What size of pipe clamp do I need – size overview for 15 mm, 18 mm and other pipes
What pipe clamp diameter do I need – Dimension overview for 15 mm, 18 mm and other pipes
One of the most common issues I encounter when advising customers is what seems to be a trivial question: "What diameter of clamp should I buy?" At first glance, the answer seems simple – you take the pipe diameter and buy a clamp with the same number. Reality is a bit more complicated, because with plastic pipes you will encounter several different marking systems, different wall thicknesses, different outer dimensions, and in addition, each clamp manufacturer may dimension their products slightly differently. In this article, I will systematically go through everything – from the basics of measuring pipes through the specific dimensions of the most commonly used sizes to practical tips from installation practice.
If you are also interested in how to choose between a simple and a double anchor, or what the correct clamp spacing is, you will find these topics in the Knowledge Centre in the articles Simple anchor vs. double anchor – when to use which and Clamp spacing on pipes – how to correctly place the fastening. This article focuses exclusively on dimensions and compatibility.
Why the pipe diameter and clamp diameter are not always the same number
The basic thing you need to understand before ordering clamps: when we talk about "pipe diameter", we can mean three different things – nominal diameter (DN or NPS), outer diameter (OD – outer diameter) and inner diameter (ID – inner diameter). The clamp is always mounted on the outer diameter of the pipe, so OD is the decisive dimension.
With steel and copper pipes, the marking according to the nominal diameter has become established, which is neither inner nor outer, but a kind of historically established convention. Plastic pipes in the Hepworth system, polypropylene (PP) and PE-RT pipes work with the outer diameter directly – the number in their designation corresponds to the actual outer diameter in millimeters. That is good news, because the selection of clamps is then more straightforward.
Complication arises in that the pipe wall has a certain thickness and this varies according to the pressure class (SDR or PN). The outer diameter remains constant for a given dimension, the inner diameter changes according to the pressure class. The clamp is therefore relevant to the outer diameter – and this is standardized for plastic systems.
Overview of standard outer diameters of plastic pipes
Before we move on to specific clamps, let's look at what outer diameters are most commonly encountered in residential and family home heating and water distribution installations. For the Hepworth system and compatible plastic pipes, the following table applies:
| Outer diameter OD (mm) | Common designation in practice | Typical use | Wall (PN10/PN16) |
|---|---|---|---|
| 15 mm | 15 / d15 | Radiator connection, manifold | 1.5 – 2.5 mm |
| 18 mm | 18 / d18 | Horizontal heating distribution | 2.0 – 3.0 mm |
| 20 mm | 20 / d20 | Main distribution, cold/hot water | 2.0 – 3.4 mm |
| 25 mm | 25 / d25 | Risers, boiler circuit | 2.5 – 4.2 mm |
| 32 mm | 32 / d32 | Boiler circuit, larger family home | 3.0 – 5.4 mm |
| 40 mm | 40 / d40 | Industry, apartment buildings | 3.7 – 6.7 mm |
| 50 mm | 50 / d50 | Collector pipes, technical areas | 4.6 – 8.3 mm |
Important note: these values apply to plastic pipes with a direct metric outer diameter. If you were to work with older pipes in the imperial system or with copper pipes, the diameters differ and mutual compatibility requires verification. More about the compatibility of plastic systems can be found in the article Clamps for the Hepworth system – compatibility with plastic pipes.
Specific Hepworth Clamps and Their Dimensional Compatibility
Now let's look directly at the products available in the category of plastic clamps. Each of the following products has a defined internal diameter of the seat (the opening into which the pipe is inserted), and this number must match the outer diameter of your pipe.
Single Clamp – Models 601004H, 601005H and 601006H
The single clamp is a basic building block for attaching one pipe directly to a wall, ceiling or other fixed structure. In the Hepworth range, three dimensions are available:
- Single Clamp 601004H – designed for pipe with an outer diameter of 15 mm. This is the smallest and at the same time the most commonly used clamp in apartment installations. It is used for radiator connections, branches from a manifold, or for short sections of cold/hot water in the bathroom.
- Single Clamp 601005H – designed for pipe with an outer diameter of 18 mm. This dimension is very common in horizontal heating circuits. It appears everywhere where a branch is taken off from the main riser pipe to individual radiators on a floor.
- Single Clamp 601006H – designed for pipe with an outer diameter of 22 mm (or 20 mm according to a specific product sheet – always check the technical documentation). These clamps are used in main horizontal circuits and supply branches of the boiler circuit.
The single clamp holds the pipe in a fixed position – it prevents movement in all directions. This is an important technical detail: in systems where it is necessary to absorb thermal expansion, sliding clamps (sliding supports) are sometimes used, but standard single clamps are fixed anchoring points. When this is an advantage and when expansion must be solved in another way is discussed in the article Installation of plastic clamps on pipe step by step.
Dual Clamp – Models 601007H and 601008H
The dual clamp allows you to attach two parallel pipes with one component. This solution is elegant wherever supply and return pipes of heating run side by side (which is practically in every apartment installation). You save installation time, reduce the number of anchoring points in the wall and the final installation looks more refined.
- Dual Clamp 601007H – for two pipes with an outer diameter of 15 mm. Classic use: supply and return lines in one room, for example along the wall leading to a radiator.
- Dual Clamp 601008H – for two pipes with an outer diameter of 18 mm. Used for dual heating lines in corridors, riser spaces and horizontal circuits from the boiler.
With dual clamps, it is also important to monitor the mutual axial distance of both seats (interaxial distance). This value must match the actual distance between the pipes in a specific installation. For 15 mm and 18 mm pipes in the Hepworth system, this distance is standardized to match common work with bends and fittings from the same system.
How to Measure the Pipe, If You Don't Know Its Diameter
This is a practical situation that is solved especially by people who take over an older installation or buy clamps for an existing pipe that has no marking. You have a pipe in front of you and you don't know what it is. How do you find out the correct diameter?
Method 1 – Vernier caliper: Place the caliper perpendicular to the pipe axis and measure the outer diameter. Round the result to the nearest standard size from the table above. If your caliper shows, for example, 17.8 mm, you probably have a d18 pipe.
Method 2 – Measuring the circumference: If you don't have a caliper and can't reach the free end of the pipe, wrap a piece of paper or string around it, mark the point of closure and measure the length. Then divide the value by π (3.1416). Example: circumference 56.5 mm → diameter ≈ 18 mm.
Method 3 – Test with an existing clamp: If you have a clamp at hand, simply try to put it on the pipe. A correct clamp must fit on the pipe without significant pressure, but must not move freely or rattle. If the clamp cannot be closed or put on at all, it is too small; if it has a noticeable clearance, it is too large.
Method 4 – Identification by marking on the pipe: Most plastic pipes have technical data embossed along their length: outer diameter, wall thickness, material, pressure class and manufacturer. It is enough to check the pipe in some hidden part (behind the cladding, in the riser space), where the marking has not been rubbed off or damaged.
Dimensions 15 mm and 18 mm in real practice – where they occur
These two dimensions are by far the most common in Hepworth system installations in apartment and family houses. Let's take a closer look at where and why.
15 mm – the smallest and most practical
The d15 pipe is practically the standard for radiator connections. From the distribution manifold or from a floor branch, it is usually this dimension that leads to the radiator. The reason is simple: a radiator does not require a higher flow – its internal resistance would limit it anyway – and a thinner pipe is cheaper, easier to bend, and installation is faster.
In practice, I have seen dozens of installations where the investor wanted to save money and bought d15 clamps in larger packages, convinced that the entire installation was in d15. Most of the time, this was correct for the connections, but the horizontal corridor distribution had d18 – and there the clamps he had bought were not sufficient. So the rule: always measure all sections, not just one.
The correct clamp for d15 is simple clamp 601004H for one-sided routing or double clamp 601007H where supply and return lines run in parallel.
18 mm – horizontal heating distribution
The d18 dimension is typical for the main horizontal distribution branches on each floor. From the main riser (d25 or d32), a branch is taken off on each floor, and this branch is usually d18. The d18 pipe runs the entire length of the corridor or under the drywall ceiling, from which individual radiators are fed via d15.
In bathrooms, where the heat source is located near the bathroom ladder radiator, d18 also appears as a direct connection to a larger device. Clamps for this dimension – simple clamp 601005H and double clamp 601008H – should always be at hand on the site, provided you are dealing with heating distribution in a residential building.
Larger dimensions – 20, 25, 32 mm and their specifics
For dimensions above 18 mm, the situation with clamps is somewhat different. These larger pipes have higher weight, a larger volume of water inside them, and are more prone to problems with expansion due to temperature changes. Therefore, higher requirements are placed on the clamps as well.
The d20 dimension is transitional – in some systems it is the standard dimension for distribution piping, while in others it skips directly from d18 to d25. It depends on the performance of the installation and on how many radiators are connected to one branch.
The d25 dimension is typical for risers and boiler loop lines. In a family house with underfloor heating combined with radiators, d25 is very common for the primary boiler–distribution loop. Clamps for d25 and d32 must be more robust, as they support the greater weight of the pipe and, in hot water with higher temperatures, also greater thermal movement.
For dimensions d32 and larger, metal clamps or combined systems with plastic sealing inserts are used more frequently. The topic of comparing plastic and metal clamps is expanded in the article Plastic vs. metal clamps for pipes – advantages, disadvantages and suitable use.
Incorrect diameters – the most common mistakes and how to avoid them
From practice, I know that there are several typical mistakes even experienced installers make, not just DIYers. I will list them so you can avoid them.
Mixing up d15 and d16
In some older systems (e.g., copper or certain PE-X pipes), an outer diameter of 16 mm occurs. A clamp for d15 will not fit this pipe – it is too tight by 1 mm. A clamp for d18 would be too loose. In this case, you must look for a specific clamp for d16. Before purchasing, verify whether you actually have d15 or d16.
Mixing up d20 and d22
A similar situation applies between d20 and d22. Hepworth clamps are dimensioned according to a specific product range – if you are working with a system that has d20, do not assume that a clamp marked "for 20 mm" from another system will fit the same. Always combine clamps and pipes from the same system, or verify compatibility in the catalog.
Mixing up internal and external diameter
This is a classic mistake during phone consultations: the customer tells me "I have a pipe with a diameter of 12 mm" and I immediately ask "is that the internal or external diameter?" If the internal diameter is 12 mm and the wall thickness is 2 mm, the external diameter is 16 mm and we need a clamp for d16, not d12. In the category of plastic pipe systems from Hepworth, labeling is consistently based on the external diameter, which makes things easier – but when combining with other systems, be careful.
Mixing up nominal and external diameter in copper pipes
If you are reconstructing an old installation made of copper pipe and want to use plastic clamps on it, be careful: copper pipe has a dimensional system where DN15 (nominal diameter) corresponds to an external diameter of 15 mm – this is an exception, where they match. However, DN22 in copper pipe corresponds to an external diameter of 22 mm, while in plastic pipe d20 or d25 may be a different number. Always measure physically.
Selection of clamps according to pipe material and temperature conditions
Dimension is important, but it is not the only criterion. For plastic clamps in the Hepworth system, another key parameter is the temperature of the medium in the pipe. Plastic clamps are designed for a specific temperature range – typically up to 80–95 °C for most common heating applications. At higher temperatures (e.g., in solar systems or steam distribution), plastic clamps could soften and lose their gripping force.
With PE-RT pipe, which is widely used in the Hepworth underfloor heating system (medium temperature around 35–45 °C), the temperature requirements for clamps are not critical. A different situation arises with the boiler's primary circuit pipe, where the temperature can reach 75–85 °C with a condensing boiler or even 90 °C with older non-condensing boilers.
Most Hepworth clamps are made of polypropylene or a similar material with sufficient thermal resistance for standard heating conditions. However, always check the product documentation for the maximum operating temperature of the clamp – you will find it in the technical data sheet of the product.
Mounting tolerance and proper clamp fit
A properly dimensioned clamp must fit the pipe in such a way that:
- It can be mounted without excessive force, meaning the clamp opening must not be smaller than the pipe's outer diameter (except for clamps with rubber seals, where a slight deformation of the seal is expected).
- After closing (clamping or screwing), it holds the pipe without play – it must not rattle, sway, or shift along the axis (for fixed clamps).
- It must not hang into the pipe – this would indicate that the clamp is too large. The pipe could then vibrate and cause noise in the installation.
The tolerance for plastic clamps is typically ±0.5 mm. This means that a clamp for d15 should have an opening in the range of approximately 14.5–15.5 mm in the neutral position (before fitting onto the pipe). With plastic materials, a slightly elastic behavior is expected – the clamp will slightly stretch during installation and then resiliently hold the pipe.
If you were to fit a d18 clamp onto a d15 pipe, the clearance would be 3 mm – which is too much. The pipe would move and, during thermal expansion, could make noise or damage the insulation jacket. Problems related to an unsuitable clamp size and their solutions are discussed in detail in the article Common problems with plastic clamps – cracking, loosening, unsuitable size.
Overview – which clamp to order for a given diameter
For quick orientation, the following table shows which Hepworth product corresponds to which dimension and application:
| Outer diameter of the pipe | Single clamp | Double clamp | Typical place of use |
|---|---|---|---|
| 15 mm | 601004H | 601007H | Radiator connections, bathroom, branches from manifold |
| 18 mm | 601005H | 601008H | Horizontal distribution branches on the floor, corridors |
| 22 mm | 601006H | — | Main distribution, boiler supply, level above the floor |
| 25 mm and more | Other products – check current range | Vertical pipes, boiler circuit, industrial distribution | |
Practical scenarios from installation practice
A few real-life situations that I have resolved or seen among colleagues – each of them illustrates some practical aspect of choosing the correct clamp size.
Scenario 1 – renovation of a residential core: A customer was replacing water supply lines in the bathroom of a panel apartment. Old steel pipes were being replaced with plastic Hepworth pipes. Cold water supply was designed in d15, hot water also in d15. He used 601004H single clamps and 601007H double clamps where cold and hot lines ran side by side. A problem arose with the supply pipe from the riser, where the diameter was d20 – he forgot to order the correct clamp and we had to make up for it. Lesson learned: don't forget the supply section from the riser, not just the distribution in the bathroom.
Scenario 2 – new construction of a single-family house, two-pipe system: The entire house had a two-pipe system designed with horizontal branches in d18 and radiator connections in d15. The installer ordered double clamps 601008H for d18 in the corridors and double 601007H for d15 radiators. The result looked neat, the pipes were run parallel along the ceiling and fixed at the same heights. The total number of clamps for one house in this case was approximately 80–120 pieces, depending on the spacing (1.0 m for horizontal d18 runs).
Scenario 3 – purchasing clamps during construction: A typical situation when clamps run out and need to be reordered. The installer calls and says: "I need clamps for an 18 mm pipe." I ask whether they are single or double and how many pieces. The answer determines whether it is 601005H or 601008H and what the order is. This is a case where knowledge of the model number significantly speeds up the entire communication and prevents errors in delivery.
Scenario 4 – mixed installation of old and new system: During the renovation of a central heating system in an older house, the original copper pipe d15 (coincidentally matching the outer diameter) was preserved and new plastic branches in d18 were added. Clamp 601004H fit the existing copper d15 – this was a lucky case. However, always verify that the outer diameters actually match, as this is not always the case.
Most frequently asked questions (FAQ)
What clamp size do I need for a d15 pipe?
For a pipe with an outer diameter of 15 mm, you need a clamp dimensioned for d15. From the Hepworth range, this is the single clamp 601004H for one pipe or the double clamp 601007H where two d15 pipes run in parallel (e.g., supply and return to a radiator).
Can I use a d18 clamp on a d15 pipe?
No, it won't help. A d18 clamp has an opening 3 mm larger than the outer diameter of a d15 pipe. The pipe would have unnecessary clearance in the clamp, move around, possibly vibrate, and make noise during water circulation. In the worst case, the pipe could shift along the wall during thermal expansion and damage the surface or insulation jacket. Always choose a clamp exactly for the given dimension.
How can I determine the outer diameter of the pipe if it is not marked?
The most reliable method is to use a sliding caliper placed perpendicular to the pipe axis. If you don't have a caliper, measure the pipe circumference using a tape measure or string and divide the result by 3.1416. For example, a circumference of 56.5 mm → diameter ≈ 18 mm. You can also look for embossed markings on an undamaged part of the pipe in hidden areas (behind cladding, in the wall).
Are Hepworth clamps compatible with pipes from other manufacturers if they have the same outer diameter?
In general – yes, the clip is concerned only with the outer diameter of the pipe, not the material or manufacturer of the pipe. If the outer diameter is identical (e.g., d15 is d15 regardless of the plastic pipe manufacturer), the clip will work. An exception are special clips with integrated sealing or insulation, where in some cases it is necessary to maintain a combination from one manufacturer. For standard non-insulated Hepworth plastic clips, compatibility is based on the outer diameter.
How many clips do I need for the entire installation?
This depends on the length of the pipe and the spacing of the clips. For horizontal routing of plastic pipe d15–d18, a spacing of 0.8–1.2 m is recommended, for vertical routing 1.2–1.5 m. For larger dimensions (d25 and above), the spacing is reduced to 0.8–1.0 m horizontally. Calculation: measure the total length of each pipe dimension, divide by the chosen spacing and round up. A reserve of 10–15 % for unforeseen situations and route changes is always reasonable. A detailed calculation procedure for spacing is in the article Spacing of pipe clips – how to correctly place the fastening.
Why did my plastic clip loosen over time, even though it was correctly dimensioned?
The most common reason is material fatigue of the fastening element (screw or anchor in the wall), not the clip itself. If the anchor is in hollow masonry or in plaster without a solid base, the screw will loosen. Another reason may be thermal expansion – if the pipe is fixed as a rigid point, but expansion is not solved by bends or compensators, the force from expansion can mechanically loosen or crack the clip. Causes and solutions are detailed in the article Common problems with plastic clips – cracking, loosening, incorrect size.
Conclusion – dimension is the basis, but not the only thing
The correct diameter of the clip is a necessary, but not sufficient condition. You must choose a clip with the correct outer diameter of the seat corresponding to the outer diameter of your pipe – this is the basis, around which this entire article revolves. For the Hepworth system, a clear rule applies: the number in the designation of the plastic pipe (d15, d18, d20...) corresponds to the outer diameter, and it is precisely according to this that you choose the clip.
Along with the dimensions, also consider whether you need a simple or double anchor, what is the recommended spacing for the given dimension and temperature conditions, and finally, what material the clip is made of and what is its maximum operating temperature. All these topics are thoroughly covered in other articles in the Knowledge Center – for example How to choose the correct plastic clip for the pipe – simple vs. double or Maintenance and inspection of pipe clips – what to pay attention to.
If you are unsure before ordering, always measure the outer diameter of the pipe physically – do not estimate by eye. Three minutes of measuring will save you a trip to the site with incorrect clips and delay the entire installation.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.