PPR Pipe Sizes & Pressure Ratings: DN, OD and PN Explained

The single most common mistake buyers make reading PPR pipe sizes is assuming DN is the inner diameter. It isn't. For PP-R, DN is the outer diameter in millimeters — DN20 is 20mm across the outside, DN32 is 32mm, and the bore inside is whatever's left after the wall. Get that one fact wrong and you order the wrong size, or worse, the wrong fittings.

The second thing nobody spells out: the pressure class (PN) changes the wall thickness, not the outer diameter. A DN25 PN16 pipe and a DN25 PN25 pipe are both 25mm on the outside — the PN25 just has a thicker wall and a narrower bore. That's why the same DN always mates with the same fittings no matter which pressure class you run.

DN, OD and Inches: What the Numbers Mean

DN stands for "diameter nominal." On metal pipe it roughly tracks the bore, which is where the confusion starts — because on PP-R, DN is the outer diameter. A DN20 PP-R pipe measures 20mm on the outside. The inch equivalents are approximate trade sizes, not exact conversions: DN20 is sold as ¾", DN25 as 1", DN32 as 1¼".

Every compliant pipe prints its own spec down the side. A marking like "PN25 DN32 x 5.4mm" reads as: pressure class PN25, 32mm outer diameter, 5.4mm wall thickness. From those three numbers you can work out the bore (32 minus two walls) and confirm you got what you ordered. If a pipe has no clear printed marking, treat that as a red flag before anything else.

PPR Pipe Size Chart

The diameters that move on most water-supply jobs, with the trade-size equivalent and where each one typically lands.

Outer diameters and wall thicknesses are fixed by ISO 15874-2 and DIN 8077/8078, so a DN25 from one compliant factory has the same outside as a DN25 from another. That's what lets fittings and pipe from a properly made system go together. It is not a license to mix random brands — quality and wall class still have to match, as the PPR fittings guide covers.

Pressure Class (PN): Pick the Wall, Not Just the Diameter

PN is the pressure the pipe holds at 20°C. Because capacity drops as water gets hotter, the class you pick depends on whether the line is cold, hot, or a demanding hot riser.

For most homes, PN20 on hot lines and PN16 on cold covers it. Reach for PN25 where the system runs hot and tall. The point to hold onto: because PN only changes the wall, you can step a line up to a higher class for a hot riser and still use the exact same DN-matched fittings.

How to Choose the Right Size

Sizing is about flow, not guesswork. Too small and fixtures starve when two taps open at once; too big and you've paid for pipe and fittings you don't need.

Size the mains first, then branch down

Start at the incoming main and work outward. A DN25 main feeding DN20 branches to individual taps is the standard residential pattern. Sizing the other way — picking branch sizes first and adding them up — usually leaves a main that bottlenecks the whole system.

Match diameter to flow demand

As a rough guide, a DN20 line carries about 15-20 L/min comfortably, while DN32 handles 40-50 L/min without noticeable pressure loss. A two-bathroom house typically runs DN25 for the main distribution and DN20 for the drops. More simultaneous fixtures or longer runs push you a size up to cover friction loss.

Oversize one step for the future

If a building might add bathrooms, a water heater, or an extension later, step the mains up one size now. Going from DN25 to DN32 on the main costs little at install and saves tearing open walls when demand grows. For long exposed hot runs, pair the sizing decision with the right pipe type — covered in the PPR vs PPRC comparison.

Conclusion

PPR sizing comes down to two ideas: DN is the outer diameter, and PN sets the wall, not the OD. Hold those and the chart stops being intimidating — you read the print on the pipe, match the diameter to the flow, and pick the pressure class for the temperature.

When you build an order, list each run's diameter and pressure class, then confirm the printed spec on arrival matches what you bought. That single check catches most of the sizing and quality problems before the pipe ever goes in the wall.

Full PPR Pipe Dimension Chart: Wall Thickness and Weight by PN Class

The size chart above tells you the outside diameter. What actually goes on a purchase order — and what changes your freight weight, flow, and price — is the wall thickness, and that is set by the pressure class, not the diameter. Under ISO 15874-2 and DIN 8077/8078 the outside diameter (OD) is fixed for each DN, while the wall thickness steps up as you move from PN12.5 to PN25. The table below gives the standard wall in millimetres for every common size and the approximate pipe weight in kg per metre, which is what determines how many metres fit in a container.

Walls follow the standard SDR (dimension ratio) series: PN12.5 is SDR11, PN16 is SDR7.4, PN20 is SDR6, and PN25 is SDR5, so wall thickness is simply OD divided by the SDR number. Weights are nominal for PP-R at roughly 0.905 g/cm3 and vary a few percent with resin and manufacturing tolerance. IFAN produces PPR pipe across DN20–DN160 in PN12.5 through PN25 to ISO 15874 and DIN 8077/8078, so any cell in this chart is a stocked wholesale option. For full material and testing detail see our complete PPR pipe guide.

PPR Pipe Size in mm to Inches: Exact Conversion Table

PPR is a metric product, so the OD in millimetres is the real number and the inch label is a plumbing trade size, not a literal conversion. One inch is exactly 25.4 mm, but a "1 inch" PPR pipe is 32 mm OD, not 25.4 mm, because the inch name refers to the old iron-pipe nominal bore. Use the table below to translate between what a spec sheet prints (mm) and what a fitter asks for (inches), in both directions.

The practical takeaway: when you buy or match fittings, use the mm/DN figure, because that is what mates. Ordering by the literal inch conversion is where mismatches happen. Note that 75 mm is often labelled 2 1/2" or 2 3/4" depending on the supplier's naming, so confirm the OD in mm on any cross-brand order.

Inner Diameter and Flow Capacity by Size and Pressure Class

Because the OD is fixed and the wall thickens with pressure class, the bore (inner diameter) shrinks as you go from PN12.5 to PN25. This matters for sizing: a 50 mm PN25 pipe flows less than a 50 mm PN16 pipe of the same OD. Water flows through the bore, not the OD, so the effective inner diameter is the number to check against your design flow. The table gives the calculated ID for the two most-ordered hot-water classes, PN20 and PN25, plus a practical continuous-flow band at typical velocities of 1–2 m/s.

Flow figures are guidance at cold-water velocity; on hot lines engineers usually hold velocity to about 1 m/s to limit noise and erosion, so read the lower end of each band for hot risers. If two projects both call for a 50 mm line but one is PN25 and one is PN16, the PN16 run carries more water. When in doubt, size on the bore and step up one DN rather than pushing velocity past 2 m/s. Our PPR installation guide covers how fusion depth also slightly reduces bore at each joint.

PPR Fitting Sizes and Thread Connections

Pipe and fittings share the same DN, so a DN25 elbow fuses to DN25 pipe with no adapter. Socket-fusion fittings are made across the full DN20–DN110 socket-weld range; DN125 and DN160 typically move to saddle or electrofusion. Reducing fittings let you step between sizes (for example DN32×25 or DN50×32), and transition fittings carry a metal thread to connect PPR to brass valves, meters, and manifolds. The chart below maps the common socket-fusion fitting sizes and the standard threads on transition pieces.

Male and female transition adapters use BSP (parallel or taper) threads in most export markets; confirm BSP versus NPT before ordering for North America. Metal-threaded transition fittings from IFAN use lead-free CW617N DZR brass inserts, so the pipe-to-valve joint stays potable-safe. For the full fitting range and fusion detail, see our PPR fittings guide.

Frequently Asked Questions