Ball Valve FF vs. FM: What's the Difference and When to Use Which

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Ball Valve FF vs. FM: What's the Difference and When to Use Which

When a customer sits down at the computer for the first time and starts looking for a ball valve for their home water supply, pump or plumbing system, sooner or later they will come across two abbreviations: FF and FM. Sometimes they are written next to dimensions, sometimes in product names, sometimes only in technical documentation. And this is where the confusion begins – many people assume it is some kind of internal manufacturer coding or meaningless technical jargon. In reality, these two letters describe something absolutely specific and practical: what type of thread is on each side of the valve.

If you buy the wrong type, the valve simply won't screw in – or you will have to buy additional reducers and adapters, which unnecessarily increase the cost of installation and complicate the piping. That is why in this article we will look at the matter from the basics: what exactly FF and FM mean, what the real technical difference between them is, where each type is used and how to properly connect it to common plumbing situations in practice.

What do the abbreviations FF and FM mean for valves

Both abbreviations come from Latin, or from the European technical terminology for valves and pipes:

FF (Femina-Femina) – both ends of the valve have an internal thread. Femina in Latin means woman, in a technical context "female thread", i.e. internal (the thread is cut into the inside of the valve body).
FM (Femina-Maschio) – one end has an internal thread (femina) and the other has an external thread (maschio = male, the thread is on the outside of the valve body projection).

In practice, these abbreviations are used as a standard throughout Europe, although you may also come across their English equivalents: FF = F/F or "female-female", FM = M/F or "male-female". In the German standard you will commonly see IG/IG (Innengewinde) for FF and IG/AG or AG/IG for FM. All these markings refer to the same thing.

Internal and external thread – basics for every plumber

Before we get into the comparison, it is good to recap the basic physics of threads, which every plumber knows, but not every layperson realizes when making their first purchase. A thread is a spiral protrusion or groove on a pipe or valve, which allows two parts to be mechanically connected by turning.

External thread (AG, maschio, male) is the one that is cut on the outer surface of the pipe or valve neck – the thread points outward, it can be felt with your fingers from the outside. You typically have it on the end of a copper or steel pipe, on the output of a pump, on the input of a filter or on the male side of an FM ball valve.

Internal thread (IG, femina, female) is the opposite – the thread is inside the hole, on the wall of the housing or valve. You screw an external thread into it. Practically every FF ball valve has an internal thread on both sides, into which the external threads of the pipes or fittings enter.

The basic rule of connection: an external thread of the same size must go into an internal thread. Two internal threads cannot be connected, two external threads cannot either – you always need one of each. That is why it matters what type of end the pipe has, to which you are connecting the valve.

What an FF valve looks like in practice – internal thread on both sides

Valves with the designation FF are by far the most popular type in the area of home plumbing and water supplies. The reason is simple: the vast majority of copper, plastic (PPR, PEX) and galvanized piping is connected via fittings with an external thread – i.e. the pipe "protrudes" with an external thread and you simply screw it into the FF valve from both sides.

A typical example from practice: you are installing a home water supply in the basement. A hose or copper pipe with an external 1" thread comes out of the pump. A 1" external thread also enters the pressure tank. Between them you place a 1" FF,P ball valve – both external threads are simply screwed into both valve ends and that's it. No additional reducer, no adapter.

It works the same way with smaller dimensions. For taps, filters or water meters in apartments, the ball valve 1/2" FF,P or ball valve 3/4" FF,P is used by default, because the distribution pipes are standardly terminated with an external thread of G 1/2" or G 3/4".

When is FF better and why it is the default in domestic installations

The FF valve is considered the standard, default type in apartment and house installations. The reasons are purely practical:

Most pipes have an external thread at the end – copper pipes with pressed fittings, PPR pipes with fittings, galvanized pipes are commonly terminated with an external thread of G or Rp. These can be directly connected to an FF valve.
Easier installation for a layperson – there is no need to worry about which side of the valve points where. The FF valve is symmetrical, the orientation does not matter, both ends are the same.
Less chance of error when purchasing – since the FF valve is symmetrical, there is no risk of turning it "wrongly". Each side accepts an external thread in the same way.
Greater availability of sizes – from 1/2" up to 6/4", FF valves are commonly in stock, for example ball valve 5/4" FF,P or ball valve 6/4" FF,P for larger industrial pipelines.

From a functional point of view, there is no difference between FF and FM valves. The ball inside, the seat, the lever, the body – everything is identical. The difference is exclusively in the type of thread on the necks. So if you have doubts and don't know what to buy, in most cases the correct answer is FF.

FM valve – where it makes sense and what it solves

The FM valve has an internal thread (femina) on one side and an external thread (maschio) on the other side. This means that one side connects to an external thread of a pipe or fitting, and the other side connects to an internal thread – so the FM valve itself "sticks out" with an external thread and you can directly screw in an element with an internal thread.

Where does this make sense? Imagine such a situation (which I see quite often during installations): you have, for example, a pressure switch or a pressure gauge, which has an internal thread on the input. The pipe has an external thread. If you were to use a standard FF valve between them, you would have to additionally purchase an FM adapter or a coupling with an external thread. If you instead purchase an FM valve, it already has an external thread on one side – you can directly screw it into the internal thread of the switch or pressure gauge, and on the other side you can screw in the external thread from the pipe. The system is more compact, fewer connections, fewer chances of leakage.

Another example: devices that have an internal thread on the input/output – some filter heads, pressure-reducing valves or expansion vessels. The manufacturer designed them with an internal thread, because they expect that a pipe or fitting with an external thread will be directly screwed into them. If you want to insert a closing valve between the filter and the pipe, an FM valve allows you to do this without any additional reducing elements.

Ball valve dimensions – from 1/2" to 6/4" and what they affect

Along with the difference between FF and FM, the dimension of the thread is also key when selecting a valve. In domestic installations and water systems, you will most often encounter these values:

1/2" (G 1/2") – the smallest common dimension, typically for check valves, taps at appliances, before water meters in an apartment. In water systems, it is used for side branches and technological inputs.
3/4" (G 3/4") – very common, especially in apartment cold and hot water distribution, garden water systems, and water heating systems.
1" (G 1") – standard dimension for home water systems, pumps, discharge pipes, and input of filtration devices. Higher flow, lower resistance.
5/4" (G 5/4") – for larger home water systems, garden irrigation of larger areas, and pumps with higher performance.
6/4" (G 6/4") – thick pipes in agricultural facilities, gardening, at group pumping systems, or in industrial filtration.

For more detailed information on how to choose the right dimension, see the article What ball valve size do you need – from 1/2" to 6/4" in this Knowledge Center. Here we just emphasize: the thread dimension must fit exactly with the dimension of the opposite thread on the pipe or device. An incorrect dimension cannot be sealed even with force or additional sealing – it simply will not screw in.

Material and construction – why it is important what is inside

For FF and FM ball valves in the category of home water technology, the valve body is almost always made of brass (a copper and zinc alloy, usually CW617N or similar). Brass has excellent properties for plumbing: corrosion resistance, mechanical strength, ability to precisely machine threads, and long service life.

The ball inside the valve is chrome-nickel (chromed steel) or fully brass – in quality valves, the surface roughness is less than 0.4 μm, which ensures a hermetic seal on PTFE seats. Precisely this PTFE (Teflon) sealing of the seats is key: it is not sensitive to chemicals in water, does not stick after long periods of inactivity, and allows easy opening even after years. More about materials can be found in the article Brass or plastic ball valve – comparison for plumbing.

Valve operation is another factor. All products mentioned in this article have a lever – which is a clear choice over knobs for home water systems. A lever allows quick closure with one motion (90°), can be operated with wet hands or with work gloves, and the position (open/closed) is immediately visible at a glance. More on this topic in the article Lever or knob on a ball valve – what is better for plumbing.

Practical scenario 1: Installation of a shut-off valve at a home water system

Let’s take a typical practical situation: a customer installs a pressure tank and pump in the basement of a family house. The pump has a discharge pipe Cu 28 with an external threaded neck G 1" (external thread). The customer wants to add a shut-off valve to the system so that in the case of service work, they can turn off the water without having to dismantle the entire pump.

Solution: 1" FF,P ball valve. The external thread of the discharge pipe (G 1" AG) is screwed into the left neck of the valve, and the continuation of the system is connected to the right neck of the valve with an external thread G 1". Sealing is done with hemp thread and Teflon paste or Teflon tape. The valve is installed at an easily accessible height, with the lever pointing perpendicular to the pipe (position "open"). Done.

Where would the problem be? If the customer mistakenly bought an FM valve. One side of the valve would have an external thread – which could not be directly connected to the external thread of the pipe. They would have to additionally purchase a coupling (FF adapter) as a transition, which would add one more connection to the entire installation, and every unnecessary connection is a potential leak point.

Practical scenario 2: Connecting a mechanical impurities filter

Second scenario: the customer has a mechanical impurities filter (screen filter) installed after the water meter. The filter head has an input neck with an internal thread G 3/4" (IG). The pipe has an external thread G 3/4" (AG). The customer wants to insert a shut-off valve between the pipe and the filter so that the filter can be cleaned without having to turn off the entire water supply.

Here you have two options:

Option with FF valve: The external thread AG from the pipe goes into the IG side of the FF valve – this works. On the other side of the valve, there is also IG, but the filter head also has IG. These two IG threads cannot be directly connected. You must additionally purchase an AG/AG coupling (external thread on both sides) as a transition between the valve and the filter.
Option with FM valve: The external thread AG from the pipe goes into the IG side of the FM valve – this works. On the other side of the FM valve, there is AG (external thread) – which you can directly screw into the IG input neck of the filter. No transition needed, one fewer connection.

The conclusion is clear: when one side of the device has an internal thread and the other side of the pipe has an external thread, an FM valve is a more elegant and compact solution.

Most common mistakes when choosing FF vs. FM valve

From practical experience with customers in the field of domestic water supply technology, several typical mistakes emerge when people purchase ball valves:

They do not check the type of thread on the existing pipe or device. They assume that a "classic valve" will fit everywhere. Result: purchase, installation, realization that it does not fit, return to the store or ordering a reducer.
They confuse the thread size with the pipe diameter. G 3/4" is not the same as a pipe with an outer diameter of 3/4". Thread size and pipe outer diameter are different things – G 3/4" has an outer thread diameter of approx. 26.44 mm. More on this in the article Internal thread on fittings – how to properly seal and connect the pipe.
They buy an FM valve thinking it is "universal". FM is not better or worse than FF – it is just a different type for a different situation. If you have external threads on both sides, an FM valve will create a problem on one side when connecting.
They underestimate sealing. Even the best valve will not hold pressure if the thread is insufficiently sealed. Hemp thread with paste or Teflon tape – always, without exception. Details in the article Installation of a ball valve on a water supply pipe step by step.
They buy a valve that is too small. In a domestic water supply system with an 800 W pump and a flow rate of 60 l/min, you should definitely not go for a 1/2" valve. Hydraulic losses in a small valve are significant and can affect the performance of the entire system.

Sealing FF and FM valve threads – practical tips

Regardless of whether you are installing an FF or FM valve, sealing the threaded connection is equally important. An internal thread is sealed by applying the seal to the external thread of the opposite part (pipe, fitting, bushing) – that is, the thread that is screwed into the valve.

Common sealing methods:

Teflon tape (PTFE) – wrap 3–5 layers in the direction of the thread (in the direction of tightening). Suitable for low and medium pressures, up to approx. 10–16 bar for cold water.
Hemp fiber + sealing paste – a traditional method, works well even for larger threads (3/4" and more). Hemp is applied thinly, evenly, without lumps.
Liquid sealants (Loctite 577, Tangit Uni-Lock) – a modern solution for professional plumbers, suitable for larger sizes and higher pressures.

Warning: no type of sealant can replace a proper thread. If the thread is rough, difficult to turn, or turns only 2–3 times and reaches the end too quickly, the problem is in the wrong size or damaged thread – not in the lack of sealant.

Combination of FF and FM valves in one system

In practice, it is nothing unusual in plumbing installations to use both types of valves in one system. For example, in a typical domestic water supply setup, you may have:

FF valve after the pump (on the discharge pipe, both sides AG from the pipe),
FM valve before the filter head (one side AG from the pipe, the other side AG male thread goes into the IG female thread of the filter head),
FF valve after the filter (both sides AG from the supply pipes of the distribution system).

It is always key to check both ends of each connection before purchasing, best by touch or sight – an external thread "sticks out" and has visible thread grooves on the outside, while an internal thread is "cast" into the housing and the grooves are on the inner wall. If you are unsure, bring a piece of pipe to the store or take a photo of the existing connection and the salesperson will help you.

Pressure, temperature and lifespan – what else you should know

Brass ball valves FF and FM in the category of water supply technology are standardly dimensioned for:

Maximum working pressure: 10–16 bar (PN 10 or PN 16 according to the manufacturer). Domestic water supplies typically operate at 2–5 bar, so there is a reserve.
Medium temperature: up to 90–120 °C – valves for water are primarily for cold potable water, but they are technically compatible with hot water in apartment distribution systems (max. 70 °C in standard installations).
Lifespan: 10,000–30,000 openings/closings for quality products with PTFE seats and chrome-plated ball. In a typical domestic water supply, where you turn the valve a few times a year, this is practically unlimited lifespan.

Problems with valves – why a connection leaks or the lever cannot be turned – are the subject of the article Common ball valve faults – why it leaks or cannot be turned, where these situations are analyzed in detail including repair procedures.

Most frequently asked questions (FAQ)

Is an FF valve always better than an FM valve, or vice versa?

Neither type is "better" – both are equally high-quality and functional. The difference is exclusively in the type of thread on the necks. An FF valve (both internal threads) is suitable when you have pipes or fittings with external threads on both sides. An FM valve is suitable when you have a device with an internal thread on one side (e.g. a filter head, pressure switch, manometer) and you want to avoid buying an adapter. In practice, FF is much more common, as most pipes are terminated with external threads.

Can I use an FM valve instead of an FF valve if I buy an adapter?

Yes, technically it is possible – you can add a reducer or bushing and connect an FM valve into a situation where an FF valve would be needed. But it makes no sense, neither economically nor technically. Every unnecessary connection is a potential leak point. Buy the correct type from the start, it is cheaper and more elegant.

How can I tell if an existing thread on the pipe is internal or external?

An external thread (AG, male) is visible on the outside of the pipe or neck – the thread grooves can be felt with fingers from all sides, usually protruding like a "screwed-out bump". An internal thread (IG, female) is inside the hole – the grooves are on the wall of the cavity into which the wrench is inserted. A simple test: try to place another thread – an external one will go into an internal one, but two of the same type will not connect.

What size ball valve should I choose for a domestic water supply with an 800 W pump?

For domestic pumps up to 1 kW and standard domestic distribution, 1" (G 1") is standard. For larger pumps (over 1.5 kW) or group feeding of multiple consumption points, consider 5/4" or 6/4". You never want the valve to be the "bottleneck" of the entire system – the valve size should match the pump discharge pipe size or be one step larger. A more detailed guide is in the article How to choose a ball valve for a domestic water supply or pump.

What should I use to seal the thread of a ball valve – Teflon or hemp?

Both work well. Teflon tape is easier for beginners and does not require any additional tools. Hemp fiber with sealing paste is traditional, slightly more robust for larger diameters (3/4" and more) and better fills possible irregularities in the thread. Professional plumbers increasingly use synthetic sealant (e.g. Tangit Uni-Lock), which is easier to apply and does not dry out. Never seal "by force" – if the thread fits properly, 3–5 turns of Teflon or a thin layer of hemp is enough.

Can I use an FF or FM ball valve in filtration circuits with chemically treated water?

It depends on the specific chemicals. Brass valves with PTFE seats are resistant to standard potable water, softened water (salt regenerations from softeners) and chlorine in the network. For aggressive chemicals (acids, highly concentrated disinfectants), you should consider stainless steel or plastic versions of fittings. More on this in the article Brass or plastic ball valve – comparison for plumbing.

Conclusion: FF vs. FM – a conclusion you will remember

The entire topic of FF vs. FM can be summed up in one sentence: an FF valve has internal threads on both sides (external pipe threads are screwed into it), while an FM valve has one internal and one external thread (the external thread of the valve goes into the internal thread of the device). Choosing the correct type takes only five seconds if you know what threads are on the sides you want to connect.

For the vast majority of household plumbing, pumps, and apartment piping systems, the correct choice is an FF valve—whether it's a 1/2" FF,P valve, 3/4" FF,P valve, or 1" FF,P valve. An FM valve has its irreplaceable place where you need to connect a pipe with an external thread directly to a device with an internal thread—without unnecessary adapters.

If you're still unsure exactly what you need before installation, do one thing: check both ends of the installation, take photos of the threads, and then read through other related articles in this Knowledge Center—such as Step-by-step installation of a ball valve on a water pipe or Common questions about fittings for home plumbing and filtration. The right valve, the right size, and the right sealing—this is how an installation works smoothly for decades.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us—we're happy to help.