What diameter of press fittings do I need for my pipeline

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What diameter of press fittings do I need for my pipeline?

This question seems simple at first glance, but in practice it causes considerable problems for customers and even experienced installers. All it takes is confusing the outer and inner diameter, mixing up the pipe size and the fitting size, or failing to notice that the pipeline changes dimension at the manifold - and the result is either a leaking joint, the need to reorder parts, or in the worst case an entire pipeline that has to be redone. In this article, we'll therefore cover the topic from the very basics: what the numbers on fittings mean, how PEX/AL/PEX pipe is measured, where the size changes and where it doesn't, and how to choose the right diameter for each part of the installation.

How is the diameter of pipe and fittings marked - what do the numbers mean?

The first thing to understand: for PEX/AL/PEX pipes (and for press fittings used with them), the size is given as the outer diameter of the pipe in millimeters. So when we talk about pipe 16, we mean a pipe with an outer diameter of 16 mm. A fitting marked as 16 is shaped exactly to this outer diameter.

This is a fundamental difference compared to threaded fittings, where the size is given in inches (1/2", 3/4", 1") and refers to the inner diameter. For press fittings for PEX pipes, we always talk about the outer size of the pipe. This is exactly where a lot of misunderstandings arise, when a customer says "I have a half-inch pipe" - which can mean something completely different than expected.

Common sizes of PEX/AL/PEX pipes that you'll encounter in Slovak installations:

16 mm - standard for underfloor heating, pipelines to radiators, hot water in apartments
20 mm - risers, main pipelines in apartments, connection of multiple branches
25 mm - family houses, main pipelines from the source, sections with higher flow
32 mm - utility rooms, manifold connections, larger systems
40 mm and 50 mm - apartment buildings, industrial pipelines, less common in ordinary households

In practice, in a family house you'll most often encounter a combination of 16 and 20 mm, or 20 and 25 mm. Apartment installations are almost exclusively 16 mm.

Wall thickness and actual inner diameter - what does it affect?

The outer diameter of the pipe is decisive for selecting the fitting. But the wall thickness affects flow resistance and the capacity of the pipeline. Common sizes of PEX/AL/PEX pipes have these typical parameters:

Outer Ø (mm)	Wall thickness (mm)	Inner Ø (mm)	Cross-section (cm²)
16	2,0	12,0	1,13
20	2,0	16,0	2,01
25	2,5	20,0	3,14
32	3,0	26,0	5,31

The table shows why it is important to correctly size the pipeline: going from 16 to 20 mm doesn't just mean a 4 mm larger diameter, but nearly double the cross-section. This has a direct effect on how much water (and thus heat) a given branch can transport per unit of time.

How to determine the correct pipeline diameter?

This is the core of the whole topic. You don't choose the fitting size based on what you happen to have in stock or what seems suitable - you choose it based on the specific parameters of your installation. The key factors are:

1. Heat output and flow rate

Basic principle: the higher the required heat output of a branch (W or kW), the greater the water flow you must ensure, and thus the larger the pipe (and fitting) diameter you need. Simplified: 16 mm pipe is suitable for branches with output typically up to 4-5 kW, 20 mm pipe for branches up to 8-10 kW, 25 mm pipe for a family house as a whole (10-20 kW). These are rough guideline values - the precise hydraulic calculation also depends on temperatures and flow velocity.

2. Branch length and pressure losses

The longer the pipeline branch, the greater the pressure losses. For long branches (over 30-40 m) it is therefore sometimes necessary to use one dimension larger diameter than you would expect based on output alone. This is a typical mistake in designing underfloor heating, where pipes 80-100 m long are designed at 16 mm, even though with a smaller diameter the circulation pump would not be able to overcome the resistance.

3. System type - heating vs. sanitary water

Different rules apply to sanitary pipelines (hot and cold water) than to heating. For sanitary water, the number of outlets and their simultaneous use is decisive. For an apartment with 2 bathrooms and a kitchen, a common size is 20 mm for the riser and 16 mm for branches to individual fixtures. For a family house with higher consumption, the riser may be 25 mm.

4. Manufacturer and pipe type - compatibility

Important: PEX press fittings must fit the pipe exactly. Different pipe manufacturers may have slightly different tolerances. It is therefore always recommended to use fittings and pipes from proven manufacturers, ideally from the same system (or verified compatible systems). You can find more on this topic in the article How to choose PEX/AL/PEX press fittings: type, size and compatibility.

Sizing underfloor heating - a special case

Underfloor heating is probably the most common reason today why customers are interested in press fittings in the 16 mm size. Here are a few specific recommendations from practice:

Loops in underfloor heating fields are almost always Ø 16 mm. The length of one loop should not exceed 80-100 m (at Ø 16 mm). For longer loops, pressure losses increase disproportionately.
Supply pipes from the manifold to individual loops are also 16 mm, but the riser that supplies water to the manifold may be 20 or 25 mm, depending on the number of loops.
For a manifold with 8 or more loops (e.g. in a larger family house), a 25 mm riser is a necessity, not a luxury.
Fittings within the floor (e.g. for connecting pipes) must be of high quality and correctly pressed - inspection is difficult because they are embedded in concrete. Recommended procedure for press joints can be found in the article Installing PEX press fittings step by step: tools and procedure.

When to use a reducing fitting and when not?

A reducing fitting is a fitting that connects two pipes of different diameters. Typically this is a transition from 20 to 16 mm or from 25 to 20 mm. These fittings are used in specific situations:

Connecting a branch to the main pipeline - if the main pipe runs at Ø 20 mm and you branch off to 16 mm to a radiator or an underfloor heating loop.
Reducing diameter after the manifold - after the manifold of a family house, the outlets for individual loops are usually Ø 16 mm, even though the riser is 25 mm.
Connecting a fixture with a smaller port - e.g. a valve or fitting with a smaller inlet.

Practical example: Press elbow 90° reduced PEX/PEX (20x16) is used exactly where you need not only to change the direction of the pipeline by 90°, but also to transition from a 20 mm dimension to 16 mm in one piece. This is more elegant and hydraulically more advantageous than combining a reducer and an elbow separately. The article Reducing tees and elbows: when and why to use them in a pipeline covers this topic in more detail.

Sizes of threaded connections - how to read them?

Many press fittings have a thread on one or both sides - either internal (G - female) or external (AG - male). These threads are in inches according to the BSP (British Standard Pipe) standard. Most commonly you'll encounter:

1/2" (half inch) - standard for connecting radiators, valves, thermostatic heads, taps
3/4" - for larger fittings, manifolds, groups
1" - for boiler groups, larger manifolds, fixtures with higher flow

So if you have a Ø 16 mm PEX pipe and want to connect it to a radiator with a 1/2" internal thread, you need a fitting marked, for example, as 16x1/2" - where 16 is the outer diameter of the PEX pipe and 1/2" is the thread size on the other end. An example of such a fitting is Press elbow 90° PEX/INTERNAL THREAD (16x1/2") - ideal for connecting a side valve to a radiator, where you need to change the direction of the pipeline and switch to a threaded connection at the same time.

For straight connections (not angled), straight fittings with an internal or external thread are used. If you need a wall-mounted version (the fitting is anchored into the wall and the pipe exits perpendicularly), there is also the Press wall elbow PEX/INTERNAL THREAD (16x1/2"). The proper use of threaded fittings is discussed in more detail in the article Press fittings with internal and external thread: how to properly connect a pipeline.

Practical scenarios from installation practice

Scenario 1: Apartment renovation - new bathroom and toilet

A customer is renovating a 3-room apartment. The hot and cold water pipeline enters the apartment at Ø 20 mm (riser). Inside the apartment, it splits to individual fixtures: bathtub, sink, toilet, kitchen mixer. All branches are at Ø 16 mm. For the tees splitting the flow, a 20×16×16 tee is needed (main branch 20, branch 16). Each fixture connection is handled with an elbow or a straight fitting with an internal thread 16×1/2".

Scenario 2: New family house construction - two-circuit heating

The house has an 18 kW boiler, two floors. The supply from the boiler runs at Ø 25 mm to the utility room. Here there is a manifold for 6 underfloor heating loops (Ø 16 mm) and 2 circuits for radiators upstairs (Ø 20 mm). The manifold connection to the supply is Ø 25 mm, the manifold outlets are 20 mm for radiator circuits and 16 mm for underfloor heating. The fittings for connecting radiators are 90° elbows with an external thread 16×1/2" or 20×3/4", depending on the specific radiator. An example is the Press elbow PEX/EXTERNAL THREAD (16x1/2"), which is suitable for connecting to valves with an internal thread.

Scenario 3: Staircase pipelines in an apartment building

An apartment building has 6 apartments, each apartment is supplied from the main riser Ø 32 mm. Branches to apartments are at Ø 20 mm. Inside each apartment, they further split to Ø 16 mm. Here it is important to correctly size each level and correctly choose tees with reducers - the most common mistake is that the whole pipeline is built at 16 mm on the assumption that "it's enough after all," but then the circulation pump can't keep up with supplying the last apartment in line.

Scenario 4: Gardener and summer pipeline

A fairly unusual but real case - a gardener is building a summer cold water pipeline for irrigation and washing tools. A 16 mm PEX/AL/PEX pipe is more than sufficient for this purpose. Only simple fittings are needed - couplings, tees and an end cap. A diameter of 16 mm is the right choice; an unnecessarily larger diameter would only make the project more expensive without any practical benefit.

How to measure an existing pipe when you don't know the size?

This is a situation we encounter very often. A customer comes with a piece of old pipeline and doesn't know its diameter. The procedure is simple:

Calipers - place them against the outside of the pipe and read the outer diameter in mm. That's the number you're looking for. If you measure a pipe and get, say, 15.9 or 16.1 mm, it's a Ø 16 mm pipe (normal manufacturing tolerances).
Plastic gauges - some fitting manufacturers supply simple plastic templates through which you slide the pipe to determine the size.
Comparison with a fitting - if you have a fitting available, you can hold the pipe up to the press socket. The pipe should slide into the fitting's socket freely, but without slack.

Never measure the inner diameter as the size for selecting a fitting! A fitting is sized according to the outer diameter of the pipe, not the inner one. Customers who measured the inner diameter of the pipe and ordered a fitting based on it received the wrong fitting and had to reorder.

Most common combined sizes in practice

Based on experience from real installations, we can list these most common combinations that appear in orders:

Installation type	Main pipeline	Branches	Fixture connection
Apartment in a panel building	20 mm	16 mm	16 × 1/2"
Family house - heating	25 mm	20 mm	16 × 1/2" or 20 × 3/4"
Underfloor heating	20-25 mm	16 mm (loops)	16 mm to manifold
Apartment sanitary pipeline	20 mm	16 mm	16 × 1/2"
Apartment building - riser	32-40 mm	20-25 mm (apartments)	depending on fixture

What if you're not sure - how to proceed?

If you're installing a new pipeline and are not sure what diameter to choose, we recommend this procedure:

Find out the heat output or consumption - from the boiler, from the project, from an energy specialist. Without this number, sizing is not possible.
Count the number of outlets or circuits - how many radiators, how many underfloor heating loops, how many fixtures in the sanitary pipeline.
Draw a diagram of the pipeline - even a simple one, but marking where the pipeline branches and where it runs straight.
Start from the source - from the boiler or riser, always use a larger diameter and gradually reduce it toward the outlets.
When in doubt, choose one dimension larger - an oversized pipeline works, an undersized one causes problems with output and noise.

The basic rule states: a diameter that's too small is a bigger mistake than one that's too large. Too small a diameter causes high flow velocity (above 1 m/s it's noisy and erosive), large pressure losses and the inability to achieve the required output. Too large a diameter is a bit more expensive, but functional.

Couplings, elbows, tees, reducers - how the size affects the choice of fitting type

The pipe diameter directly determines what type of fitting you'll use at a given point. It's not enough to know just the size - you also need to know the function of that point in the pipeline. For more information about types of fittings, we recommend the article Elbow, tee or reducer: which fitting belongs where in an installation.

In short: for a straight section you need a coupling, for a change of direction an elbow (also available at various angles, but 90° is the most common), for a branch a tee, and for a transition between sizes a reducer - or a reduced version of an elbow or tee, if you need to change both direction and size at the same time. The basic fitting for a straight connection of the same diameter, for example Press elbow 90° PEX/PEX (16), is ideal for all corner turns in Ø 16 mm pipelines - the most popular type of all.

Inspection, pressure tests and service

After installing the pipeline, it is mandatory to perform a pressure tightness test. For PEX/AL/PEX pipelines, a water test is standardly performed at 1.5 times the working pressure (usually 6-9 bar for pipelines with a working pressure of 4-6 bar) for at least 30 minutes, ideally longer. Correctly choosing the fitting diameter is a precondition for the pressing to be done correctly and for the joint to be durable and tight. Symptoms of an incorrectly selected or pressed fitting, as well as the procedure in case of suspected leakage, are described in the article Inspection and service of press joints: what to do if leakage is suspected. Typical installation mistakes are covered in Common mistakes when pressing PEX fittings and how to avoid them.

Frequently asked questions (FAQ)

How do I find out what pipe diameter I have in my pipeline if I don't know the manufacturer?

The most reliable way is to measure the outer diameter with calipers. The outer diameter is decisive for selecting the fitting. Common sizes in Slovak households are 16, 20 and 25 mm. If the measured number doesn't match any standard (e.g. 18 mm or 22 mm), it may be copper or plastic pipes of a different system - in that case you need to verify whether they are even compatible with PEX press fittings.

Can I use a 20 mm fitting on a 16 mm pipe if I add a gasket or wrap the pipe?

No, under no circumstances. Press fittings are sized for the exact outer diameter of the pipe. Any padding or wrapping of the pipe will cause the press tool to fail to create a proper joint, and the joint will be neither tight nor mechanically strong. Each pipe diameter requires its own fitting.

Why does my diagram show 16 mm, but the installer suggests 20 mm?

The installer is probably considering the specific installation conditions - the length of the run, the number of outlets, working pressure or required flow. If the branch length is longer (over 30-40 m) or if additional circuits branch off from that branch, 16 mm may not be sufficient. Ask the installer to explain the specific reason and verify it, or consult the project with a building services designer.

Can I combine fittings from different manufacturers at the same diameter?

Theoretically yes, if both manufacturers meet the same standards (typically EN 1254-2 and EN ISO 15875). In practice, however, it's always safer to use fittings and pipes from a verified combination - either the same manufacturer, or a combination explicitly certified by the manufacturer. Different tolerances can cause imperfect pressing. The installer bears responsibility for the correctness of the joint.

Is the pipe diameter the same for heating and for drinking water (sanitary)?

Pipe diameter doesn't depend on the medium (heating water vs. drinking water), but on the required flow and pressure. For the same flow rates, the sizes are the same. What differs is the pipe type - for drinking water, the pipe must be certified for contact with drinking water (DVGW approval or similar). Most quality PEX/AL/PEX pipes meet this, but always check the certificate.

What if I need to transition from a PEX pipeline to copper or steel?

Transition fittings with a thread (internal or external) are used for transitioning from PEX to a metal pipeline. The diameter on the PEX side is chosen according to the outer diameter of the PEX pipe (e.g. 16 mm), the threaded part according to the size of the metal pipeline (e.g. 1/2" or 3/4"). Such transitions are common when connecting to existing boiler circuits, groups or fittings. More in the article Press fittings with internal and external thread: how to properly connect a pipeline.

Conclusion: The right diameter saves time, money and nerves

Choosing the correct diameter of press fittings is not just a technical formality - it's the foundation on which the functionality, efficiency and lifespan of the entire pipeline depends. An undersized pipeline will be noisy, underperforming, and in extreme cases will require a complete rebuild. An oversized pipeline will be more expensive, but will work. A correctly sized pipeline will be quiet, economical and trouble-free for decades.

The rules are clear: measure the outer diameter of the pipe, create a correct pipeline diagram, take into account the length of branches and the number of outlets, and always use certified fittings and pipes. If you're not sure, it's better to consult - either with a building services expert, or browse other articles in our Knowledge Center. The entire PEX/AL/PEX press fittings category is structured so you can quickly find the right fitting for your specific size and connection type.

Do you have a question about this topic?

Not sure what to decide, or dealing with a specific situation in your household? Write to us - we'll be happy to help.