PPR Pipe 20, 25 or 32 mm: Sizes, PN and Fittings You Need
If you see PPR 20, PPR 25 or PPR 32 printed on a white pipe and don’t know which diameter to buy, the short answer is that the number indicates the pipe’s outside diameter, not the opening through which water actually flows. The right choice depends on the route: for the main line supplying the whole house, you generally choose the largest available diameter, 32 mm, while branches to a single tap, washbasin or washing machine use 20 or 25 mm. The pressure class (PN10, PN16, PN20 or PN25) and the pipe series (SDR6 or SDR7.4) matter just as much as the diameter, because they determine the wall thickness and, consequently, the actual inside diameter.
- The diameter on the label (20/25/32 mm) is the outside diameter; the actual inside diameter depends on the wall thickness, and therefore on the pipe’s SDR series.
- PN10/PN16/PN20/PN25 indicate the pressure and temperature the pipe can withstand, but the commercial label does not always match the value in the technical datasheet for the same product, so it’s worth checking before you buy.
- The mm-to-inch pairings (20≈1/2″, 25≈3/4″, 32≈1″) are a commercial convention, not a mathematical conversion; confirm the exact thread specified on the fitting or tap you choose.
- Exposed hot-water or heating runs benefit from a pipe with a fibreglass core, which expands much less than a standard PPR pipe.
What PPR 20, 25 and 32 actually mean
The numbers 20, 25 and 32 on a PPR pipe indicate its outside diameter, which can be measured with a caliper at the end of the pipe, not the diameter of the opening through which water flows. The wall thickness, which determines how large the inside opening remains, depends on the pipe’s SDR series (the ratio between the outside diameter and the wall thickness), not just the number printed on the label. So two pipes both sold as “32 mm” can have visibly different inside openings if one is SDR6, with a thicker wall and higher pressure rating, and the other is SDR7.4, with a thinner wall.
Outside diameter vs. actual inside diameter
The difference matters most with a thick-walled pipe used on the main line, where a smaller inside opening over a long length can noticeably reduce the flow available at taps at the end of the installation. The following table gives real examples, verified against a manufacturer’s technical datasheet, for the two series most commonly found on the Romanian market.
Size table: outside diameter, wall thickness, inside diameter
The examples below come from a manufacturer’s technical datasheet, cited beneath the table, and show how the same outside diameter produces a different inside opening depending on the series.
| Series | Outside diameter | Wall thickness | Inside diameter (approx.) |
|---|---|---|---|
| SDR6 | 20 mm | 3.4 mm | ≈13.2 mm |
| SDR6 | 25 mm | 4.2 mm | ≈16.6 mm |
| SDR6 | 32 mm | 5.4 mm | ≈21.2 mm |
| SDR7.4 | 20 mm | 2.8 mm | ≈14.4 mm |
| SDR7.4 | 32 mm | 4.4 mm | ≈23.2 mm |
Source: VALDuotherm GREEN PPR Pipe D. 32x5.4 mm SDR6 and VALDuotherm Grey PPR Pipe D.20x2.8mm SDR7.4.
Why inside diameter matters for flow
At the same outside diameter, SDR6 has a thicker wall and withstands higher pressures, but leaves a smaller inside opening than SDR7.4. On a short branch to a single tap, the difference is rarely noticeable; on a long main run supplying several bathrooms at the same time, a smaller inside opening can mean a noticeable pressure drop at the end of the installation. When comparing offers from different manufacturers, ask for the full technical table, not just the outside diameter on the label.
PN10, PN16, PN20, PN25: what pressure and temperature each can withstand
The PN class, from “nominal pressure”, indicates the working pressure the pipe can withstand at a given safety factor, and decreases as the water temperature rises. In an actual technical datasheet for the SDR6 series, the pipe is rated PN20 (safety factor 1.5) or PN25 (factor 1.25) for cold water, and PN10 for hot water in class 1, up to 60°C, and PN8 for class 2, up to 70°C. For the SDR7.4 series, the same logic gives PN16 or PN20 for cold water and PN8 and PN6, respectively, for hot water.
One useful detail to check before you buy: for two Wavin products inspected directly, the page’s commercial title said “PN10”, but the technical table on the same page gave a pressure class of PN12.5; for another product, the title said “PN16”, while the technical table gave PN20. This doesn’t mean the label is always wrong, but the example shows why it’s worth opening the datasheet for the exact product you’re buying, rather than relying only on the number in the listing title.
Inch equivalents: why 20 mm does not mean exactly 1/2 inch
When connecting a welded PPR pipe to a fitting or tap with a metal thread, sizes are expressed in inches, not millimetres. In stores, the usual pairings are 20 mm with a 1/2″ thread, 25 mm with 3/4″ and 32 mm with 1″, but this is a convention observed on actual products, not an exact mathematical conversion from the pipe’s outside diameter. One concrete example shows why checking matters: a white PPR elbow with an external metal thread is sold as 20 mm × 3/4″, so it has a larger thread than the “typical” pairing above for the same pipe diameter. Before you buy, read the exact thread specified on the packaging of the fitting or tap you choose, not just the diameter of the pipe you already have installed.
Standard PPR pipe vs. composite-fibre pipe for hot water and heating
A standard, single-layer PPR pipe expands by approximately 0.15 mm per metre for every degree Celsius increase, according to a manufacturer’s technical datasheet for a 25 mm pipe in the SDR6 series. A three-layer pipe, with a PP-R core reinforced with fibreglass between two layers of standard PP-R, lowers this coefficient to around 0.045 mm per metre per degree—more than three times less—according to the technical datasheet for the same manufacturer’s range, for a 20 mm pipe in the same series.
Thermal expansion and why it matters for exposed installations
Thermal expansion is rarely noticeable on a short cold-water run. On a long exposed hot-water or heating run—that is, one that is visible and not buried—a pipe that expands significantly can bend or put stress on the joints if it isn’t left room to move, which is why composite-fibre pipe is often preferred for such runs. For runs buried in screed or in a wall, the minimum installation depth recommended in a manufacturer’s technical datasheet is at least 1.5 times the pipe’s outside diameter, and threaded joints are never buried: they remain accessible or are protected with an expansion sleeve that accommodates movement.
PPR fitting glossary: what each name means
Besides the pipe, a PPR installation is assembled from several parts with standard commercial names that are worth knowing before you go to the store.
- The socket joins two pipe ends of the same diameter in a straight line, without changing direction.
- The elbow changes the direction of the run, usually by 90° or 45°.
- The tee creates a branch in three directions, equal or reduced—for example, a 25/20/20 mm tee, where the main run remains 25 mm and the branch is 20 mm.
- The reducer changes from one diameter to another along the same line, without a branch.
- The socket or elbow with an external or internal thread connects welded PPR to a threaded metal fitting, usually with a plastic body and a brass insert inside. One real example: a white 90° elbow with an external metal thread, 20 mm × 3/4″, a maximum operating temperature of 90°C and a working pressure between 0 and 20 bar.
- The PPR ball valve completely shuts off the flow of water; it isn’t designed for fine flow adjustment. One real model, made of brass and plastic and measuring 20 mm, is described as suitable for water and heating and used to supply taps, boilers, central heating boilers and washing machines.
- The quick-connect or repair socket joins two ends without welding, which is useful for a repair or a connection you know in advance you’ll need to take apart—the guide to hose connectors and quick-connect fittings for water describes other detachable couplings for non-permanent connections, too.
Socket with an external vs. internal thread: how to choose
The choice depends on the part you’re connecting to, not on your preference: if you’re connecting a valve, tap or boiler that already has an external thread on its body, buy a socket or elbow with an internal thread that screws onto it; if the part already has an internally threaded opening, buy the version with an external thread. Always check both the PPR diameter and the thread size in inches on the packaging, because the same diameter can come with different threads from different manufacturers, as the elbow example above shows.
Where to use each diameter: main run vs. branches
The main run supplying the whole house carries the greatest simultaneous flow, especially if the installation has several bathrooms that can be in use at the same time. That’s why it is usually sized using the largest diameter available in the range you’ve chosen, typically 32 mm. Branches running to a single washbasin, kitchen tap or washing machine need much less flow and are usually made with 20 or 25 mm pipe, which also reduces the cost and the space the pipe takes up on the wall.
A smaller inside diameter over a long length means a greater pressure drop by the last tap on the run. For an exact calculation for your installation, using its actual lengths and number of outlets, a plumber can check the run against the flow table for the pipe manufacturer you’ve chosen, which takes into account both the inside diameter and the distance covered.
Socket fusion welding: what to know before you start
For socket fusion, the end of the pipe and the fitting socket are heated simultaneously on a welding tool fitted with heads of the right size, then joined without twisting and held still until they cool. The exact heating, joining and cooling times vary depending on the diameter and wall thickness, so they must be taken from the table for the welding tool being used and the pipe’s technical datasheet, not generalised across all situations.
Common welding mistakes
The most common mistakes are welding on a surface that hasn’t been cleaned of dust or moisture; moving or twisting the joint as it cools, which weakens the weld even if it looks fine at first; and overheating a thin-walled pipe, which can narrow or partially block the inside opening right at the joint.
Pressure testing before finishing
Before you close the wall or cover the installation with screed, the run must be pressure-tested. A manufacturer’s technical datasheet confirms only that testing is done before final covering, without specifying an exact test pressure or duration, so follow the pressure and duration recommended by the manufacturer of the pipe and fittings you actually used. For continuous pressure monitoring on a filtration system once it’s installed, the guide to water pressure gauges and solenoid valves covers that topic separately. For a main run or work on the house’s riser, calling a licensed plumber remains the safe choice, especially if you’re not familiar with welding or pressure testing.
Where a shut-off valve or an automatic shut-off leak detector comes in
Hydrion does not sell PPR pipe or fittings, and the information above remains a general, neutral guide for anyone renovating their water installation, regardless of the pipe brand they choose. Where the PPR run reaches the main riser through a threaded connection, however, you can install a standard shut-off valve or an automatic shut-off leak detector, which turns off the water on its own if it detects an abnormal flow.
The product description mentions included adapters for 3/4″ and 1″ pipes, so check these sizes against the thread on the connection you’ve chosen for your PPR installation, not against the pipe’s outside diameter in millimetres. For full details on how the sensor and shut-off valve work, the guide to automatic shut-off leak detectors continues the topic beyond this short bridge. A shut-off valve or a detector of this type reduces the risk of an unnoticed leak turning into a flood, but it does not replace an installation that has been correctly fitted and tested from the outset.
In short, the diameter on the label tells you only how thick the pipe is on the outside; the SDR series, PN class and technical datasheet for the exact product you’re buying tell you everything else, from the actual inside opening to the thread on the fitting you need. Check these figures before you buy, not just the diameter on the box, and leave welding and pressure-testing the main run to a licensed plumber if you haven’t done them before.