Thread 3/8" or 1/2" – how to find out what you need

We are still preparing products.

But you can look at other categories.

3/8" or 1/2" thread – how to find out what you need

This is probably the most common question people face when replacing a hose under the sink, connecting a toilet tank, or installing a new faucet. It may seem like a small detail, but buying a hose with the wrong thread is frustrating – you have to return to the store, wait for delivery, and in the meantime, you have no water. That's exactly why we're addressing this topic in depth.

In practice, we often see people either estimating the thread by eye (which rarely works) or writing down something like "thread about 1 cm" and wondering why the hose doesn't fit. Threads of 3/8" and 1/2" look very similar at first glance, but they are not interchangeable. This article will guide you through the whole issue – from what a thread actually is, through where each is used, to a specific step-by-step method to determine the correct size without special tools.

What do 3/8" and 1/2" actually mean – basics without unnecessary theory

An inch (symbol ") is an English unit of length, 1 inch = 25.4 mm. When we talk about threads on water supply hoses, we are referring to the so-called BSP thread (British Standard Pipe), which is commonly marked as G thread in Europe. This is an important detail – the thread size does not refer directly to the pipe's outer diameter or the thread's own diameter, but is a historical reference to the inner diameter of the pipe from which the thread evolved.

That is why the outer diameter of a 1/2" thread is larger than 12.7 mm (which would be half an inch). In reality:

3/8" thread has an outer diameter of the threaded part (thread peaks) of approximately 16.66 mm
1/2" thread has an outer diameter of the threaded part (thread peaks) of approximately 20.96 mm

The difference is about 4.3 mm in diameter – not much to feel, but a 1/2" thread simply won't screw into a 3/8" hole and vice versa. If you try to connect the wrong threads by force, you risk damaging the thread on the valve or the shut-off box – and that is a much more expensive repair.

Where each thread is commonly used – overview by device

Before you start measuring, take a look at the following overview. In practice, many things can be estimated based on where you need to connect the hose. This is not a 100% rule, but it applies in 80–90% of cases:

3/8" thread – where you'll find it

Connecting a toilet (WC tank) – the valve under the tank almost always has a 3/8" internal thread (on the hose side). This is probably the most common use of a 3/8" hose in a household.
Water supply to a sink faucet – most sink faucets (including bidet faucets) have a M10 or directly a 3/8" external thread on the bottom side. Therefore, hoses to faucets are typically 3/8" on at least one end.
Angle valves (shut-off valves) under the sink and WC – the output from the angle valve towards the device is standard 3/8" external thread.
Hand showers, bidets – the supply to the hand shower or bidet behind the wall or on the wall can be 3/8".
Pressure vessels and small boilers – connections to small flow heaters or under-sink storage heaters.

1/2" thread – where you'll find it

Main water supply in the wall/floor – pipes in most apartment water supplies are 1/2" (Rp 1/2"), so all angle valves, direct shut-off valves, and wall-mounted valves have an input thread of 1/2".
Angle valves on the wall side – the input to the angle valve from the installation side is 1/2" (usually internal thread, which fits the 1/2" external thread of the pipe).
Kitchen faucet – supplies to kitchen faucets, especially those with higher flow, are mostly 1/2".
Shower and bathtub faucets – supplies are standard 1/2".
Washing machines and dishwashers – water supply to the washing machine is 1/2", so the washing machine hose (the thick, braided one) has a 3/4" or 1/2" thread.
Garden valves and outdoor taps – standard 1/2".

How to physically determine the thread size – step-by-step procedure

Having an overview by device is fine, but it doesn't always apply – in older apartments, during renovations, and with non-standard installations, you may encounter anything. That's why it's good to know how to measure or identify a thread directly.

Method 1: Measuring with a caliper (most reliable method)

A caliper is a tool that every person who occasionally deals with home repairs should have at home. It can be purchased for just a few euros and is easy to use.

What to measure? Measure the outer diameter of the thread on the valve – that is, on the angle valve, at the outlet of the valve, or at the inlet of the faucet. The thread must be dry and clean.

If measured ~16.5–17 mm → thread 3/8"
If measured ~20.5–21 mm → thread 1/2"
If measured ~26–27 mm → thread 3/4" (for washing machines, garden taps)

Be careful – you are measuring the outer diameter, not the diameter of the hole. That means on the external thread (male), you measure the maximum diameter. On the internal thread (female), you measure the diameter of the hole, but it is less accurate.

Method 2: Using a coin or calibrated object

If you don't have a caliper, you can roughly compare the thread diameter to a known object. For example, a one-euro coin has a diameter of 23.25 mm – a 1/2" thread (20.96 mm) will be slightly smaller. A 3/8" thread (16.66 mm) corresponds roughly to the diameter of a CR2032 battery (20 mm) – so it will be visibly smaller. This is only an approximate method, not accurate.

Method 3: Testing with old hoses or connectors

If you are replacing the original hose, simply bring it to the store or take a photo of its thread and the number on the hose body. Most hoses have the thread marking printed or stamped on them (e.g., "3/8" FF" or "1/2" FF"). FF means Female-Female, i.e., both ends have internal threads (female).

Method 4: Using thread gauge templates

Thread gauge templates are used in specialized stores, service centers, and by experienced plumbers. You simply place the template against the thread and see which one fits. For home use, this is an unnecessary investment, but if you are managing an apartment building or a larger property, it is worth having this tool.

What FF, MF, MM means – hose end markings

When choosing a hose, you will not only encounter the thread size, but also the markings FF, MF, or MM. This describes the combination of threads on both ends of the hose:

F (Female = female = female thread) – internal thread, the thread is inside the hole. It is placed over an external thread on the valve.
M (Male = male = male thread) – external thread, the thread is on the outer surface. It is screwed into an internal thread.
FF – both ends have internal threads. This is the most common type for connecting hoses to faucets and WC valves – the angle valve and the faucet have external threads, and the hose connects them from the inside.
MF – one end has an external thread, the other has an internal thread. Less common, used for specific transitions.
MM – both ends have external threads. Rare in standard household hoses.

In the vast majority of cases, standard stainless steel connecting hoses are FF – both ends have internal threads. For example, the PROFI MW hose 3/8" FF has internal 3/8" threads on both ends – and fits an angle valve and a faucet with external 3/8" threads.

Can both ends of the hose have different threads? – Reducing hoses and adapters

Yes, there are also hoses with different thread sizes on each end, for example 1/2" on one end and 3/8" on the other. Such hoses are useful when you have an angle valve with a 1/2" output (which can happen if the valve is not properly sized or is an older model) and a faucet with a 3/8" inlet.

Another option is reducing couplings (adapters) – metal or brass rings that convert the thread from one size to another. I do not recommend them in practice, if you can avoid it – each additional connection is a potential leak point. It is much better to choose the right hose from the start.

If you are 100% sure of the thread size on the valve and the faucet, and they are different, look for a hose marked, for example, "3/8" × 1/2" FF". If you are unsure, measure both ends before ordering.

Concrete examples from practice – what we have solved with customers

Theory is fine, but real situations vary. Here are a few typical cases that repeat themselves.

Case 1: Replacing the hose to the WC cistern

A customer buys an apartment in a building from the 80s, replaces the plumbing, but the old hose to the WC still works. Looking at the angle valve, we see a 3/8" external thread, and on the bottom of the cistern, also a 3/8" external thread. The correct choice: a 3/8" FF hose. The length depends on the distance between the valve and the cistern – usually 30–60 cm. If the distance is variable or imprecise, an adjustable hose is ideal – for example, the PROFI MW 3/8" FF in the range 30–60 cm covers most standard situations.

Case 2: New kitchen faucet

A customer replaced the kitchen faucet. The wall inlets are 1/2", and the new faucet has 1/2" external threads on the inlets (typical for kitchen faucets). The hoses will therefore be 1/2" FF, longer (40–80 cm, or 80–160 cm if the faucet is further from the wall). For example, the PROFI MW 1/2" FF 40–80 cm or for longer distances the PROFI MW 1/2" FF 80–160 cm would work well.

Case 3: Faucet with two inlets

A faucet typically has two inlets – hot and cold water. From the bottom of the faucet, hoses with threaded ends emerge. For most standard faucets, this is an M10 × 1 or 3/8" external thread – depending on the manufacturer. Wall-mounted angle valves have a 3/8" outlet. Therefore, the hose is 3/8" FF. A length of 30–50 cm is usually sufficient if the valve is directly under the sink.

Case 4: Customer with an older piping system and non-standard valves

In older panel buildings from the 60s–70s, we often encounter angle valves with different spacing or non-standard dimensions. In such cases, it is always recommended to measure physically – do not rely on assumptions. One customer had an angle valve with a 1/2" outlet instead of the standard 3/8" – either the valve was incorrectly selected during a previous renovation or it was an older dimension. Solution: a hose 1/2" FF or a direct 1/2" → 3/8" reduction on the valve.

Case 5: Thread mix-up – what happens

We have seen several times that a customer mistakenly bought a 1/2" hose for a 3/8" thread. During installation, it seemed that the thread "fit", but after tightening, it started to leak – the thread was crossed, and the thread grooves were damaged. Result: not only the hose but also the angle valve had to be replaced. Checking before purchase is definitely worth it.

Markings on the hose itself – where to find the information

If you are replacing an original hose that is still somewhat functional, try to find the marking directly on it. On most certified products, you will find the marking embossed or printed on the hose body (or on the coupling), containing:

Thread size – e.g., "3/8" or "1/2"
Thread type – FF, MF, or just F/M on each end
Length – in centimeters
Certification – e.g., DVGW, WaterMark, KTW – relevant for potable water
Maximum pressure and temperature – e.g., PN 10 or PN 16, 70°C

If the marking is missing or unreadable, measurement is the only reliable option.

Differences between 3/8" and 1/2" hoses – flow and practical consequences

The question of whether the thread size matters in terms of performance (water flow) is justified. Technically, yes – a hose with a larger internal diameter will carry more water per unit of time at the same pressure. However, in practice, for standard hoses under a sink, to a toilet or a faucet, it does not play a decisive role. These hoses are short segments, and the narrowest point in the system is usually elsewhere (e.g., in the faucet or valve itself).

More important is to choose a hose with the correct thread matching the fitting and sufficient pressure resistance (standard PN 10, i.e., 10 bar, which is always enough in domestic systems – typical pressure in apartments is 2–5 bar).

Thread and sealing – what else must be correct besides the size

Having the correct thread is not enough on its own. To prevent leaks, the connection must also be properly sealed. For stainless steel hoses with F-thread, sealing is typically handled as follows:

Flat rubber gasket (flat washer) – inserted into the female part (F) of the hose. This is the most common method. During installation, make sure the gasket is in place and does not fall out during tightening.
Cone-shaped sealing – less common, used with certain special fittings.
Teflon tape – not added for F×M connections with flat gaskets, as it is unnecessary and may interfere with the seal. Yes for cone-shaped and threaded connections without gaskets.

If the hose leaks despite the correct thread, check whether the gasket is in place and not damaged. More on this issue can be found in the article Why the hose drips or leaks at the connections – causes and repair in our Knowledge Center.

Most common thread selection mistakes – and how to avoid them

After years of communication with customers and monitoring complaints and returns, we still see the same mistakes. Let’s summarize them:

Estimating by eye without measuring – threads look very similar, your eye can deceive you.
Mixing up external and internal threads – the customer measures the hole diameter on the valve (internal thread) and assumes it is the hose diameter – but always measure the external thread of the fitting.
Ignoring the FF/MF marking – a hose with MF is bought, but the connection does not fit on one side because the fitting has a different thread type.
Reliance on "I saw on the internet that the toilet always has 3/8" – mostly true, but not always. Always verify your specific installation.
Ignoring the length in combination with the thread – the correct thread is bought, but the hose is too short, and during installation, you have to bend it into a sharp angle, which shortens its lifespan.

For more on hose lengths, see the article What hose length suits me – fixed or adjustable?

Product overview and which thread belongs to which hose

For quick orientation, here are specific products from our range and their threads:

Stainless steel adjustable hose PROFI S for potable water with stainless steel ends (1/2" FF, 50–100 cm) – both ends 1/2" internal thread, stainless steel nuts, for kitchen, bath, shower. Adjustable, covers a larger range of distances.
Stainless steel adjustable hose PROFI MW for potable water covered with plastic and with brass ends (3/8" FF, 30–60 cm) – both ends 3/8" internal thread, brass nuts covered with plastic. Ideal for toilets and sink faucets.
Stainless steel adjustable hose PROFI MW for potable water covered with plastic and with brass ends (1/2" FF, 30–60 cm) – both ends 1/2" internal thread, short range.
Stainless steel adjustable hose PROFI MW for potable water covered with plastic and with brass ends (1/2" FF, 40–80 cm) – both ends 1/2" internal thread, medium range for kitchen or sink with greater distance.
Stainless steel adjustable hose PROFI MW for potable water covered with plastic and with brass ends (1/2" FF, 80–160 cm) – both ends 1/2" internal thread, long range for larger distances or kitchen with longer routing.

Common Questions (FAQ)

How can I determine whether my angle valve has a 3/8" or 1/2" outlet if I don't have a measuring tool available?

If you don't have a measuring tool, try an approximate method – place a ballpoint pen (diameter approx. 7–8 mm) or a 1 euro coin (25.75 mm diameter) against the thread and estimate the comparison. However, the most reliable method is always physical measurement using a sliding caliper – you can buy one for 3–5 € at any hardware store. If you really can't access the valve, look at the original hose – the body of the hose or the nut should be stamped with "3/8" or "1/2".

Can I use a 1/2" hose instead of a 3/8" one if I use a reducer?

Technically yes, but it is not recommended if there is another option. Every additional connection is a potential weak point in terms of sealing and durability. If your device has a 3/8" thread, buy a 3/8" hose. Reducers are only a solution when you have no other option – for example, in atypical combinations of fittings.

What if one side is 3/8" and the other is 1/2"?

In that case, you need a reducing hose with a marking such as "3/8" × 1/2" FF" – one end is 3/8", the other is 1/2", both with internal threads. Such hoses are available and commonly found. Alternatively, you can use a 3/8" hose and attach a brass reducer from 1/2" to 3/8" to the valve.

Why is a 3/8" thread called 3/8" when its actual diameter is almost 17 mm and not ~9.5 mm (which would be 3/8 of an inch)?

This is a historical matter from the British system of BSP (British Standard Pipe). The thread designation refers to the nominal internal diameter of the pipe, not the actual thread diameter. A pipe with an internal diameter of 3/8" has a larger outer diameter due to the wall thickness – and the thread diameter is even larger. It is confusing, but it is an old industrial standard that cannot be easily changed, as it is used globally in plumbing installations.

Do I need to use Teflon tape during installation?

Do not use Teflon tape with hoses that have flat rubber sealing (which is the case for most stainless steel connecting hoses). The sealing is achieved by a flat rubber ring inside the nut. Adding Teflon tape on top may deform the seal or prevent the flat surfaces from seating properly. Teflon tape is used only for tapered threads or threaded connections without rubber seals.

Is an M10 thread the same as a 3/8" thread?

No, M10 is a metric thread with an outer diameter of 10 mm – it is a completely different type of thread. Some basin taps have an M10 × 1 thread on the inlet side, and special hoses with an M10 nut on one end and a 3/8" nut on the other (for the angle valve) are supplied. Therefore, always check what thread your specific tap has on the inlet side when choosing a hose – it can be M10, 3/8", or even 1/2", depending on the manufacturer.

Conclusion – a short summary

Selecting the correct thread is not difficult, but it requires a minute of attention and ideally a sliding caliper. The key points from this article that everyone should remember:

A 3/8" thread (outer diameter ~16.7 mm) is the standard for toilets, basin taps, and angle valves on the device side.
A 1/2" thread (outer diameter ~21 mm) is the standard for kitchen taps, showers, bathtubs, and the supply side in most installations.
The marking FF means an internal thread on both sides – this is the most common type for connecting hoses.
Measuring with a sliding caliper is always the most accurate – don't rely solely on assumptions.
Do not use Teflon tape with hoses that have rubber seals.
Reducers are a last-resort solution – if possible, choose a hose with the correct thread on both ends.

If you are unsure about the hose selection, length, or type of fitting, we recommend reading other articles in our Knowledge Centre – for example, How to choose the right stainless steel hose for drinking water, Stainless steel vs. brass fittings – what is the difference and what to choose, or Plastic-coated stainless steel hose vs. bare stainless steel – which is better. Each of them will help you make an informed decision and avoid unnecessary returns or repeated exchanges.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.