External vs. internal thread on screw fittings – what's the difference and when to use which

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External vs. internal thread on screw fittings – what's the difference and when to use which

If you've ever found yourself holding a screw fitting and not knowing whether it has the "right" thread for what you need – you're not alone. The relationship between external and internal threads is one of the most common sources of confusion when purchasing fittings for heating and plumbing systems. At first glance it seems like a minor detail, but in practice a single wrongly chosen piece can bring an entire installation to a halt, or worse – cause a water leak after you've already covered the wall with plasterboard.

In this article, we'll take a detailed look at what external and internal threads actually mean, how to reliably tell them apart, which type is suitable for which situation, and how this all relates to the specific pipe diameters and thread sizes you'll encounter most often when installing underfloor heating, radiators, or water supply systems.

What is a thread and why does its type matter

A thread is a helical groove milled or cut into a cylindrical or conical surface. Two threaded elements – external and internal – screw into each other to create a mechanically strong, hermetically sealable joint. The relationship is always paired: an external thread screws into an internal thread, and never the other way around.

So why does this cause so much confusion? Because catalogs, packaging, and everyday speech use a whole range of terms for threads – external, male, AG (from German "Außengewinde"); internal, female, IG ("Innengewinde") – and customers don't always realize that all these names refer to the same thing, just in different languages or traditions. Add to that the fact that a thread can be parallel (cylindrical) or tapered, and that sizes are given in inch fractions that don't correspond to the actual diameter – and you have full potential for buying the wrong part.

How to reliably tell an external thread from an internal thread

The rule is simple and never lies:

External thread (AG, male) – the thread ridges are on the outer surface of the part. When you look at the end of a fitting, you'll see a raised spiral running around a rod or protrusion. The diameter with the thread is larger than the diameter of the smooth part of the body.
Internal thread (IG, female) – the thread is cut inside the opening. When you look into the opening, you'll see a spiral groove inside the cylinder. From the outside, the opening looks smooth or has a hexagon for a wrench.

The quickest field test: put your finger on the thread. If the thread ridges "go outward" and you can move your finger along them like along a helix on the outer surface – it's an external thread. If you have to put your finger inside the opening – it's an internal thread.

A second, even more definitive method: take a common nut or fitting nut. Does it screw onto your fitting? If yes – the fitting has an external thread. If you took a bolt and it screws into the fitting – the fitting has an internal thread.

Standards and thread designation on installation fittings

For screw fittings used in heating and plumbing systems, we mostly encounter the Whitworth pipe thread (G) according to ISO 228 / DIN 2999. This thread has a 55° profile angle and its size is given in inches – but beware: this number does not correspond to the actual outer diameter of the thread.

That's why a fitting marked "G 1/2"" (half an inch) does not have an outer diameter of 12.7 mm – it's actually 20.96 mm. This is a historical convention derived from the inner diameter of a steel pipe, not from the diameter of the thread itself. This fact is a source of enormous confusion when shopping, so we also recommend reading our separate article Screw fitting dimensions – how to correctly measure and match 1/2" and 3/4" to 16 and 20 mm pipes.

In practice, for plastic and multilayer pipes (PEX-AL-PEX, multilayer), the following applies:

Pipe 16 mm outer diameter → thread G 1/2"
Pipe 20 mm outer diameter → thread G 3/4"
Pipe 25 mm outer diameter → thread G 1"

External thread on screw fittings – where and why you need it

Screw fittings with an external thread are by far the most widespread type used to connect plastic and multilayer pipes to fittings, valves, manifolds, and – very commonly – to threaded connections inside a wall or behind tiling. The principle is simple: an external thread screws into the female (internal) thread of the opposing part.

Typical situations where you'd choose a fitting with an external thread:

Connection to a wall valve or ball valve – for example, in front of a radiator or behind a mixer tap. The valve body has an internal thread, so your fitting must have an external thread.
Connection into a manifold – connections on underfloor heating manifolds usually have an internal thread, so the pipe connects via a fitting with an external thread.
Connection to a wall outlet holder – the body of the universal wall outlet holder for water, single has an internal thread on the pipe connection side, so the fitting must have an external thread. The same applies to the universal wall outlet holder for water, double, where both connections work the same way.
Connection into a filter body, pressure reducing valve, or expansion unit – these fittings typically have an internal thread G 1/2" or G 3/4".
Extension using a sleeve – when you need to extend a pipe run through a wall or a tile panel, the external thread screws into a reworked sleeve or reducer with an internal thread.

Specific products covering the most common combinations:

Screw fitting 1/2x16 with external thread – designed for a pipe with an outer diameter of 16 mm and thread G 1/2" (external). Ideal for thinner domestic hot water lines and underfloor heating.
Screw fitting 1/2x20 with external thread – 20 mm pipe, G 1/2" thread. This combination occurs, for example, when connecting a 20 mm pipe to a fitting with a narrower 1/2" connection.
Screw fitting 3/4x20 with external thread – 20 mm pipe, G 3/4" thread. This size is standard for central heating distribution lines and circulation branches with higher flow.

Internal thread on screw fittings – where and why you need it

Fittings with an internal thread are somewhat less common than fittings with an external thread when it comes to plastic pipes, but there are scenarios where they are necessary or clearly more advantageous.

An internal thread comes into play in these situations:

Connection to fittings with an external thread – if, for example, a fitting, valve, or manifold has an external thread on the outlet (less common, but occurs with some types of valves and filters), you need a fitting with an internal thread.
Connecting two pipes via an intermediate piece with an external thread – for example, a steel sleeve or galvanized extension piece has an external thread on both ends, and you need two fittings with an internal thread.
Direct installation into a wall via a threaded anchor with an external thread – for wall outlets or concrete penetrations, a fixed element with an external thread is sometimes used, onto which a fitting with an internal thread is screwed as the connection point for the pipe.
Adapters and reducers – when reducing the thread diameter, you may have an internal thread on one end and an external thread on the other (a so-called reducing insert or reducing sleeve).

An important practical note: in a typical home installation, where you're going from a plastic pipe (PEX, PEX-AL-PEX, PPR) to a standard installation fitting, you'll be working with external-thread fittings 80–90% of the time. Fittings with an internal thread are more likely to be encountered in special connections or when working with steel piping.

When confusion arises and how to prevent it

In installation practice, I've seen several situations that recur in almost every other job where a customer purchased the material themselves. A summary of the most common mistakes:

1. The customer confuses "1/2" thread" with "1/2" pipe"

This is probably the most common mistake. The customer has a pipe with an outer diameter of 16 mm and says: "I need a 16 mm thread." In reality, they need a G 1/2" thread – and these two numbers seemingly have nothing to do with each other, even though they're an absolutely standard combination. Always ask (and clearly tell the seller) the outer diameter of the pipe, not the "inch size", to avoid a double mistake.

2. The customer buys two pieces of the same thread

If you buy a fitting with an external thread and want to screw it to another fitting with an external thread – they won't screw together. An external thread needs a counterpart with an internal thread, and vice versa. In practice, it happens that a customer buys two fittings "from the same box" because they look the same, but doesn't realize one has an external thread and the other an internal one.

3. Confusing tapered and cylindrical threads

The G pipe thread (ISO 228) is cylindrical – it has the same diameter along its entire length. There is also a tapered pipe thread R (Rc, BSPT) – this one narrows toward the end. These two types are NOT interchangeable, even though they share the same size designation. With common screw fittings for plastic pipes, you'll almost always encounter the cylindrical G thread, and the sealing effect is achieved not by the geometry of the thread but by sealant (teflon tape, hemp fiber). For more on this topic, see the article Sealing materials for screw joints – teflon, hemp fiber, or gasket.

4. Incorrect orientation when installing behind tiling

When installing piping before tiling a wall with ceramic tiles, a special situation applies: the fitting must protrude from the tiling to the correct depth so that an angle valve or mixer cover can be screwed onto it. If you choose the wrong type of thread, either the valve won't fit, or you'll have to fix a wall outlet that's been pulled out by a few millimeters with an extension insert. Therefore, we recommend measuring the tile thickness in advance and choosing the correct length of the threaded part or the correct type of wall outlet accordingly – see more in the article Universal wall outlet holder for water – single vs. double and when to use which.

Material aspects and thread durability

Screw fittings for plastic and multilayer pipes are made predominantly of brass or nickel-plated brass, and less commonly of polymer materials (PP, PPSU). The choice of material also affects the character of the thread:

Brass fittings – the thread is precise, corrosion-resistant, with a long service life. Suitable for both drinking water and heating. Disadvantage: higher price and weight.
Nickel-plated brass – the same properties, better appearance, higher resistance to surface oxidation.
Polymer (plastic) fittings – lighter, cheaper, but the thread is less resistant to over-tightening (so-called stripping). Never over-tighten a plastic thread. They are commonly used for cold water lines and less demanding applications.

In terms of the thread, the following applies: the external thread is always the more stressed point compared to the internal thread, because it bears the torque when screwing and nothing protects it from the outside. That's why the external thread should always be made of higher-quality, stronger material if heavier operation is expected (higher pressure, frequent temperature fluctuations).

Practical scenarios – when external, when internal

Let's look at a few specific situations commonly encountered in apartment and house installations:

Scenario A: Connecting underfloor heating to a manifold

You have a plastic manifold with 8 circuits. Each manifold outlet ends in an internal thread G 3/4". Each circuit uses a PEX 16 mm pipe. You need: a screw fitting 3/4"×16 with an external thread – the external thread screws into the manifold's internal thread, and on the other side, the fitting grips the 16 mm pipe using a press or compression mechanism.

Scenario B: Connecting a radiator valve to a multilayer pipe

The radiator valve has a G 1/2" connection with an internal thread. The pipe comes out of the wall with a diameter of 16 mm. You need a 1/2"×16 fitting with an external thread. The external thread of the fitting screws into the valve, while the other end grips the pipe.

Scenario C: Connecting two steel sections via a plastic pipe

An old steel line ends with an external thread G 3/4" on both sides. You want to insert a flexible plastic section between them. In this case, you need fittings with an internal thread G 3/4" on both ends of the plastic pipe, which screw onto the external thread of the steel pipes.

Scenario D: Installing a bathroom faucet mount via a wall outlet

When installing a bathroom faucet above a shower (e.g. a rain shower head), two pipes pass through the wall. Each pipe ends directly in the wall in the form of a wall outlet connection. The wall outlet holder – for example the universal wall outlet holder for water, double – is fixed to the wall and has an external thread G 1/2" on the outer side for the faucet connector itself. On the pipe connection side it has an internal thread, so the pipe must be connected here via a fitting with an external thread.

Sealing aspects and the effect of thread type on tightness

Whether you use an external or internal thread, the tightness of the joint depends mostly on the quality of the sealant, not on the thread itself. For a cylindrical G thread, the thread itself does not seal – sealing is provided either by a flat sealing washer (at the location of the front sealing surface), or by thread sealant applied directly onto the thread (teflon tape, hemp fiber with paste).

Practical rules for tightness:

Always wrap the teflon tape in the direction of the thread's winding (clockwise when looking at the thread head-on). If wrapped in the opposite direction, the tape unwinds during screwing instead of being pressed into the grooves.
Sealant – tape or hemp fiber – is applied to the external thread. Sealant does not belong on the internal thread, because it would be torn off or unevenly distributed during screwing.
A flat rubber sealing washer (if present in the fitting head) replaces the thread sealant – in that case, don't use teflon tape at the same time, otherwise you would over-tighten the joint and risk cracking the fitting body.
Check that the thread is not damaged or deformed. Even a small deformation in the thread will mean the joint will never be hermetic, regardless of how much sealant you use.

More details on sealants can be found in the article Sealing materials for screw joints – teflon, hemp fiber, or gasket and on the installation procedure in Installing a screw fitting on a pipe step by step – what you need and how to proceed.

Visual inspection before installation – quick checklist

Before you start screwing, do a quick visual audit of each fitting and the opposing element:

✅ Is the thread type (external/internal) compatible between the two (every external must have a matching internal)?
✅ Is the thread size designation the same (G 1/2" to G 1/2", G 3/4" to G 3/4")?
✅ Is the pipe diameter correct for the press/compression mechanism of the fitting?
✅ Are the thread grooves undamaged, uncut, or clean?
✅ Is the sealant correctly applied (on the external thread, in the correct direction)?
✅ Try tightening by hand first – does it screw easily and without resistance? If not, it may be the wrong thread type or a mismatch in the thread.

Frequently Asked Questions (FAQ)

What do "AG" and "IG" mean on fitting packaging?

AG is an abbreviation of the German "Außengewinde" – external thread. IG is "Innengewinde" – internal thread. In English labeling, "male" (external) and "female" (internal) are used. All these terms refer to the same thing – just in different language traditions. When you buy a screw fitting labeled, for example, "1/2" AG × 16", it means an external pipe thread G 1/2" on the threaded side and a press mechanism for a 16 mm pipe on the other side.

Can I screw an external thread G 1/2" together with an internal thread G 3/4"?

No – these two threads will not screw together. Although G 1/2" and G 3/4" belong to the same family of Whitworth pipe threads, they have different diameters (G 1/2" has an outer diameter of 20.96 mm and G 3/4" has 26.44 mm) and a different number of threads per inch (G 1/2": 14 TPI, G 3/4": 14 TPI, but the diameter is different). When you try to connect them, you'll feel that the thread "won't go" or immediately jams. Never force the connection – you risk damaging the thread.

How do I know if my fitting has an internal or external thread?

Open the connection opening of the fitting and look inside. If you see a thread spiral inside the opening – internal thread (IG). If the thread ridges protrude outward from the body of the fitting – external thread (AG). Another simple test: hold a G 1/2" or G 3/4" nut against it – if it can be screwed onto the fitting's connection, the connection has an external thread.

Is a screw fitting with an external thread always the "correct choice" for connecting to a valve?

In the vast majority of cases, yes – standard installation valves, ball valves, thermostatic heads, and most manifolds have an internal thread on the supply side. A fitting with an external thread is therefore the correct choice for connecting a plastic pipe to these fittings. Exceptions exist (some special valves or reducers have an external thread), but these are clearly marked in the fitting's technical documentation.

Do I need to use a sealant when installing an external thread? Isn't the thread itself enough?

The cylindrical G pipe thread (ISO 228) does not seal on its own. You must use teflon tape, hemp fiber with paste, or a flat rubber washer (depending on the type of joint) to seal it. The opposite is only true for tapered R/BSPT threads, where the conical geometry can provide a basic seal on its own – but additional sealant is recommended here too. Basic rule: never leave a joint without sealant, and always apply it to the external thread.

What if the external thread on my fitting gets cut or damaged during installation?

A damaged threaded part cannot be reliably repaired – the only solution is to replace the entire fitting with a new one. Thread damage most commonly occurs from screwing it in at an angle (the thread gets "cut" crooked), using the wrong tool (for example a jaw wrench instead of a pipe wrench), or tightening a dry thread without sealant, where metal-on-metal friction damages the grooves. Never try to "fix" a damaged thread by tightening it further – a leak will always occur. The topic of causes and solutions for leaks is covered in more detail in the article Causes of water leaks at screw joints – common mistakes and their repair.

Conclusion – how to reliably choose the correct thread type

Choosing between an external and internal thread on a screw fitting is not rocket science, but it does require diligence and a moment to check what the opposing part has. The rule is simple and works without exception in practice: an external thread always looks for an internal thread, and vice versa. If a fitting or valve has an internal thread – and this is true for the vast majority of common installation fittings – your fitting must have an external thread.

For typical home installations using plastic and multilayer pipes, you will most often work with combinations such as fitting 1/2×16 with external thread for thinner lines or fitting 3/4×20 with external thread for main heating branches with higher flow. Check every element of the installation before mounting – thread type, thread size, and pipe diameter – and you'll save yourself unnecessary trips back to the store as well as frustration from a water leak after closing up the wall.

For more on choosing the right fitting based on diameter and material, see the article How to choose the correct screw fitting for pipes – diameter, thread, and material. If you have questions about connection spacing and mounting, also check out How to correctly mount a wall outlet holder – spacing, anchors, and position.

Do you have a question about this topic?

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