Extension Sizing - How to Calculate the Correct Length and Diameter for Your Piping

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Extending Pipes – How to Calculate the Correct Length and Diameter for Your Piping

Extending pipes belong to those fittings that are constantly used in practice, yet their dimensioning is often underestimated more than any other connecting elements. The plumber arrives at the site, estimates with the eye, takes what is available – and a week later the connection is leaking or the valve is not seated properly. This article is about how to correctly dimension extending pipes based on physical facts and real practical experience, not just by estimation.

If you are not familiar with the basic terminology of threaded standards (BSP, NPT, metric thread), we recommend reading the article BSP, NPT and Metric Thread Standards – What Size Fitting Do You Need? in our Knowledge Center. This topic directly follows on from dimensioning – without the correct thread standard, you simply cannot choose the right fitting.

What is an extending pipe and when do you actually need it

An extending pipe (also called a "nipple" in the trade, from the German "Nippel") is a short cylindrical piece of pipe with an external thread on both ends. Its task seems simple at first glance: to bridge the spatial gap between two valves or fittings with internal threads. In practice, however, you encounter extending pipes in several distinctly different situations:

Connecting a valve to a manifold or collector – when a direct pipe is not enough and you need to "extend" the valve thread to reach the next connection.
Space compensation during boiler, water heater or tank installation – the equipment has fixed connections, but the service valves need to be moved by a few centimeters.
Intermediate layer between two fittings of different types – e.g. elbow + ball valve, where without an extending pipe the two parts would collide with each other.
Repairing damaged threads on an existing pipe – when the original thread is too short or damaged and must be "extended" outward.
Installing meters, temperature or pressure sensors into a pipe, where the exact position of the bleed valve, thermometer, etc. is required.

On the topic of when to reach for an extending pipe instead of a coupling or a reducing fitting, a more detailed discussion is available in the article Extending Pipes vs. Couplings vs. Reducing Fittings – When to Use Which Type of Fitting. Here we will focus exclusively on dimensioning.

Diameter of the extending pipe: how to determine it correctly

The diameter of the extending pipe must match the diameter of the threads of the opposite valves. It is determined according to the nominal thread diameter in inches (BSP – British Standard Pipe), with the most common sizes in typical water and heating systems being: 3/8", 1/2", 3/4", 1", 1¼", 1½" and 2".

A fundamental mistake many people without professional experience make: they measure the outer diameter of the extending pipe with a caliper and compare it to the inch value. For example, a 1/2" BSP thread has an outer diameter of about 20.96 mm – not 12.7 mm, which would be the geometric equivalent of 0.5 inches. The inch designation of the thread comes from the historical internal diameter of the pipe, not the external diameter of the thread. If you are unsure, always measure the external thread diameter and compare it with the BSP standard table.

As you can see in the diagram, the external diameter of the thread is always significantly larger than the geometric inch value. This very regularity causes confusion when purchasing without measuring or without a table – and is the reason why it is worth having a caliper with you and knowing how to use it.

For typical central heating and cold/hot water systems in family homes, 3/8" extending pipes are suitable for smaller valves, bleed valves, thermometers and sensors. Extending pipes in sizes 1/2" and 3/4" cover most standard water and heating systems. Extending pipes in sizes 1" and larger are used for manifolds, main shut-off valves and in industrial applications.

Length of the extending pipe: exact calculation step by step

Calculating the required length of an extending pipe seems trivial at first glance – you measure the gap and that's it. In practice, it is more complicated, because the length of the extending pipe given by the manufacturer is always the total length of the fitting body, but during tightening part of the thread is embedded into the mating part. This so-called "thread engagement" can vary from 6 to 15 mm on each side, depending on the thread size and the quality of the mating part.

From the diagram, the key rule becomes clear: if we measure the gap between two fittings and it turns out to be, say, 24 mm, we cannot reach for an extension of 24 mm in length. We must add the thread depth on both sides – typically 7–10 mm on each side for BSP threads. Result: we need an extension of approximately 38–44 mm. In such a case, it is ideal to use for example Extension 3/8" – 40 mm, or alternatively Extension 3/8" – 30 mm if the gap is smaller.

Formula for calculating the required extension length

The procedure is as follows:

Measure the free gap between the face of one fitting and the face of the other fitting (that part which is not covered by the thread). Let's denote it as M.
Determine the thread depth – for 3/8" BSP it is typically 7–8 mm per side, for 1/2" BSP about 8–9 mm, for 3/4" BSP about 9–11 mm, and for 1" BSP about 10–13 mm.
Calculate the total extension length: L = M + 2 × thread depth
Select the next higher standard size from the available extensions and adjust the position if necessary using a gasket or insert.

Practical example: You are installing an air vent on a radiator. You measure the gap: 16 mm. Thread depth for 3/8" BSP: 8 mm per side. Calculation: 16 + 8 + 8 = 32 mm. The nearest standard size: Extension 3/8" – 30 mm or Extension 3/8" – 40 mm. If the thread is tightened 2 mm deeper (which is common with calibrated emergency fittings), the 30 mm option is suitable. Otherwise, the safer choice is 40 mm with gasket compensation.

Overview of standard lengths of 3/8" extensions and their typical use

In the range of 3/8" extensions, the following standard lengths are available, each with its typical application:

Extension 3/8" – 12 mm – the shortest variant, used where the fittings almost fit together and you only need a minimal transition. Typically used for air vents on a manifold, pressure sensors in tight installations. The thread depth on both sides absorbs most of the length, the actual free gap bridged is only 2–4 mm.
Extension 3/8" – 15 mm – a standard short size for small spatial corrections. Bridged free gap: approx. 4–6 mm. Commonly used for installing thermometers, expansion vessels with small connections, etc.
Extension 3/8" – 20 mm – a universal medium size. Bridged free gap: approx. 6–10 mm. Suitable for most common installation situations where it is necessary to shift a fitting one "step" from the wall or from another fitting.
Extension 3/8" – 30 mm – a longer size for larger spatial gaps. Bridged free gap: approx. 14–18 mm. Used for connecting fittings to a larger manifold, for routing the thread through a wall or thermal insulation cover.
Extension 3/8" – 40 mm – the longest commonly available size in this range. Bridged free gap: approx. 22–26 mm. Suitable for routing the thread through thicker walls, for installing fittings where there is a larger space between the connection and the fitting (e.g., storage heaters with thick insulation).

Effect of diameter on pressure losses and flow

Most plumbers consider only the length and thread when dealing with extensions. The diameter from a hydraulic perspective is usually not considered – and they are right for short extensions. However, there are situations where the choice of extension diameter will affect the hydraulic conditions of the entire circuit:

When the extension is part of a measurement point – for example, with flow meters or differential pressure sensors. The diameter of the extension before and after the sensor must match the design documentation, otherwise the measurements will be distorted.
When the extension is longer than 100 mm (unspecified order, or multiple extensions in sequence) – here, pressure losses and friction add up, which can affect the balancing of the circuit at low flows.
With reduced diameter extensions – when transitioning from a larger to a smaller diameter (which is actually a combined piece, not a pure extension), the pressure loss is significant and must be included in the calculation.

For standard apartment heating systems (3/8", 1/2", 3/4") and extension lengths up to 40 mm, the effect on pressure losses is negligible and does not need to be calculated explicitly. For larger diameters and industrial applications, we recommend consulting with the designer.

Measuring in practice: how to measure the gap without error

Theoretical calculation is good, but in the field, most errors come from incorrect measurement. Here is a procedure that works:

Make sure both fittings are in their place and securely fastened. If one is hanging in the air, the measurement will be inaccurate.
Use a steel sliding caliper or a depth gauge – not a tape measure, not a flexible tape. A sliding caliper is ideal for this task.
Measure from the face of one internal thread to the face of the other internal thread. This is the free gap M that the extension must bridge.
Add the thread depth according to the table below and select the next higher standard size.
If you are unsure, choose the longer size – a long extension can be compensated by tighter tightening or a transition insert, but a short extension that does not reach cannot be compensated by anything.

BSP Thread Size	Thread Engagement Depth (min.)	Thread Engagement Depth (typ.)	Thread Engagement Depth (max.)
3/8" BSP	5 mm	7–8 mm	10 mm
1/2" BSP	6 mm	8–9 mm	11 mm
3/4" BSP	7 mm	9–11 mm	13 mm
1" BSP	8 mm	10–13 mm	15 mm
1¼" BSP	9 mm	11–14 mm	17 mm
1½" BSP	10 mm	12–15 mm	18 mm

Thread engagement depths are approximate and depend on the quality of the fitting thread and the extension. New fittings from European manufacturers have precisely machined threads, and the engagement is often at the upper limit of the typical value. Older, worn fittings from the era of socialist production may have shorter engagement, but the threads may be "worn out," making sealing more problematic.

Typical installation scenarios from practice

Scenario 1: Radiator air vent

A classic situation – the radiator has an internal thread of 3/8" BSP, and the air vent is directly threaded (external thread of 3/8"). There is no gap between them. Why would you need an extension then? The most common reason is that the radiator is against the wall and the air vent is too close, making it inconvenient to operate with a wrench. Solution: a short extension of 12 or 15 mm creates the necessary space and allows for convenient installation and maintenance. A 20 mm extension is usually unnecessary and visually unattractive in such a situation.

Scenario 2: Installing a thermometer on a boiler or tank

The hot water tank has a 1/2" BSP connection in the wall of insulation 30 mm thick. The thermometer has an external thread of 1/2" and a short immersion well of 50 mm. Without an extension, you would not be able to get the thermometer through the insulation at all. Measuring the gap: 30 mm (insulation thickness). Thread engagement depth for 1/2" BSP: 8 mm on each side. Required extension length: 30 + 8 + 8 = 46 mm → you choose an extension of 40 mm or 50 mm depending on availability, with the 6–10 mm difference being compensated by deeper threading into the fitting.

Scenario 3: Adjusting the position of a ball valve on a manifold

The floor heating manifold has branch connections of 3/4" BSP. Each loop has a ball valve with 3/4" BSP, but due to space in the recess, it is necessary to shift each valve by 25 mm away from the manifold. Measuring the gap is not necessary – you know exactly how much you want to shift. Calculation: extension = 25 mm (required shift) + 2 × 10 mm (thread engagement depth for 3/4") = 45 mm → the nearest standard size is 40 mm, which shifts the valve by about 20 mm. If you need exactly 25 mm, you use a 50 mm extension (if available) or a 40 mm straight piece with an adjusting nut.

Scenario 4: Routing a thread through a wall when transitioning to an adjacent room

The pipe must pass through a 70 mm thick drywall partition. On one side is a ball valve (internal thread 1"), on the other side is an elbow (internal thread 1"). There is an opening in the wall between them. Without an extension, you have no way to bridge the wall. Required extension length: wall thickness (70 mm) + 2 × 12 mm (thread engagement depth for 1" BSP) = 94 mm. An unusual size – in this case, two extensions are usually used in sequence (e.g., 2 × 40 mm with a spacer), or a custom-made extension is ordered, or a short piece of pipe with threads on both ends (so-called double-ended nipple) is used.

Sealing with extensions – what affects the choice of length

The sealing of a threaded connection (hemp fiber, teflon tape, anaerobic sealant) directly affects how deep the extension is screwed in. Hemp fiber wound in several layers can add 0.5–2 mm to the length of each engagement. Teflon tape is thinner and has minimal effect. Anaerobic sealant (e.g., Loctite 577) adds a negligible layer, but it can make the thread easier to turn – this means the extension is screwed in deeper, and the resulting length may be 1–2 mm shorter than expected.

This is a practical reason why experienced plumbers prefer to extend by one size longer and compensate for it with the gasket thickness. More on this topic can be found in the article What gasket to use for brass fittings – hemp fiber, teflon or O-ring.

With O-ring extensions (if available), the situation is different – the engagement is fixed and the length of the extension corresponds to the exact dimension without compensation. These types are, however, less common with standard BSP threads and appear more often with metric threads or in hydraulics.

Common mistakes in dimensioning extensions and how to avoid them

Measuring the total distance between valves instead of the free space – this mistake leads to buying an extension as long as the total distance, which is always too long.
Ignoring the thread engagement depth – the opposite mistake, where you buy an extension exactly as long as the free space, and during installation you find out that the thread does not catch.
Mixing up internal and external diameter – the extension has an external thread on both sides. If you are looking for a transition with an internal thread, you need a different component (nipple, coupling).
Using an extension behind another one without a coupling – two extensions cannot be directly screwed together, because both have an external thread. A coupling (slip) with an internal thread on both sides must be placed between them.
Choosing the wrong thread standard – BSP and NPT threads have a different angle and pitch. Although they may look similar at first glance, a combination of a BSP extension with an NPT valve will always leak. More in the article Thread standards BSP, NPT and metric thread – what fitting dimension do I need.
Underestimating thermal expansion – in hot lines (heating, steam), the pipes expand during operation. An extension rigidly built into the structure without an expansion joint can transfer mechanical stress to the threaded connection and loosen it.

Brass, iron or stainless steel – does the material of the extension matter?

When choosing an extension, you mainly encounter brass and galvanized iron parts, less often stainless steel. Brass is the clear preferred choice for most applications in domestic and commercial water and heating systems. Resistance to corrosion, compatibility with copper and polymer pipes, and long service life are its main advantages. Galvanized steel is cheaper, but in systems with drinking water or in a corrosive environment, it degrades quickly. Stainless steel is ideal for aggressive media or food industry applications, but its price is significantly higher. A detailed comparison can be found in the article Brass vs. chrome vs. stainless steel fittings – material comparison for heating and water supply.

Important warning: never connect materials that are galvanically incompatible without a transition element. A direct connection between brass and stainless steel in the presence of an electrolyte (which is any water) causes galvanic corrosion. In practice, this is solved by using dielectric couplings or a short plastic transition.

Frequently asked questions (FAQ)

How can I find out what diameter of extension I need if I can't read the thread?

The easiest way is to measure the external diameter of the thread on an existing valve with a caliper and compare it with the BSP standard table. If it measures approximately 14.95 mm → 3/8" BSP; approximately 20.96 mm → 1/2" BSP; approximately 26.44 mm → 3/4" BSP; approximately 33.25 mm → 1" BSP. Alternatively, you can physically bring the valve to the store and ask for advice. Never measure the internal diameter of the thread – the value in inches does not correspond to the geometric internal diameter.

Can I use two shorter extensions instead of one long one?

Technically yes, but you must insert a coupling (nipple) with an internal thread between the two extensions, because the extensions have an external thread on both sides. Each additional threaded connection is a potential leak point, so always prefer one extension of the correct length over a combination of multiple pieces if possible.

Why is the extension leaking, even though I tightened it to the end?

The most common cause is insufficient sealing of the thread (too little hemp fiber, improperly applied teflon), or a damaged thread. Another possibility is that the extension is too short and the thread does not engage sufficiently into the mating part – when tightening, you hit the bottom before achieving sufficient engagement. A detailed diagnostic procedure can be found in the article Common faults in brass fittings – why the connection is leaking and how to fix it.

Is it important how much the extension is screwed into the valve if the space is tight?

Yes. Too little engagement (less than 4–5 threads) leads to leakage under pressure even with proper sealing, because the thread simply cannot withstand the mechanical load. The optimal engagement is 6–8 threads for BSP, which corresponds to an engagement depth of 7–12 mm depending on the size. If the extension is so long that you cannot screw it in at least 6 threads, it is too long and must be replaced with a shorter piece.

Can I use a BSP 3/8" extension in a valve with a metric thread M16?

No, without an adapter. BSP 3/8" and metric thread M16×1.5 have different profiles (55° vs. 60°), different thread pitches, and different external diameters. Combining these two threads without a transition adapter will never be tight and can damage both threads. Always check what type of thread your valve has and choose the corresponding extension accordingly.

How long do brass extensions last in a central heating system?

With proper installation, proper sealing, and chemically treated circuit medium, brass extensions can last for decades – practically for the entire lifetime of the installation. Problems arise with aggressive media (unauthorized additives, high oxygen content, low pH of water), or with mechanical stress (vibrations from the pump, thermal expansion without compensation). More on corrosion prevention is discussed in the article Corrosion and deposits on brass fittings – how to avoid problems and when to replace fittings.

Conclusion: measure twice, buy once

Dimensioning extensions is not rocket science, but it requires discipline and understanding of what the numbers in the product name actually mean. The total length of the extension includes the engagement parts on both sides – the "visible" part that fills the gap is always shorter. The engagement depth depends on the thread diameter and ranges from 5 to 15 mm per side. Determine the diameter by comparing the external thread diameter with the BSP table, not by estimation.

If you work with 3/8" extensions – whether it's 12, 15, 20, 30 or 40 mm – in the atria.sk range you will find all common sizes directly available. For most air vents, thermometers and small valves, the range from 12 to 40 mm is sufficient and will cover almost all situations you will encounter in practice. If you are dealing with larger sizes (1/2", 3/4" and more), the calculation principle remains the same – only the engagement depth values change according to the table provided in this article.

If you have any doubts about choosing the right fitting, we recommend reading the articles How to choose brass threaded fittings – what to focus on before buying and Installation of brass threaded fittings – procedure, sealing and common mistakes. Correct dimensioning is only the first step – proper installation will determine whether the installation will last for years without problems.

Do you have a question about this topic?

Can't decide or are dealing with a specific situation in your home? Write to us – we are happy to help.