drinking-water pipe. DVGW is the German rule-setting association for the gas and water industry, so this is the credential a German or Austrian specifier will look for by name on a potable line.
• An NSF laboratory report to BS 6920, the British standard for how a non-metallic product affects drinking-water quality — run for both cold and hot contact, hot being defined here as up to 65 °C. That temperature is the number to check, because a room-temperature-only report says nothing about a hot line running Class 2.
• WRAS product approval on the PP-R and PEX lines. WRAS product approvals run to a maximum five-year validity and are revalidated on a cycle, so ask for the certificate reference and check it yourself in the public WRAS directory rather than trusting any supplier’s date, ours included.

One wording check while you are reading certificates. The Hitze trademark is registered at the DPMA in Munich, which supports German brand and engineered to German standards — it is not the same claim as “Made in Germany”, and a supplier who blurs those two on a datasheet is worth reading more carefully everywhere else.

BSP or NPT: reading the thread on a PPR transition fitting

A correctly built PP-R system has almost no threads in it. Pipe fuses to fitting, the joint becomes one continuous wall, and there is nothing mechanical inside it to age or loosen. Threads appear at exactly one place: the transition to something that is not PP-R — a boiler, a meter, a brass valve, a pump. That transition is made with a fitting carrying a brass insert, moulded into the body with a fusion socket on one side and a machined thread on the other.

Two thread families dominate that machined end, and the sizes on both are named in inches — which is the second place this subject trips people up. A 25 mm PP-R male adapter typically carries a 3/4 inch thread, but the 3/4 there refers to the thread designation, not to the pipe, and the two inch labels are unrelated systems that happen to use the same words.

ISO 7-1 is the international family, widely called BSP. It covers sizes 1/16 to 6 for joints made pressure-tight by the mating of the threads themselves, and it uses three designations you will see stamped or printed on parts: R for a taper external thread, Rp for a parallel internal thread, and Rc for a taper internal thread. The standard also carries a note worth remembering when someone offers you a parallel male fitting for a sealing duty: parallel external pipe threads are not suitable as jointing threads. If the seal is meant to be made on the thread, the male part is tapered.

NPT is the North American family, defined by ANSI/ASME B1.20.1. Same idea, different geometry. Both taper at the same rate — 1 in 16 on diameter — which is part of why they feel so similar in the hand and why the mistake is so easy to make.

ISO 7-1:1994 Table 1 major diameter at the gauge plane; ANSI/ASME B1.20.1 outside diameter converted at 1 inch = 25.4 mm.

The diameters are close enough to be indistinguishable by eye. The forms are not. ISO 7-1 defines its thread height as h = 0.640 327 P, and that constant is not arbitrary — it is exactly two thirds of 0.960 491 P, the theoretical triangle height of a 55° symmetric thread. A 60° form would give 0.866 025 P instead. So the standard’s own formula fixes the flank angle at 55°, the Whitworth form, with rounded crests and roots. NPT is a 60° Sellers form with flattened crests and roots.

Put a 55° male into a 60° female and the flanks touch at the wrong angle. The two surfaces meet along a line rather than bedding together, so there is no continuous contact band for a sealant to bridge. That is a geometric incompatibility, not a tightness problem, and no amount of PTFE tape or extra torque converts one form into the other.

The half-inch trap: where BSP and NPT quietly leak

Here is the part that costs money, and it runs against intuition: the small sizes are the dangerous ones, precisely because they are the ones that seem to work.

Look again at the pitch column. At 1/2 and 3/4 inch, ISO 7-1 and NPT both run 14 threads per inch. Identical pitch. The major diameters differ by 0.381 mm at 1/2 inch and 0.229 mm at 3/4 inch — differences you will not see and will not feel. So a mismatched pair engages smoothly, turns several times, and comes up snug against the wrench. Everything about the assembly says it is correct.

It is not correct. The flank angles still disagree by 5°, so the mating surfaces are only touching along a line, and the joint is being held by whatever sealant happens to be trapped in the gaps. It will usually pass a brief pressure test. It fails later — often after the first few thermal cycles, once the brass and the sealant have moved a little — which is the worst possible time, because by then the wall is closed and the ceiling is up.

Now compare 1 inch and above. ISO 7-1 runs 11 tpi; NPT runs 11.5. The pitches no longer match, so the threads fight each other within a turn or two and the part visibly refuses to seat. The mistake is caught on the bench by a fitter who swears at it, not on site by a facilities manager six months later. A 1/2 inch mismatch ships; a 1 inch mismatch gets returned.

How to keep it off your order in the first place

The fix is administrative rather than technical, and it takes one extra field on a line item.

• Write the designation, not the size. “1/2 inch male adapter” is ambiguous. “PP-R male adapter, 20 mm socket × R 1/2 (ISO 7-1) taper male” is not. Use R, Rp or Rc for the ISO family and NPT for the American one.
• State the destination market on the enquiry. Thread convention follows the market the goods are installed in, not the market they are made in, and a supplier who knows where the container is going can flag a mismatch before production rather than after.
• Ask what the mating part is. If the PP-R fitting joins a specific valve, meter or boiler, the thread on that device decides the answer. Send its model number with the enquiry.
• Check a physical sample before volume. A thread gauge on one real fitting settles what a datasheet leaves ambiguous, and it costs a courier fee rather than a container.
• Use a union where the two systems meet. A union seals on a gasket face and uses the thread only for clamping, which both removes the form question at the joint and lets the assembly be broken later for service.

On the third point, note what a sample actually has to prove. Gauging one adapter settles the thread form, but the joint it lands in is a brass insert moulded into a plastic body, and that interface is the one carrying a 50-year product warranty on the Hitze range. So gauge the thread, then check the insert seats square in the body and shows no moulding gap at the shoulder — a warranty on the pipe wall is not a warranty on a transition fitting that was over-torqued into a boiler.

Then read what the catalogue does not say. The PP-R pipe and fittings range was checked against this article on 29 August 2026, and it publishes the threaded fittings by size and gender but contains the strings BSP, NPT and ISO 7 exactly zero times. That absence is deliberate rather than an oversight, and it is the honest state of most PP-R catalogues: thread designation is set per order and per destination market, so confirm R, Rp, Rc or NPT explicitly on the quotation rather than reading a default into any supplier’s size table, this one included.

If you are planning the transition points themselves, the guide to connecting PP-R to metal pipe covers where to put them and how to support them. The PP-R supplier RFQ checklist covers what else belongs on the enquiry beside the thread designation.

A worked example, end to end

An importer receives a mechanical schedule for a four-storey apartment block. The riser is marked 2 inch, the floor branches 1 inch, and the final connections to the water heaters are marked 1/2 inch BSP. The building is in a market that uses ISO 7-1 threads. Working the chart from top to bottom:

• Riser, 2 inch: 63 mm outside diameter, DN50. At SDR 7.4 the wall is 8.6 mm, so the bore is about 45.8 mm. Gauge incoming pipe against 63.0 to 63.6 mm.
• Floor branches, 1 inch: 32 mm outside diameter, DN25. At SDR 7.4 the wall is 4.4 mm, giving roughly 23.2 mm of bore — not the 25.4 mm an arithmetic conversion of “1 inch” would have promised.
• Heater connections, 1/2 inch BSP: a 20 mm PP-R socket on one end, and on the other an R 1/2 taper male thread to ISO 7-1 — written that way on the line, because this is precisely the size where an NPT part would have fitted and failed.
• On arrival: read the printed marking for 63 × 8,6, the dimension class and the application class, then caliper two axes on a sample from several bundles.

The order that comes out of that exercise names three millimetre sizes, one wall series and one thread designation. Nobody downstream has to guess, and nothing in it depends on the word “inch” meaning the same thing twice.

Where this page ends and another begins

Two boundaries are worth stating plainly, because getting them wrong is how a correct chart still produces a wrong order.

The first is the SDR column. This page tells you what each wall series measures; it does not tell you which one your system needs. That depends on operating temperature, design pressure and service class, and it is genuinely a different calculation — set out in the SDR and PN ratings guide.

The second is dimension class. Everything here is class A, the metric series. ISO 15874-2 also defines a class B1 built on copper pipe sizes — 10, 12, 15, 18, 22, 28 and 35 mm — which is why a European system occasionally shows PP-R in 22 or 28 mm, and why those sizes will not fuse into class A fittings. If your drawing carries those numbers, you are in a different table and should say so on the enquiry.

One size gets misfiled here often enough to name: 16 mm. Sitting between 15 and 18 mm on a merchant’s shelf, it reads as one of the copper numbers — but it is class A, Table 5, mean outside diameter 16.0 to 16.3 mm. It is simply the size below 20 mm in the metric series, and it fuses into ordinary class A sockets.

FAQ: PPR pipe sizes in mm and inches

What to do next

Working through this in order takes about ten minutes and removes most of the ways a size conversion turns into a wrong container.

• Translate the drawing. Convert every inch label to its millimetre outside diameter using the first table. Do not calculate; look it up.
• Fix the wall series against operating temperature and pressure, using the SDR and PN ratings guide rather than this page.
• Mark every transition point and decide its thread designation — R, Rp, Rc or NPT — from the device it connects to and the destination market.
• Write the purchase line in full and ask your supplier to confirm the thread designation per size in writing before production.
• Request the manufacturer’s technical datasheet and check its declared dimensions and tolerances against Tables 5 and 9. Gauge a sample against the marking when it lands.

If you are a wholesaler, importer or stocking distributor building a PP-R range across the 20 to 110 mm sizes, the distributor and importer programme page covers how a range is put together. Bring your size mix and the markets you ship to; the thread question is easier to settle before the first order than after it.