What diameter of clip or strip do I need for my pipe

We are still preparing products.

But you can look at other categories.

What pipe clamp or strap diameter do I need for my pipe – the complete guide to selection

This question may seem trivial at first glance. But in practice, choosing the correct clamp or strap diameter is one of the most common causes of problems during installation – and paradoxically one of the easiest to solve, if you know what to look for. In this guide, we will explain how to correctly measure the pipe, what manufacturer markings really mean, where hidden pitfalls lie with different materials and dimensions, and how to choose the right tool without unnecessary guesswork.

This topic relates to the entire area of mounting tools – if you are interested in a broader context, see our other articles: Clamps vs. straps for pipe mounting – when to use which and How to choose the right mounting tools for Hepworth pipe. Here we focus purely on dimensional selection – numbers, tolerances and practical scenarios from real customer cases.

Why the clamp diameter is a critical parameter – and not just "approximate"

An experienced plumber knows that a clamp that does not fit precisely is not just an aesthetic problem. A clamp that is too tight deforms plastic pipe with every thermal expansion, which over the course of several years leads to cracks in the pipe wall – precisely at the point of attachment. A clamp that is too loose, on the other hand, stops fulfilling its function: the pipe moves, hits the walls, noise is created, and in the worst case, the connections also shift.

With metal pipe, for example copper pipe, this is less dramatic – copper does not have the same thermal expansion as plastics. But even here it is not indifferent: a loose clamp on a copper pipe under vibrations from a pump will cause a characteristic rattling sound, which customers later describe as "something is rattling in the wall" and the service technician will then spend an hour looking for the cause.

Conclusion: the diameter of the clamp or strap must match the outer diameter of the pipe – not the nominal, not the inner, not the thread dimension. Outer diameter. This is the basic rule, from which everything else follows.

Outer diameter vs. nominal dimension – where confusion arises

Most confusion when selecting the clamp diameter stems from the fact that pipes have historically been marked differently. We have three different systems that mix with each other:

DN (Diamètre Nominal / Nominal diameter) – a nominal value that refers to the inner diameter and is primarily used for hydraulic dimensioning. DN15 is not a pipe with an outer diameter of 15 mm.
OD (Outer Diameter / Outer diameter in mm) – the actual outer diameter, most important for selecting the clamp. Always look for this number on the pipe packaging or in the technical data sheet.
Inch marking – mainly used for steel threaded pipes, where "1/2 inch" does not mean the outer or inner diameter in inches in a direct sense, but refers to the historical English pipe system (British Standard Pipe).

For common heating and plumbing pipes, the following typical outer diameters apply:

From the table it is clear that for example copper Cu 15×1 has an exact outer diameter of 15 mm, while a steel threaded pipe DN15 (½") has an outer diameter of 21.3 mm and requires a 22 mm clip for it. This is a crucial difference that is easily overlooked when purchasing without verification.

How to correctly measure the pipe – without errors

If you do not have the technical data sheet of the pipe at hand, measure the outer diameter with a caliper (vernier caliper). Do not forget: measure perpendicular to the pipe axis, at a place without deformations and without insulation. For plastic pipes, make sure to measure on a non-plasticized section – in older installations, pipes may slightly deform at the sites of old clips, and such a section is not representative.

If you measure using a tailor's tape or string (circumference method), divide the circumference by the value π (3.14159) – the result is the outer diameter. For common dimensions: circumference 50 mm = diameter approx. 15.9 mm (copper 16 mm), circumference 56.5 mm = diameter approx. 18 mm (copper 18 mm).

Mounting strips for pipe – what dimensions exist and what for what applies

Mounting strips (also called mounting rails or pipe clamping rails) are long profile elements into which the pipe is simply clicked or secured. Their main advantage compared to individual clips is the speed of installation when running multiple parallel pipes and a more aesthetically clean result – the pipes are in one line, evenly held along the entire length.

In the range of Hepworth/atria.sk you will find for example Mounting strip for pipe 16 mm intended for pipes with an outer diameter of 16 mm – typically for plastic pipes PERT, PB or multilayer 16 mm. For a combination of copper and plastic pipe or in installations where two common dimensions meet, there is Mounting strip for pipe 16–18 mm, which covers both dimensions in one system – which is particularly practical in renovations, where an older distribution in copper 18 mm is supplemented by a newer plastic one 16 mm.

When to use 16 mm strip?

You choose the 16 mm strip whenever you have a pipe with an outer diameter exactly or very close to 16 mm. Typical applications:

Plastic pipe PB 16 mm (polybutylene, Hepworth system)
PERT (temperature-resistant polyethylene) 16 mm
PEX-Al-PEX multilayer 16 mm
Certain types of flexible hoses with an outer diameter of 16 mm

Attention: if you have a copper pipe Cu 16×1, its outer diameter is 16 mm and the 16 mm strip is the correct choice. But Cu 15×1 has an outer diameter of 15 mm – for this pipe the 16 mm strip is too loose and you need a clip/strip 15 mm or a combination 16–18 mm with an insert. This is one of the most common mistakes when ordering.

When to use combination 16–18 mm?

The strip or clip 16–18 mm is designed for flexible use in this range. It is suitable for:

Copper pipe Cu 18×1 (OD = 18 mm)
Plastic pipes with OD 16–17 mm with a thicker wall
Situations where you run two pipes of different diameters in one strip (if the strip allows combined mounting)
Renovations, where an old distribution (typically copper 18 mm) is supplemented by a new one (plastic 16 mm)

Pipe clips – differences from strips and dimensional selection

Clips are individual clamping elements – each holds one pipe at one location. They are mounted on the wall, ceiling or structure via a screw or bolt. Compared to the strip, they offer greater flexibility when running the pipe in different directions and heights.

The dimensional selection of the clip follows the same rule as with the strip: the outer diameter of the pipe = the inner diameter of the clip (with minimal tolerance). Clips are mostly manufactured in 2 mm increments: 12, 14, 15, 16, 18, 20, 22, 25, 28, 32 mm etc. Some types have a variable diameter thanks to an elastomeric insert.

Special case: Compression fittings and their diameter

Compression fittings (also called compression adapters or EK couplings) are not clamps in the true sense of the word – they are used to connect pipe to fittings, to transition between different materials, or to close off branches. However, the outer diameter of the pipe is also crucial when choosing them.

Compression fitting 15×1 EK for copper pipe is intended for copper pipe Cu 15×1, i.e., for an outer diameter of 15 mm with a wall thickness of 1 mm. The marking "15×1" tells you directly: 15 is the outer diameter and 1 is the wall thickness. This fitting will not fit a 16 mm pipe or a 15 mm pipe with a wall thickness of 1.5 mm – the compression mechanism only works within a narrow tolerance range.

Similarly, Compression fitting 15×1 EK for copper or PB pipe covers both materials while maintaining the same outer diameter of 15 mm. The reason it can be used for both materials is that copper 15×1 and PB pipe 15 mm have the same outer diameter – the compression ring fits correctly on both.

If you are unsure, please refer to the article Compression fittings for copper pipe – selection and installation in our Knowledge Centre, where these differences are discussed in greater depth, including the installation procedure.

Material factor: why it matters what the pipe is made of

The same outer diameter does not mean the same clamp for all materials. The reason is thermal expansion and the mechanical properties of the material. When selecting a support, consider the following:

Copper

Copper has a thermal expansion of approximately 17 μm/(m·K). For a 10-meter pipe run and a temperature fluctuation of 50 °C (from 10 °C to 60 °C in the season), the pipe will elongate by 8.5 mm. A support for copper pipe must be sufficiently rigid, but for pipe expansion it is necessary to alternate fixed (rigid) and sliding (movable) support points. A sliding support must fit precisely – not loosely – but the material of the support (rubber insert, metal ring) must allow axial movement of the pipe.

Plastic pipes (PB, PERT, PEX)

Plastic expands significantly more: PB has an expansion of approximately 130 μm/(m·K), PERT and PEX approximately 140 μm/(m·K). This is 7–8 times more than copper. The same 10-meter pipe run and 50 °C temperature fluctuation = elongation of up to 65–70 mm! Therefore, plastic pipes require carefully designed expansion joints and sliding supports mostly every meter (or according to calculation). A support or strip for plastic must not grip the pipe – it must guide it without lateral pressure, only preventing lateral movement.

Multilayer (PEX-Al-PEX, PE-Al-PE)

The aluminum layer reduces thermal expansion to approximately 25–30 μm/(m·K) – significantly less than pure plastic, but still more than copper. A support for multilayer pipe must be dimensioned for the outer diameter of the entire composite wall, while it should be noted that the pipe is stiffer than pure plastic – do not use tighter supports, as the aluminum layer can be irreversibly damaged during deformation.

Fasteners for mounting clamps and strips – what not to forget

The clamp or strip itself is not enough – you need to securely anchor it into the wall, ceiling, or load-bearing structure. For most standard clamps and strips, M8 anchoring screws are used. If you are mounting into concrete or brick, you need a metal or plastic anchor system with a drill bit diameter matching the anchor. For drywall, use drywall anchors (so-called butterfly anchors or chemical anchors for heavier loads).

For standard installation of Hepworth strips and compatible clamps, the KOMBI M8 screw is suitable – a universal mounting screw that fits most standard anchors and allows quick installation and removal without special tools. Its advantage is the combination head, which can be tightened with a flat or cross screwdriver, or an Allen key depending on the version.

The depth of anchoring depends on the substrate and the load: for standard pipe distribution up to DN25 in apartment distribution, an anchoring depth of 40–50 mm in solid masonry, or 60–80 mm in hollow masonry or aerated concrete (Ytong) is sufficient. Never anchor clamps into plaster without a load-bearing substrate – plaster does not have sufficient shear strength.

Practical scenarios from installation practice

Scenario 1: Apartment renovation – old copper piping + new plastic underfloor heating loop

This is a typical situation in most apartments built after 2000. The original heating piping is in copper 15×1 (OD = 15 mm), while the new underfloor heating loop runs in PERT 16 mm (OD = 16 mm). Both piping systems run parallel along the wall to the manifold.

Solution: For the copper pipe, you need a clamp for 15 mm, for the PERT pipe a clamp for 16 mm. If you want to combine both piping systems in one bracket, consider using brackets with different inserts, or use a bracket 16–18 mm and center the copper pipe using a sealing insert 15/16 mm (reduction rubber strip). The cleanest solution, however, is a separate 16 mm bracket for PERT and individual 15 mm clamps for copper.

Scenario 2: New family house – Hepworth PB system throughout the building

The Hepworth system is designed for PB piping, with the basic dimension for apartment piping being 15 mm (OD = 15 mm) and 22 mm (OD = 22 mm) for main risers. Here the choice is simple: clamps and brackets exactly for the OD of the piping. For PB 15 mm, you use a 15 mm clamp/bracket, for the riser 22 mm a 22 mm clamp/bracket. It is important with the Hepworth system to follow the recommended attachment distances – for PB 15 mm vertical piping every 1.0–1.2 m, horizontal piping every 0.5–0.8 m.

Scenario 3: Technical room – dense routing of multiple pipes

In a technical room, you typically encounter pipes of various dimensions: heating supply and return in copper 22×1, boiler connections in copper 28×1.5, safety valve and expansion tank in copper 18×1. Each requires the correct clamp. Here it makes sense to do a thorough inventory before ordering – measure each pipe individually and record the list: "X pieces of 18 mm clamp, Y pieces of 22 mm clamp, Z pieces of 28 mm clamp." Mixing diameters and ordering one size "by estimation" is a recipe for unnecessary trips to the store.

Scenario 4: Repair – loose clamp on piping behind a radiator cassette

Customer calls: "The pipe behind the radiator is rattling." On site, we find that the clamp has cracked and the pipe is hanging freely. The original installation was in copper Cu 18×1, but the installer used a 20 mm clamp "that was on hand" during the repair. Result: a loose clamp, the pipe moves during water flow and pump vibrations. Solution: replace with a clamp of the exact 18 mm diameter. For a closer look at common faults, see the article Loose or cracked clamps – causes and solutions.

Most common mistakes when selecting diameter – and how to avoid them

Confusing OD and DN: Always verify the outer diameter, not the nominal. DN15 ≠ OD 15 mm for steel piping.
Ordering by estimation "by eye": Plastic pipes 16 mm and 18 mm look very similar at first glance. A caliper takes 2 minutes and saves an hour of unnecessary returns.
Ignoring insulation thickness: If you plan to insulate the pipe later, keep in mind that the clamp must be dimensioned for the insulated pipe instead of the bare one – or install sliding clamps after insulation.
One size for everything: "I have various pipes, I'll buy 18 mm clamps and somehow secure them." This does not work in the long run – too loose clamping of a smaller diameter causes movement and noise, too tight clamping for smaller diameters causes deformation.
Forgetting about expansion: Forgotten sliding points on plastic and composite pipes lead to bulging and deformation of the pipe. See the article Common mistakes in pipe clamping and how to avoid them.

Quick selection process – 5 steps to the right clamp/bracket

Determine the outer diameter (OD) of the pipe – using a caliper or from the technical data sheet.
Identify the pipe material (copper, PB, PEX, PERT, multilayer, steel).
Decide whether you need fixed or sliding clamping – based on the length of the section and the material (see the spacing diagram above).
Select a clamp or bracket with an inner diameter = OD of the pipe (or with an allowed tolerance of ±1 mm for sliding clamping).
Check the anchoring – wall material, depth, screw type (e.g. KOMBI M8 for standard anchors).

Frequently asked questions (FAQ)

Can I use a 16 mm clamp for a copper pipe Cu 15×1?

Technically yes, but it is not the correct solution. A copper pipe Cu 15×1 with an outer diameter of 15 mm will be held loosely by a 16 mm clamp – a 1 mm gap around the circumference will result, which may not look like much, but is enough to cause noise and instability of the connection under vibrations and thermal movement. The correct choice is a clamp exactly for 15 mm, or a bracket 16–18 mm with a sealing rubber insert to reduce the diameter.

Does the diameter marking on the bracket/clamp refer to the outer or inner diameter of the pipe?

The marking on the clamp or bracket always refers to the outer diameter (OD) of the pipe for which the clamp is intended. The inner diameter of the pipe is not known to the clamp manufacturer – it depends on the wall thickness, which varies according to material, manufacturer, and pressure rating. Therefore: clamp "16 mm" = for pipe with OD 16 mm.

What is the difference between a fixed and a sliding clamp and how do they affect the choice of diameter?

A fixed (rigid) clamp holds the pipe without movement – it serves as a fixed point from which spacing is calculated. A sliding clamp allows longitudinal movement of the pipe (spacing), but prevents lateral buckling. In terms of diameter, the same rule applies: clamp diameter = OD of the pipe. The difference lies in the material and construction of the clamp – a sliding clamp has a smoother inner surface or a special sliding insert. More on this distinction can be found in the article Clamps vs. brackets for pipe clamping – when to use which.

Bracket 16–18 mm – can I insert a 15 mm pipe into it?

No, not directly without modification. A bracket 16–18 mm is designed for pipes with an OD of 16 to 18 mm. A pipe with an OD of 15 mm would be too small and loose. If you need to combine 15 mm and 16/18 mm in one bracket, use a reducing insert (rubber profile), or solve each diameter with separate clamps.

How many clamps/brackets do I need per meter of pipe?

It depends on the diameter, material, and direction of the run. For plastic piping horizontally, we recommend a clamp every 0.5–0.8 m, for copper every 1.0–1.5 m, for multilayer every 0.8–1.0 m. Vertical runs can have a larger spacing – for plastic pipes about 1.0–1.2 m, for copper up to 2.0 m. The detailed calculation also depends on the weight of the pipe filled with water and the presence of insulation.

What if I cannot determine the material of the old installation?

First of all, measure the outer diameter – this is the decisive factor for selecting the clamp regardless of the material. To determine the material, try: copper is orange-brown and shiny after grinding; PB is firm, slightly flexible, typically white or gray; steel is magnetic (check with a magnet); PEX is harder than PB, does not crack when bent. If you are still in doubt, consult an installer or read the article Common questions about installation tools for Hepworth piping.

Conclusion: measure first, order later

The correct diameter of a clamp or bracket is not a matter of estimation or routine experience – it is a matter of a specific number that you determine with a measuring tool or from the technical data sheet. The outer diameter of the pipe is always the decisive parameter. Everything else – material, direction of the run, type of clamping, anchoring – are important factors, but secondary to this basic number.

If you are still looking for an answer to the question of which specific product to choose, visit the category installation tools, where you will find a complete range of clamps, brackets, and accessories for the Hepworth system and general heating applications. And if you find yourself in a situation where you are dealing with a specific dimension and are unsure – measure, write down the OD, and choose according to this article. It really is that simple.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.