To choose an MDPE compression fitting, match the compression end to the pipe's outside diameter, then choose the connection type you need. A modern metric 25mm MDPE pipe needs a fitting specified for 25mm polyethylene pipe. Joining two pipes, reducing the pipe size and connecting to a threaded valve each require a different fitting.
This FAQ explains how to choose fittings for modern metric MDPE (medium-density polyethylene) pipe, including couplers, reducers, threaded adaptors and pipe liners.
What size compression fitting do I need for MDPE pipe?
For modern metric MDPE pipe, the stated size refers to the outside diameter. Read the pipe markings and check the outside diameter with callipers before ordering. Do not use the internal bore measurement to choose the compression end.
- 20mm MDPE pipe: choose a compression end specified for 20mm PE pipe.
- 25mm MDPE pipe: choose a compression end specified for 25mm PE pipe.
- 32mm MDPE pipe: choose a compression end specified for 32mm PE pipe.
Size alone does not confirm suitability. Check the fitting's accepted pipe material, pressure and temperature limits, and intended application.
Which type of MDPE compression fitting should I buy?
- Join two pipes of the same size: use an MDPE compression coupler.
- Join two different metric pipe sizes: use a compression reducer, with one end matching each pipe.
- Connect to a compatible female threaded port: choose a compression male adaptor.
- Connect to a compatible male threaded port: choose a compression female adaptor.
A threaded adaptor must match both the pipe and the threaded connection. Check the thread specification and sealing arrangement as well as the nominal thread size.
How do I connect 25mm MDPE pipe to 32mm MDPE pipe?
Use a compression reducer specified for 32mm and 25mm PE pipe. Each end grips and seals around its matching pipe size; tightening a 32mm fitting further does not turn it into a 25mm fitting.
Select the 32mm x 25mm option on the compression reducer product page. Confirm the complete connection is suitable for your pipe specification and operating conditions.
What does 25mm x 3/4 inch mean on a compression adaptor?
These numbers describe two different ends. The 25mm measurement identifies the metric PE pipe connection. The 3/4 inch measurement identifies the nominal thread size.
It does not mean that 25mm pipe is interchangeable with every pipe described as 3/4 inch. It also does not mean the thread measures exactly 19.05mm across. Identify the mating thread before choosing the adaptor.
For more help with thread identification, read our BSPP vs BSPT thread guide.
Do MDPE compression fittings need pipe inserts or liners?
Use the liner required by the fitting manufacturer for your pipe and application. A liner fits inside the pipe end to provide support at the joint. Requirements vary between fitting systems, pipe sizes and uses, so do not assume that all compression fittings follow the same instructions.
T-mex Express lists Plasson pipe liners. Confirm that the selected liner suits your pipe dimensions and fitting system before ordering. Do not assume a liner is included with a fitting or that liners from different systems are interchangeable.
Can I use these fittings on old black Alkathene pipe?
Do not choose a modern metric fitting solely from an old pipe's nominal inch size. Older imperial polyethylene pipe can have different outside diameters and may require a dedicated transition fitting or conversion components.
If you are repairing older pipework, start with our old Alkathene pipe and modern metric MDPE guide. Identify the existing pipe before buying a connector.
Can the same compression fitting be used for MDPE and HDPE?
Some PE compression fittings accept both MDPE and HDPE, but compatibility depends on the particular fitting and pipe specification. Check the manufacturer's accepted pipe types and dimensions rather than assuming that matching diameters guarantee a suitable joint.
Our MDPE, HDPE and LDPE comparison explains the material differences.
Are all MDPE compression fittings rated to the same pressure?
No. Check the exact product and size. For example, our MDPE Compression Coupler listing specifies 16 bar for sizes up to 63mm and 10 bar for sizes from 75mm to 110mm. Do not apply those figures to every fitting in another range.
Confirm the manufacturer's permitted pressure at your operating temperature. The fitting rating does not increase the pressure capability of the pipe, valve or any other component in the system.
What should I check before ordering?
- Pipe identification: record the material, markings and outside diameter.
- Both connection ends: note each pipe size or the required thread size and type.
- Application: identify the fluid, working pressure and temperature.
- Product suitability: confirm any drinking-water approval needed for the exact product and use.
- Installation components: check liner requirements and the manufacturer's assembly instructions.
For example, joining two confirmed 25mm metric MDPE pipes starts with a 25mm coupler. Connecting 32mm pipe to 25mm pipe starts with a 32mm x 25mm reducer. Connecting to a valve also requires its thread details.
Where can I buy MDPE compression fittings?
Browse the T-mex Express compression fittings range to choose your connection type and size.
Need help identifying a fitting? Email sales@t-mex.co.uk with photographs of the pipe markings and connection, your outside-diameter measurements and the intended application. You can also call 01622 470047.