How to determine the correct diameter and elbow angle for your piping

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How to determine the correct diameter and elbow angle for your piping system – a complete guide

Selecting the correct elbow for a pipe may seem simple at first glance: you take a fitting with the correct diameter and that's it. In practice, however, it is not that simple. I regularly encounter situations on jobs where an installer used an elbow with the wrong angle, forcing the pipe to go around an obstacle with two extra elbows, or where someone bought an elbow with the wrong diameter, confusing the outer diameter of the pipe with the nominal diameter of the fitting. The result: unnecessary costs, time-consuming rework, and sometimes even pressure issues in the entire system.

This article will step by step explain how to correctly measure the pipe diameter, how to determine the required elbow angle, and how to choose a fitting that fits the first time – without unnecessary trips to the hardware store.

Why is the selection of diameter and angle complicated at all?

The first reason is a terminological confusion. Plastic HEPWORTH piping is labeled according to the outer diameter in millimeters (15 mm, 22 mm, 28 mm, etc.), while threaded connections are labeled in inches according to the BSP standard (1/2", 3/4", 1"...). These two systems do not intuitively overlap – an outer diameter of 15 mm roughly corresponds to a 1/2" BSP thread, but only "roughly," because a 1/2" BSP thread has an outer diameter of about 20.9 mm and an inner threaded cross-section of about 16 mm. If you therefore buy an elbow with a 1/2" x 15 thread, it means that one side is for a 15 mm plastic pipe and the other has an outer 1/2" BSP thread for connecting to a radiator or valve.

The second reason is geometric. Most people imagine an "elbow" as only 90°, that is, a right angle. In reality, 45° elbows are also commonly used, and these are not interchangeable – the difference between them affects not only the pipe route, but also the hydraulic resistance of the entire system, and thus the heating efficiency. If you are unsure when to use 45° or 90°, also read the article "45° Elbow or 90° Elbow – When to Use Which Type" in our Knowledge Center.

Step 1: Accurately measuring the pipe diameter

Before you go to the store or shop online, you need to know the diameter of the pipe you are working with. Here is the most important principle: always measure the outer diameter, not the inner one. HEPWORTH fittings are dimensioned according to the outer diameter of the pipe that enters them.

How to measure the outer diameter:

With a caliper (slide gauge) – the most reliable method. Place the jaws perpendicular to the pipe axis and read the value in millimeters. For standard HEPWORTH pipes, you should get values close to 15, 22, 28, 35, or 42 mm.
With a tape measure or string (circumference) – if you don't have a caliper, wrap a string around the pipe, measure the length of the circumference, and divide by π (3.1416). You will get the diameter. This method is less accurate, but sufficient to distinguish 15 mm from 22 mm.
By comparing with an existing fitting – if you have a fitting that fits the pipe, look at its labeling. HEPWORTH fittings always have the diameter embossed or printed directly on the body of the fitting.

A common mistake I repeatedly see: a customer comes with a pipe and says it is an "eighteen." I measure the outer diameter – it is 22 mm. It turns out that the "eighteen" was a colloquial name based on the inner diameter of an old steel pipe. Plastic HEPWORTH pipes are not named this way – always work with the outer diameter.

Step 2: Understanding the nomenclature of fittings – what do the numbers in the name mean?

When you look at the HEPWORTH fittings catalog, you will see labels such as "15 x 15," "22 x 22," "22 x 15," "1/2" x 15," and so on. Here is the key to reading these labels:

15 x 15 – the same diameter on both sides, 15 mm. This is a common symmetrical elbow without reduction.
22 x 15 – a reduced elbow, one side 22 mm, the other 15 mm. Used when switching between two different pipe diameters at one bend.
1/2" x 15 – a combined elbow: one side is a threaded connection (BSP thread), the other is for a 15 mm plastic pipe.

If you are working with the same pipe diameter on both sides of the bend, look for example at a 15 x 15 elbow for 15 mm pipes or at a 22 x 22 elbow for 22 mm pipes. If you need to change both the direction and the pipe diameter at the same time, reach for a reduced elbow – more about this option can be found in the article "Reduced Elbow or Separate Reducer – What is More Cost-Effective."

Step 3: Determining the correct elbow angle – 45° or 90°?

The angle of the elbow is not just an aesthetic choice. It has a direct impact on the hydraulic resistance of the system, on where the pipe will end after the bend, and on the ease of installation in confined spaces.

When to use a 90° elbow

A 90° elbow (also called a K-piece or 90° fitting) is the most commonly used. It is used wherever you need to make a sharp right angle – for example, when bringing a pipe from a vertical riser to a horizontal run, when branching off to a radiator, or when going around a corner of a wall. A 15 x 15 K-piece is most commonly used in practice for connections to radiator valves and thermostatic heads in standard apartment systems.

When to use a 45° elbow

A 45° elbow is often a better choice in many situations than it might seem. The hydraulic resistance of a 45° elbow is significantly lower than that of a 90° elbow – as a reference, the equivalent length of straight pipe for a 90° elbow at a diameter of 15 mm is about 0.5–0.7 m, while for a 45° elbow it is only about 0.3 m. This means that in systems with multiple direction changes, the resistances add up and can significantly affect the flow and balancing of the heating units.

Practically, a 45° elbow – 15 x 15 is used for running pipes under a sloped roof, for bypassing beams and joists, for running pipes along a wall with little space behind it, or for combined routing where two 90° elbows would take up too much space. Similarly, a 45° elbow – 22 x 22 finds its use in thicker systems, for example, in main distribution branches from the boiler.

When to use two 45° elbows instead of one 90°

This combination is sometimes the best solution. If you need to span an obstacle – for example, to bypass a load-bearing column or a pipe bend – then two 45° elbows in opposite directions create a so-called "offset," allowing the pipe to avoid the obstacle without significantly increasing hydraulic resistance. This solution is also more aesthetically clean than combining different angles.

Step 4: How to calculate the position of the elbow on paper before installation

For an experienced installer, this may seem obvious, but I have seen many beginners trying to fit pipe fittings directly on the site without any preliminary calculation – and then they are short by 5 cm and the pipe does not reach the fitting. That is why I recommend a simple procedure:

For a 90° elbow: Draw the pipe route on paper. Mark the point at the direction change. Always measure the pipe length to the center of the elbow (axis of the fitting), not to its edge. For HEPWORTH fittings, the insertion depth (the depth to which the pipe is inserted into the fitting) is approximately 15–16 mm for a 15 mm diameter and 18–20 mm for a 22 mm diameter. This value must be subtracted from the total length of the pipe section.

For a 45° elbow: The calculation is a bit more complex. If you need to offset the pipe by a distance X (perpendicular to the original pipe axis), then the length of the diagonal section between two 45° elbows is X divided by sin(45°), which is X × 1.414. For example, to offset by 100 mm, you need a diagonal section of 141 mm.

Step 5: Threaded dimensions and their relation to pipe diameters

If you need to connect plastic pipe to a threaded fitting (valve, radiator connection, heat meter...), you must know not only the diameter of the plastic pipe, but also the type of thread on the fitting. BSP (British Standard Pipe) threads are marked in inches and their actual dimensions are:

1/2" BSP – external thread diameter 20.955 mm, pitch 14 threads per inch. Used for radiator connections, valves and most household fittings.
3/4" BSP – external diameter 26.441 mm. Common for larger valves, safety valves, boiler connections.
1" BSP – external diameter 33.249 mm. Main distribution lines, boiler connections, manifolds.

This is why the Elbow with 1/2" x 15 thread exists – it combines in one fitting the transition from a 15 mm plastic pipe to a 1/2" BSP thread and a 90° direction change. Without this fitting, you would need a separate reducer and a separate elbow, which takes up more space and creates an additional potential leak point. More on why to combine vs. using two fittings can be found in the article "Reduced elbow or separate reducer – which is more cost-effective."

With threaded connections, it is also important to distinguish between external thread (M – male) and internal thread (F – female). The elbow with 1/2" x 15 has an external thread (the fitting is screwed INTO the fitting). If you need an elbow with an internal thread, that is a different fitting. Details can be found in the article "External vs. internal thread on an elbow – what difference does it make."

Typical situations from practice – where dimensions or angles are most often mixed up

Case 1: Radiator connection in a panel apartment

Classic situation: the apartment heating distribution runs behind drywall, a plastic HEPWORTH 15 mm pipe comes out of it, and the customer wants to connect a new designer radiator with 1/2" connections. They need an elbow with 1/2" x 15 thread. Without measuring, they rely on the assumption that "all connections are the same." Result: in some older apartments, the riser is still steel 3/4", and from it there is a reduction to 1/2" – so the entire geometry of the connection is shifted by several centimeters and the elbow simply does not fit. Always measure the position of the connection, not just its diameter.

Case 2: Distribution pipe in a technical room

During the installation of a condensing boiler with new distribution lines, the customer designs the pipe route "by eye." The main distribution line is in 22 mm diameter (due to the boiler's 20 kW output and the length of the circuit), but 15 mm pipe branches off to individual radiators. Each branch has a T-piece and then an elbow for direction change. Here it turns out that the route around the plumbing installation requires an offset of 80 mm. Instead of two 90° elbows, which would take up space and add resistance, two 45° elbow – 22 x 22 are used with a diagonal section of 113 mm (80 × 1.414). The result is more space-efficient and hydraulically cleaner.

Case 3: Attic with a sloped roof

In an attic apartment, the pipe runs along a sloped roof structure. The angle between the roof and the floor is 135° – so the pipe must turn 45° from the horizontal. Here, a 90° elbow is completely unusable – it would create a visible "bent piece" off the axis. The solution is a single 45° elbow, which directs the pipe precisely along the slope of the roof. The customer initially insisted on a 90° elbow, because "that's what he has at home from the beginning." After explaining the geometry, he was glad to take advice.

Dimensioning when transitioning between different pipe sizes

Reduced elbows – that is, those where each side has a different diameter – are used when the pipe size also changes at the point of direction change. This is advantageous in terms of installation space (one fitting instead of two) and in terms of the number of potential leak points.

When does reduction typically occur:

Transition from the main distribution loop (22 mm or 28 mm) to a branch to one radiator (15 mm)
Transition from a boiler with a large flange to a household distribution
Combined distribution, where some parts are in old and some in new dimensions

When dimensioning, it is crucial to follow hydraulic rules – the flow velocity of water in plastic pipe should not exceed 1.5 m/s (ideally 0.7–1.0 m/s). The smaller the diameter, the higher the velocity at the same flow rate, the higher the hydraulic resistance and noise. So if you reduce the diameter, do it at a point where the required flow rate also actually decreases (that is, after a branch, not before it). More on this topic can be found in the article "Dimensioning elbows when transitioning between different pipe sizes."

Most Common Mistakes When Choosing an Elbow – A Practical Summary

Mixing up internal and external diameter – the customer buys a fitting 22 x 22, but has a pipe with an external diameter of 22 mm (which fits), or a pipe with an internal diameter of 22 mm (which does not fit – the external diameter would be 28 mm). Always use the external diameter.
Wrong angle – the customer uses a 90° elbow where a 45° elbow would suffice, or vice versa. The result is either an unnecessarily long pipe run or an installation that is geometrically impossible.
Dependence on "standard" dimensions – the assumption that every radiator connection is 1/2" and every pipe is 15 mm. In practice, combinations such as 3/4" thread × 15 mm pipe or other non-standard situations do occur.
Ignoring insertion depth – when measuring pipe lengths, the customer forgets to subtract the insertion depth of the fitting, and the pipe ends up being 15–20 mm too short.
Buying one extra piece "just in case" without actually determining the need – a better practice is to accurately count the fittings according to the project or layout drawing.

Checklist Before Ordering an Elbow

Before you click "add to cart," double-check the following:

✓ Do I know the external diameter of the plastic pipe (15 / 22 / 28 mm)?
✓ Do I need the same or different diameter on both sides of the elbow?
✓ Does the fitting the elbow connects to have an external or internal thread, and what size is it (1/2", 3/4")?
✓ What angle of direction change do I need – 45° or 90°?
✓ Did I remember to subtract the insertion depth when measuring lengths?
✓ How many pieces do I need (based on the number of direction changes in the layout)?

Frequently Asked Questions (FAQ)

How can I tell if I have a 15 mm or 22 mm pipe if I don’t have a caliper?

The fastest approximate test: a 15 mm pipe has an external diameter that fits into the "circle" formed by the thumb and index finger of most people. A 22 mm pipe is significantly thicker – its diameter is just below that of a two-euro coin (25.75 mm). If you have both coins on hand, the 15 mm pipe is narrower than a euro coin (23.25 mm), while the 22 mm pipe is almost the same. For precise measurement, however, I always recommend using a caliper – you can buy one for a few euros at any hobby store.

Can I use two 90° elbows instead of one 45° and one reversed 45°?

You can, but it is not recommended as a primary solution. Two 90° elbows in a row will change the direction by 180° – meaning the pipe will return in the opposite direction. To create an offset, you need two 90° elbows with pipe segments in between, which takes up much more space than two 45° elbows. In addition, the hydraulic resistance of two 90° elbows is significantly higher. Two 45° elbows are almost always a better choice for offsetting the pipe run.

What is the difference between a K-piece and an elbow – are they the same?

Yes, in the context of HEPWORTH fittings, a K-piece is another name for a 90° elbow. "K" comes from the German "Knie" (knee). The term "el" or "elbow" is also used in English technical documentation. All these terms refer to the same thing – a fitting for a 90° direction change. A 45° elbow is usually not abbreviated and always includes the angle in the name.

Why is my threaded elbow leaking even after tightening? Did I buy the wrong size?

Incorrect diameter is one of the possibilities, but not the only one. A more common cause is the absence or incorrect application of sealing on the thread. A BSP thread requires either Teflon tape wound in the direction of the thread or a liquid sealant (such as Loctite 577). For tips on color-coded cross-sections of the connection and other leak diagnosis tips, I recommend reading the article Why the elbow is not sealing and how to fix it without replacing the entire joint and Seal Tightness – Most Common Problems When Installing Reduced Elbows in our Knowledge Center.

How can I tell if I need a reduced elbow or if a standard symmetrical one will do?

A simple rule: if the pipe diameter before and after the direction change is the same, use a symmetrical elbow (e.g., 15 x 15 or 22 x 22). If the diameter changes (e.g., a main line of 22 mm branches into a radiator line of 15 mm), and you also need to change direction, use a reduced elbow. An alternative is a symmetrical elbow + a separate reducer, but this takes up more space and creates an additional joint.

Is there any quick online tool to calculate pipe lengths with 45° elbows?

For a simple offset calculation using two 45° elbows, just use the formula: length of the diagonal section = desired offset × 1.414. For more complex routes with multiple angles, I recommend a simple sketch on paper with a ruler and protractor – it is faster than searching for an online calculator. Experienced installers often draw such diagrams directly on the wall with a pencil during construction and then remove them after installation.

Conclusion – Measure Twice, Order Once

Choosing the right elbow is not an intellectual quiz – it is a matter of practice and a systematic approach. It is enough to get used to three steps: measure the external diameter of the plastic pipe, determine the required elbow angle from the pipe layout geometry, and verify whether you need a reducer or a threaded connection. If you can be sure of these three things, selecting a fitting is simple.

For those who are starting with HEPWORTH installations for the first time, I recommend reading the article How to Choose the Right Elbow for HEPWORTH Plastic Pipe and Installing a Threaded Elbow on Copper and Plastic Pipe Step by Step – both are available in our Knowledge Center. A comprehensive overview of the most frequently asked questions about the entire category of fittings can be found in the article Frequently Asked Questions About HEPWORTH Reduced Elbows.

And if you are still unsure after reading all of this – take a look at our range of HEPWORTH reduced elbows, where each product lists its dimensions, material, and recommended use. Most of the time, the correct fitting is found much faster than it seems.

Do you have a question about this topic?

Still undecided or dealing with a specific situation at home? Write to us – we are happy to help.