Dimensions of compression fittings: how to correctly measure pipe diameter
Dimensions of compression fittings: How to correctly measure pipe diameter
When a customer comes to the store or orders online and says "I need a compression fitting for a pipe," without further clarification, this is not enough. In practice, I repeatedly encounter the situation where people confuse the outer diameter with the inner diameter, inch dimensions with millimeter dimensions, or mix up the nominal size with the actual physical dimension. The result? An unnecessary trip to the store, lost time, and in the worst case, a water leak in the wall or behind the tiles. This article will explain in detail how to correctly measure the pipe, what the numbers in the designation of compression fittings mean, and how to avoid the most common mistakes when selecting them.
Why are the dimensions of compression fittings so confusing
The basic problem lies in the fact that in plumbing practice, two different labeling systems coexist: the inch system for threads and the millimeter system for the outer diameter of the pipe. These two numbers are not equal and are not directly convertible without a table. In addition, historically established inch dimensions of threads are based on the inner diameter of the pipe, while today's pipes are labeled by their outer diameter. This creates confusion that troubles not only DIY enthusiasts but also experienced plumbers transitioning from one system to another.
Compression fittings are labeled in catalogs with a combination of the inch dimension of the thread and the millimeter dimension of the pipe—for example, Compression fitting 1/2" x 15. This dual designation means: the thread is 1/2 inch (BSP) and the pipe has an outer diameter of 15 mm. Both numbers describe two different parts of the same fitting, with each number measured in a different way and with a different tool.
Three types of dimensions you will encounter
1. Outer diameter of the pipe (OD – Outer Diameter)
This is the physical dimension you measure with a caliper on the pipe itself. For standard copper pipes in heating, the EN 1057 standard applies, and the most common values you will encounter are: 8 mm, 10 mm, 12 mm, 15 mm, 18 mm, 22 mm, 28 mm, 35 mm, 42 mm, and 54 mm. In residential heating installations, the most common dimensions are 15 mm and 22 mm. A 15 mm pipe is typically used for distribution to individual radiators, while a 22 mm pipe is used for the main loop or risers.
2. Nominal thread size in inches (BSP – British Standard Pipe)
This is the nominal thread size, historically derived from the inner diameter of a steel pipe. A 1/2" BSP thread has an outer diameter of the threaded part of about 20.96 mm—i.e., physically larger than half an inch (12.7 mm). This is a historical artifact that has persisted to this day and must be taken into account when working. A 3/4" BSP thread has an outer diameter of the threaded part of about 26.44 mm. These values are fixed by the BS EN ISO 228 standard.
3. Inner diameter of the pipe (ID – Inner Diameter)
The inner diameter depends on the wall thickness of the pipe, which varies according to the type of use (R220, R250, R290). For a copper pipe of 15 mm with a wall thickness of 1 mm, the inner diameter is 13 mm, and with a wall thickness of 1.5 mm, it is 12 mm. The inner diameter is important for hydraulic calculations (flow, medium speed), but not for selecting a mechanical fitting—you select the fitting based on the outer diameter.
How to correctly measure the pipe – step by step
Accurate measurement is the foundation. You do it in three steps, and we will describe each step exactly as it is done in practice.
Step 1: Choose the right tool
To accurately measure the outer diameter of the pipe, you need a caliper. A standard steel caliper for a few euros is sufficient—you don't need anything digital or extra precise. An alternative is a tailor's tape measure with millimeter markings, but it gives slightly less accurate results, especially for smaller diameters. A ruler or folding tape measure is simply not suitable for measuring the diameter.
Step 2: Measure the outer diameter (OD)
Place the jaws of the caliper perpendicular to the axis of the pipe (not at an angle!) on a clean, undamaged section of the pipe. If the pipe is embedded in the wall and you have access only to a short section, measure where the surface is undamaged. Read the value in millimeters. Example: you measured 14.8 mm to 15.2 mm—that is clearly a 15 mm pipe within normal tolerance. You measured 21.8 mm to 22.2 mm—that is a 22 mm pipe.
Step 3: Check the thread size (if replacing an existing piece)
If you are replacing an existing fitting or elbow with a thread, also measure the thread size. The outer diameter of the thread (male thread, i.e., external) is measured with a caliper on the top of the thread teeth. A value around 20.9–21.0 mm = 1/2" BSP thread. A value around 26.4–26.5 mm = 3/4" BSP thread. A value around 33.2–33.3 mm = 1" BSP thread. If you are measuring a female (internal) thread, you must measure through the holes or use a thread gauge—a caliper will give you the diameter of the hole, not the actual thread size.
Conversion of Inches and Millimeters: Table for Practical Use
The following table captures the combinations you will encounter most frequently in heating practice. It includes the outer diameter of the pipe, the corresponding BSP thread size, and typical use in the heating system.
| Pipe OD (mm) | BSP Thread (inches) | Thread OD (mm) | Typical Use |
|---|---|---|---|
| 15 mm | 1/2" BSP | 20,96 mm | Radiator connection, valves |
| 22 mm | 3/4" BSP | 26,44 mm | Main circuit, boiler, manifold |
| 28 mm | 1" BSP | 33,25 mm | Risers, larger circuits |
| 35 mm | 1 1/4" BSP | 41,91 mm | House risers, larger systems |
| 42 mm | 1 1/2" BSP | 47,80 mm | Boiler rooms, larger industrial circuits |
| 54 mm | 2" BSP | 59,61 mm | Main distribution of larger buildings |
* Thread OD = outer diameter of the threaded part (measurable with a sliding caliper on the male thread)
What the Fitting Designation Means in Practice
Let's take a concrete example: Fitting with compression 3/4" x 22. This designation is read as follows: the threaded part of the fitting is made for a 3/4" BSP thread (outer diameter of the thread approx. 26.4 mm), and the crimped or threaded part of the fitting is dimensioned for a pipe with an outer diameter of 22 mm. So if you have a heating circuit with copper pipes of 22 mm and want to connect them to an accessory with a 3/4" thread (e.g., a ball valve or manifold), this is the correct choice.
Similarly, Fitting with compression 3/4" x 15 is used to connect a 15 mm pipe to an accessory or manifold with a 3/4" thread. This is a transitional fitting where not only the diameter but also the connection system changes (from crimping / capillary to threaded). This is particularly useful, for example, when connecting individual circuits (15 mm) to a manifold with a larger thread (3/4").
Specific Cases of Measurement in the Field
Pipes Embedded in the Wall
This is the most common scenario I encounter during heating system renovations. You have an exposed end of the pipe, the rest is in the wall, and you need to find out what kind of pipe it is. Procedure: measure the outer diameter on the exposed end with a sliding caliper. If the pipe is surface oxidized or covered with paint, increase the measured values by 0.1–0.3 mm. If it is an old black steel pipe, clean the surface with fine sandpaper before measuring – the outer diameter of steel pipes is also given, but the nominal dimension differs (DN20 = outer diameter 26.9 mm, which corresponds to a 3/4" thread).
Plastic and Multilayer Pipes
Multilayer and PEX pipes use the same millimeter system for outer diameter as copper pipes, but the dimensions are not always identical. For example, a 16 mm multilayer pipe has an outer diameter of 16 mm – this cannot be connected to a fitting designed for a 15 mm copper pipe. Always verify for which material and exact outer diameter a specific fitting is certified. Hepworth fittings are primarily intended for copper piping according to EN 1057.
Old Steel Pipes
In older buildings, steel threaded pipelines are still found. Here, a different system applies: pipes are marked with a nominal diameter DN (Diameter Nominal), which roughly corresponds to the internal diameter in millimeters. DN15 is a pipe with an outer diameter of 21.3 mm and corresponding 1/2" BSP thread. DN20 has an outer diameter of 26.9 mm and a 3/4" BSP thread. So if you are replacing old steel piping with copper and need a transitional fitting, you must know exactly what type of thread (BSP) the old steel pipe has – and on the other side, connect to the new copper pipe of the correct diameter. For this, direct transitions are used, for example Direct transition 1/2" x 15, internal thread or Direct transition 3/4" x 22, internal thread.
Most Common Errors in Measuring and Ordering
From practice, I know that errors repeat themselves. Here are the most common ones:
- Measuring internal diameter instead of external. The customer measures the inside of the pipe, gets 13 mm, and orders a fitting "for a 13 mm pipe". Such a fitting does not exist – the correct one is a 15 mm pipe with an internal diameter of 13 mm and a wall thickness of 1 mm.
- Mixing up inch and millimeter. People think that 1/2 inch = 12.7 mm and look for a fitting "for a 12 mm pipe". The 1/2" BSP thread does not refer to the pipe diameter, but to the thread nominal.
- Measuring on deformed or corroded parts of the pipe. An old copper pipe that is mechanically deformed at the point where it passes through a brick will show a distorted dimension. Always measure on a free, undeformed section.
- Forgetting to distinguish internal vs. external thread. The customer sees a hole, measures it, and says "I have a 20 mm thread" – but he measured the internal diameter of the hole, not the thread dimension. I recommend always referring to our other educational materials in the category Internal vs. External Thread: What type of transition do you need?
- Assuming that "all pipes of the same diameter are the same." There are different wall thicknesses (R220, R250, R290) and different materials – a fitting certified for copper pipes EN 1057 may not be suitable for a plastic pipe with the same outer diameter.
- Mixing up mm and cm. Sounds trivial, but it happens – the customer measures 1.5 cm and thinks "15 mm, that's my dimension" – but the measured value in centimeters gives 15 mm only if correctly converted. The problem arises when someone measures 15 mm and writes down "1.5", then orders "1.5 inches" – and that is a completely different dimension.
How to verify the size without a caliper
Don't have a caliper? In an emergency, you can manage in another way, although with less accuracy. Wrap the pipe once with a thread or a thin strip of paper, mark the point where the thread meets itself, and measure the length of this thread with a ruler – you will get the circumference of the pipe. The outer diameter is calculated by dividing the circumference by π (3.1416). Example: the thread measures 47.1 mm → 47.1 / 3.1416 = 15 mm. This method is sufficient to determine whether it is a 15 mm or 22 mm pipe, but it is not reliable for less common dimensions.
Another option: take an existing fitting or piece of pipe from the system and bring it physically to the store or measure it. If you have an old coupling with a number on the body (for example, "15" or "22"), you can determine the size directly.
Difference in measurement for various types of compression fittings
Not all compression fittings are the same in construction. Some use a compression (screw) type pipe holding, while others are designed for soldering and have the threaded part only on one side. The measurement is always the same – it is based on the outer diameter of the pipe – but you must know which part of the fitting you are measuring.
In a compression (soldering) fitting, the pipe is inserted into the body of the fitting and compressed by a nut through an olive. The outer diameter of the pipe is absolutely crucial here – the olive deforms precisely around the outer surface, and if the pipe is more than 0.3 mm larger or smaller, the seal is not guaranteed. Therefore, never use worn or deformed pipe sections with these fittings – always cut a new, straight end.
More about practical installation can be found in the article "Installation of a compression fitting on copper pipe step by step," where the individual steps are described in detail, including the preparation of the pipe end, the correct placement of the olive, and the tightening torque of the nut.
Tolerances and limit values: when the dimension is still acceptable
Copper pipes according to EN 1057 have defined production tolerances for the outer diameter. For pipes up to 22 mm, the tolerance is ±0.1 mm, and for pipes 28–54 mm, it is ±0.2 mm. Therefore, if you measure with a caliper and get 14.9 mm or 15.1 mm, it is a 15 mm pipe – within the standard. If the measured value is 14.5 mm or 15.5 mm, the pipe is either worn/damaged or it is a different product/standard (for example, some foreign manufacturers of certain plastic pipes have slightly different external diameters).
Similar tolerances apply to threaded connections according to the standard BS EN ISO 228 (BSP threads). The thread pitch (number of threads per inch) for 1/2" BSP is 14 TPI (threads per inch), for 3/4" BSP it is also 14 TPI, and for 1" BSP it is 11 TPI. If you have doubts, use thread gauges or a thread plug – they are sold in every specialized plumbing store for a few euros.
Practical scenarios from real customer cases
Scenario 1 – replacing a radiator valve: The customer is replacing a thermostatic valve on a radiator. The old valve has a 1/2" external thread (male). The new valve also has a 1/2" male thread on the inlet. To connect it to a 15 mm copper pipe, a fitting is needed that has a 1/2" BSP thread on one side and a compression fitting for a 15 mm pipe on the other. The correct choice is Compression Fitting 1/2" x 15, and it is necessary to know whether the thread on the valve is internal or external – and choose accordingly whether the fitting has an internal or external thread. More about this decision can be found in the article "Difference between a compression fitting and a direct transition."
Scenario 2 – connecting a boiler to a distribution loop: A new gas boiler has an inlet and outlet with an internal 3/4" thread. The distribution in the house is a 22 mm copper pipe. The customer needs a fitting that connects an internal 3/4" thread with a 22 mm pipe – on each side of the boiler. In this case, the solution is Direct transition 3/4" x 22, internal thread. The measurement confirms two things: the outer diameter of the 22 mm pipe (with a caliper), and the outer diameter of the male threaded nipple on the boiler is approximately 26.4 mm (= 3/4" BSP).
Scenario 3 – a distributor with a larger thread and a smaller pipe: In an apartment building with floor heating, you have a distributor whose inlet connections have an internal 3/4" thread. The supply pipe is copper 15 mm. You therefore need a transition fitting that reduces the pipe from 22 mm to 15 mm and also connects the threaded part. In this case, the solution is Compression Fitting 3/4" x 15 – a fitting with a compression part for a 15 mm pipe and a 3/4" BSP threaded part. Without the correct measurement of both dimensions, you would not know whether you are in this situation or in the situation from the previous scenario.
When measurement is not enough and consultation is needed
There are situations where measurement alone is not enough and you need to consult with a technician or salesperson. For example: it is not clear what material the existing pipe is made of (copper vs. carbon steel vs. stainless steel can have the same outer diameter but different certified fittings), or the pressure conditions in the system are non-standard (e.g., industrial equipment with pressure above 10 bar), or different systems are being combined (e.g., old galvanized steel + new copper + plastic). In such cases, correct measurement is necessary, but not sufficient – a material assessment is also required. The topic "Compatibility of Hepworth fittings with other systems and pipe materials" discusses this in more detail.
Similarly, when you inspect the condition of compression fittings and soldered joints during your annual heating system service, good measurements and installation records will save you time. The correct approach to this routine inspection is discussed in the article "Inspection and tightening of compression fittings during annual heating service."
Frequently asked questions (FAQ)
How can I determine the diameter of a pipe embedded in the wall if I have no access to it?
If only the end of the pipe is exposed (e.g. behind a radiator or at the boiler), measure the outer diameter right there. If no part of the pipe is exposed, try to find the building's design documentation or consult the installer who installed the system. Another option is to use an endoscopic camera to inspect the pipe, but this is an extreme solution. In practice, most residential heating systems built since the 1980s consist of steel pipes DN15/DN20, or copper pipe 15/22 mm – so you can quite reliably estimate the size in the wall based on the system's characteristics (building age, boiler type, radiator connections).
What if the measured number does not match any standard size?
First, check that you measured correctly – perpendicular to the axis, on an undamaged part of the pipe. If the dimension is indeed non-standard (e.g. 16 mm, 20 mm, 25 mm), it is likely a plastic or multi-layer pipe, not copper. These dimensions are common in plastic systems and require a different type of fitting. Fittings designed exclusively for copper pipes (such as Hepworth fittings in this category) are not suitable for these dimensions.
Why does a 1/2" thread have an outer diameter of about 21 mm, when half an inch is only 12.7 mm?
This is a historical artifact. The BSP (British Standard Pipe) system was introduced in the 19th century, when inch-based dimensions referred to the internal diameter of a steel pipe. The outer diameter was always larger, because the pipe had a wall. These nominal dimensions have been retained even after standardization, because it would have been too great a burden to change the entire threading infrastructure. Today, they are purely agreed-upon numbers (nominal), not physical dimensions in inches.
Can I use a 3/4" thread instead of a 1/2" thread if I have higher pressure in the system?
No – the choice of thread is determined by the pipe and fitting diameter, not by pressure. Pressure parameters are determined by the material and construction of the fitting, not by the size of the thread. If you have a system with a 15 mm pipe and fittings with a 1/2" thread, you must use 1/2" x 15 fittings. Mechanically, it is not possible to force a 3/4" thread onto a 1/2" fitting – the thread simply will not fit. Pressure parameters should be addressed by selecting fittings with the appropriate certification (PN rating), not by changing the thread size.
What is the difference between a compression and a capillary joint when measuring?
In terms of size selection, the difference is minimal – in both cases you base your choice on the outer diameter of the pipe. The difference lies in the installation and in the tolerances: a compression (screwed) joint can accommodate slightly larger surface deviations of the pipe, because the olive adapts. A capillary joint (soldering) requires the pipe to have a clean and precise outer diameter within the EN 1057 tolerance, otherwise the capillary gap is not uniform and the joint may be leaky. For compression joints with ferrules, such as those in this category, always use a freshly cut and squared end of the pipe with a straight, perpendicular cut.
Where can I find all available sizes of Hepworth ferrule joints?
The full range is available in the ferrule joints category on atria.sk. If you are unsure about your choice, also check other articles in the Knowledge Centre – for example, How to choose the right ferrule joint for heating provides a systematic overview of selection criteria, and Why is my ferrule joint leaking and how to fix it addresses problems after installation, which may stem precisely from an incorrectly chosen size or incorrect installation.
Conclusion: measurement is the foundation, not a formality
Correctly measuring the pipe diameter is not just a formality before purchasing – it is the foundation on which the tightness and reliability of the entire joint depends. An error of 1–2 mm, confusing outer and inner diameters, or confusing inch-based thread markings with the millimetre-based pipe size are the most common causes of unnecessary problems on site, repeated visits to the store, and in the worst case, water leaks in a completed structure.
Invest five minutes in proper measurement, note both values (OD of the pipe in mm + thread size in inches BSP), check whether you need internal or external thread – and selecting the correct fitting will be simple and straightforward. A sliding caliper, this table and a basic understanding of the BSP system will be sufficient for 95% of situations you will encounter in a standard residential heating installation.
Do you have a question about this topic?
Not sure how to proceed or dealing with a specific situation in your home? Write to us – we are happy to help.