What diameter and length of gas flexible hose – how to measure and choose correctly

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Why choosing the right gas flexible hose is more important than most people think

The gas flexible hose is one of the components that receives the least attention – and yet it is one of the most important safety elements of the entire gas installation. Customers usually come to the store with a simple request: "I need a gas hose." When you ask them about the diameter, length or type of end fitting, they usually respond with vague shrugs. That is understandable – not everyone is a plumber. But precisely for that reason, this article was created: so that you can accurately know what to measure, what to look for and how to choose a hose that will safely function for years.

In practice, we see recurring problems: the hose is too short and stretches when the appliance moves slightly, or on the contrary, it is too long and folds into loops. Others buy a hose with the wrong thread and try to "solve" it with an additional reducer, creating additional connections and potential leaks. And some have a hose with the correct diameter but the wrong standard – which in the case of an insurance investigation of an accident can have serious legal consequences.

We will look at the entire issue systematically: from basic dimensions, through measurement, to specific product recommendations for common installation situations.

Basic terms: what is a gas flexible hose and how does it work

A gas flexible hose (also known in technical terms as a flexible connecting hose or gas connection hose) is a flexible pressure hose used to connect a fixed gas pipe to a gas appliance – most often to a stove, cooktop, boiler, flow heater or industrial burner. Unlike solid copper or steel pipes, it has the advantage of being able to compensate for appliance vibrations, allows for easy disassembly of the appliance without the need to cut the pipe, and can tolerate minor spatial deviations during installation.

The construction of a modern gas flexible hose usually consists of three layers:

Inner layer – the sealing medium is most often EPDM or a special synthetic rubber resistant to gaseous hydrocarbons (natural gas, propane-butane)
Middle reinforcement – braiding made of stainless steel (AISI 304 or 316), which ensures mechanical strength and resistance to overpressure
Outer sheath – protects against mechanical damage, UV radiation and chemical influences of the environment

Some hoses have an outer braiding made of stainless steel visible directly (so-called "open braiding"), while others are covered with a plastic sheath. In domestic installations, we most often encounter stainless steel braiding with a visible mesh.

Diameter of the gas flexible hose – what exactly to measure and what to understand

This is the point where the most confusion arises. When we talk about the "diameter" of the hose, it is important to distinguish between three different quantities, all of which are called "diameter", but mean something different:

1. Nominal diameter (DN – Diameter Nominal)

Nominal diameter is the internal diameter of the hose, i.e. the diameter of the opening through which the gas flows. For domestic gas flexible hoses, the standard nominal diameter is DN 13 (i.e. 13 mm internal diameter). This value corresponds to the gas flow capacity and must match the requirements of the appliance. For most domestic appliances (stove, cooktop, boiler up to 30 kW), DN 13 is more than sufficient.

For larger industrial burners, gas distribution systems or commercial kitchens, hoses with DN 20 or DN 25 are used, but these are outside the scope of typical domestic installation ranges.

2. Thread diameter of the end fittings

The hose end fittings (fittings, i.e. metal ends with a thread) have their own dimensions, which are given in inches according to the BSP (British Standard Pipe) standard. The most common end dimensions for domestic gas flexible hoses are:

G 1/2" (RP 1/2") – the most common dimension, corresponds to an external thread of 20.955 mm
G 3/4" – less common in domestic installations, more common in boiler rooms
G 1" – for stronger distribution systems and commercial applications
R 1/2" – conical (tapered) thread, different from the cylindrical G 1/2"

Here comes an important technical detail: the thread can be cylindrical (marked as G or Rp) or conical (marked as R or NPT). Cylindrical threads are sealed with a sealing washer or Teflon, while conical threads are sealed by the thread itself. A combination of conical and cylindrical threads without the correct sealing element is a source of leaks.

3. Outer diameter of the hose

The outer diameter of the hose (including the braiding) is important in terms of the installation space – i.e. whether the hose fits behind the stove or under the cooktop. A typical DN 13 gas flexible hose has an outer diameter of about 20–22 mm. When planning the space behind the appliance, you should also consider the minimum bending radius of the hose, which is about 50–80 mm for standard domestic hoses.

How to Determine the Correct Hose Fitting Type

Flexible gas hose fittings are combined in various ways: one end can be different from the other, because the wall connection (gas line) can have a different thread type than the appliance connection. Therefore, hoses are produced in combinations of MF (Male-Female, i.e., external-internal thread) or MM (Male-Male, i.e., external-external thread).

A typical combination for a stove or cooktop in a standard apartment installation is:

On the wall side (gas line): external thread G 1/2" or R 1/2" – so the hose must have an internal thread (Female) on this side
On the appliance side: internal thread G 1/2" – so the hose must have an external thread (Male) on this side

The resulting hose combination is therefore R 1/2"(T) – G 1/2" MF, where "T" denotes a sealing ring (tangent), "M" means Male (external), and "F" means Female (internal). You will find such a designation, for example, in the product flexible gas hose WS-CE R1/2(T)-G1/2 MF 150cm, which is suitable precisely for standard home installations.

Before purchasing a hose, always check:

Type of thread on the wall (gas line outlet) – conical or cylindrical, external or internal
Type of thread on the appliance – usually cylindrical internal G 1/2"
Thread size – G 1/2", G 3/4", or possibly another

If you are unsure about the thread type, the simplest way is to place a thread template against the thread or have it measured with a thread gauge. Alternatively – bring the old hose to the store and compare the threads physically.

Flexible Hose Length – How to Measure Correctly and Why Every Centimeter Matters

Length is perhaps an even more critical parameter than diameter, because an error in length cannot be easily corrected. You cannot extend the hose (connecting two hoses is not allowed!), and a too short hose will be constantly under tension. On the other hand, a too long hose creates loops where condensation can accumulate and the hose can be mechanically damaged by edges.

Measuring Hose Length Procedure

Hose length is measured as the distance between the centers of both connection fittings in the assembled state – not the external length of the hose including the fittings. However, in practice, this is not straightforward, because the appliance is not yet in place. Therefore, proceed as follows:

Place the stove or appliance at the planned location – but do not yet fix or install it.
Measure the distance from the axis of the gas line outlet on the wall to the axis of the connection on the appliance – ideally with a tape measure or steel tape.
Add a 10–20 % reserve to the measured distance – so if the distance is 80 cm, choose a 100 cm hose. This reserve compensates for route irregularities (the hose does not lie in a straight line, but describes a slight curve).
Consider the direction of the outlet – if the wall outlet points forward and the appliance connection points backward, the hose must describe an arc, which requires a longer hose than the direct distance between the points.
Check the minimum bend radius – never fold the hose into a sharp angle. The minimum bend radius is usually 50 mm for DN 13.

Standard Lengths Available on the Market

Flexible gas hoses are delivered in standardized lengths. On the Slovak market, the following lengths are commonly available:

50 cm – for very close connections (e.g., a built-in gas cooktop in a kitchen line directly above an outlet)
75 cm – short reach, suitable for gas flow heaters
100 cm – the most commonly sold length for standard kitchen stoves
150 cm – for situations where the gas line outlet is to the side of the appliance, or where the appliance is further from the wall
200 cm – for special situations, longer routes or industrial burners; available, for example, as Flexira xConnect Gas Standard R1/2(T)-G1/2, 2000 mm

Warning: A flexible gas hose must never pass through a wall, floor, or ceiling. It must not be embedded, covered with plaster, or run in hollow spaces without the possibility of visual inspection. The entire hose route must be accessible and visually inspectable.

Most Common Installation Scenarios and Recommended Dimensions

Scenario 1: 60 cm Kitchen Stove – Standard Situation

The gas line outlet is in the wall behind the stove at a height of 10–30 cm from the floor, and the stove is placed right next to the wall. The straight-line distance between the outlet and the stove connection is 40–60 cm. Recommended hose length: 100 cm. The hose describes a slight arc behind the stove, has enough reserve when the stove is pulled out for cleaning, and at the same time does not create dangerous loops. Thread: R1/2"(T) – G1/2" MF.

Scenario 2: Built-in Gas Cooktop in a Kitchen Line

The outlet is in the cabinet under the cooktop, and the cooktop connection is directly above it, with a distance of only 20–30 cm. Recommended hose length: 50 cm. Here, you must pay attention to the minimum bend radius – the hose must not be compressed too much. Check that the hose is not pinched by the cabinet doors.

Scenario 3: Wall-mounted Gas Boiler

The boiler is mounted on the wall and the gas supply comes from the floor or wall. The distance can be 60–120 cm depending on the room layout and boiler position. Recommended hose length: 100 or 150 cm. It is also important to check the maximum working pressure of the hose – gas boilers can operate at higher pressure than stoves. Read the technical specifications of the specific hose.

Scenario 4: Kitchen after renovation, moved gas outlet

After a kitchen renovation, it often happens that the kitchen layout has changed and the gas outlet is now further away from the appliance or located on the side of the room. The distance can be 130–180 cm. Recommended length: 150 or 200 cm. If you need a hose longer than 200 cm, it is better to consider extending the gas line with a rigid pipe – this must be done by a certified plumber.

Scenario 5: Industrial burner or commercial kitchen

Completely different rules apply here: higher gas flow, higher pressure, and different standards. For these applications, hoses with a larger diameter (DN 20 or more), higher working pressure, and special certifications are required. Always consult a specialist.

Material, certification and standards – what every legal gas hose must have

In Slovakia, the European standard EN 14800 applies to flexible gas hoses made of stainless steel with braiding, or EN 14432 for hoses used with LPG (propane-butane). Every hose sold in the EU must have the CE marking and must be accompanied by a Declaration of Performance (DoP).

You must find the following information on the hose or its packaging:

CE marking
Number of the relevant standard (EN 14800 or EN 14432)
Maximum working pressure (typically 0.5 bar for domestic installations, or 5 bar for LPG)
Type of gas for which the hose is intended (G20 = natural gas, G30/G31 = propane/butane)
Date of manufacture or expiry date (important! – see below)
Length and dimensions of the fittings

The topic of certifications and standards for gas installation components is discussed in more detail in the article Certifications and standards for gas installation components – what every part must have in our Knowledge Centre.

Lifespan of a flexible gas hose

This is a point that many homeowners completely ignore: a flexible gas hose has a limited lifespan. According to most manufacturers and recommendations from gas companies, the hose should be replaced every 5 years, regardless of whether it shows visible damage or not. The reason is the aging of the inner rubber layer, which gradually loses elasticity, cracks, and can cause a gas leak.

In practice, we often encounter hoses that are 10 or more years old. One customer brought us a hose with a production year of 2007 – it was as stiff as a wire, the inner rubber was cracked, and the hose looked quite good from the outside. This is exactly the kind of situation where an accident is imminent and the owner is unaware of it. How to recognize when it is time to replace the hose is described in detail in the article Replacement of a flexible gas hose – how to recognize when it is time to replace it.

Flexible gas hose and ball valve – complete connection

A flexible gas hose never works alone – it should always be part of a complete connection, where a shut-off valve is installed before the hose (at the gas pipe outlet). Most often, this is a ball valve, which allows for quick closure of the gas supply to the appliance without the need to interfere with the main gas shut-off valve.

When choosing a ball valve, the same thread parameters must be considered as with the hose: thread size and type (tapered/parallel, internal/external). The valve must be certified for use with gas – not every ball valve is suitable for gas. In the gas installation supplies category, you can find, for example, a gas ball valve FF 2" internal/internal thread with a lever, suitable for larger connections with inch-sized diameter.

The arrangement of a complete connection looks like this: gas pipe → ball valve → flexible hose → appliance. This arrangement is standard and recommended. If the valve at the outlet is missing or is an older type (tap instead of a ball valve), we recommend replacing it at the earliest opportunity. More on this topic can be found in the article How to choose the right gas ball valve – thread, material and sealing class.

Practical tips for selecting and purchasing a flexible gas hose

Based on experience from many customer orders, we can summarize several practical tips that will save you time, money, and help you avoid problems:

Bring the old hose with you – if you are replacing the hose, bring the old one. The thread type, length, and type of fitting are immediately readable from the original piece.
Don’t buy a hose “in bulk" – hoses have a production date, so don’t buy a large stock in advance. A hose in the packaging ages over time.
Don’t combine hoses – two hose pieces connected with a reducer are not a substitute for one correctly long hose. Every connection is a potential leak.
Check the sealing – most hoses have a rubber seal in the fitting. Before installation, check that the seal is present and undamaged.
Don’t use Teflon tape on hoses with a sealing washer – if the hose fitting has a rubber sealing washer (marked with “T” in the hose code), the sealing is ensured by this washer. Adding Teflon tape on top may actually prevent the washer from seating properly.
Follow the bending direction – don’t bend the hose right after the valve. Leave at least 5 cm of straight section before the first bend.
Check the tightness after installation – always apply a soapy water solution to the connections after installation and check for bubbles. Never use a flame to detect gas leaks. Tightness of gas connections and how to check for gas leaks is described in detail in the article Tightness of gas connections – how to check for gas leaks yourself.

What to do if you cannot determine the correct size

This happens – a customer comes in, does not know the thread type, does not know the length, and the appliance is not with them. In such a case, there are several solutions:

The first option: Measure the thread with a thread gauge – these are available in every larger hardware store. Place the gauge on the thread and read the value. The second option is to contact the appliance manufacturer and request a technical datasheet, where the connection dimensions are precisely stated. The third option is to consult a gas fitter, who can determine the correct type of hose at a glance.

In the case of installing a new connection from scratch, we recommend consulting the entire process with a certified gas fitter. Gas appliance installation is regulated in the Slovak Republic, and guidelines for the procedure, tools and safety rules are described in the article Installation of a ball valve for gas – procedure, tools and safety rules and also in Connecting an appliance via a flexible gas hose – step by step.

If you are planning to install the connection yourself (which is allowed for repairs and replacement of existing components, not for new gas lines), read Common mistakes in the installation of gas installation components and how to avoid them in advance – this will help you avoid unnecessary mistakes that we see repeatedly.

Propane-butane hoses vs. natural gas hoses – are they interchangeable?

This is a question that comes up regularly – and the answer is no, they are not interchangeable without verification. Although they may seem the same at first glance, there are technical differences:

Natural gas (methane, G20) operates at low working pressure (usually up to 25 mbar in the distribution network). Propane-butane (G30/G31) is stored and filled into cylinders at high pressure (up to 8–15 bar), although the pressure is reduced by a regulator during consumption. Hoses for propane-butane must have a higher test pressure and the inner layer material must be resistant to hydrocarbons of higher molecular weight.

Therefore, always check which type of gas the hose is certified for. The hose must indicate: G20 (natural gas), G30 (propane), G31 (butane) or a combination. A hose certified only for G20 must not be used for propane-butane appliances and vice versa.

Most frequently asked questions (FAQ)

Can I use the same hose for natural gas and for propane-butane from a cylinder?

Only if the hose is certified for both types of gas (G20 and G30/G31 are indicated on the hose or packaging). Many modern hoses are certified for both types, but always verify this in the technical datasheet or on the hose itself. Never use a hose that is not explicitly approved for the specific type of gas.

What is the difference between a hose marked R 1/2"(T) and G 1/2"?

The letter "R" indicates a conical (tapered) thread, which seals itself by cutting into the thread when tightened. The letter "G" indicates a cylindrical thread, which is sealed with an external sealing element – a sealing washer or rubber ring. The suffix "(T)" means that the fitting has an integrated sealing washer (soft rubber seal). These types of threads are not interchangeable without the use of the correct sealing element.

How long does a flexible gas hose last?

The standard recommended lifespan is 5 years. After this period, it is advisable to replace the hose preventively, even if it shows no visible damage. The reason is the aging of the inner rubber layer, which becomes brittle and cracks. Some manufacturers state a lifespan of up to 10 years, but this is only valid for hoses stored and used under ideal conditions. Under normal use in the kitchen (heat, humidity, grease in the air), the 5-year replacement interval applies.

Can I extend a flexible gas hose by connecting two shorter pieces?

No, this is not allowed. Every connection is a potential leak point. In addition, connecting two hoses using a fitting would create an unclear installation where it is difficult to check the tightness. If you need a longer route, buy a hose of the correct length (e.g. 150 or 200 cm), or have a certified gas fitter extend the fixed line.

What wrench should I use to install a flexible gas hose?

To install a flexible gas hose, use two wrenches: one to hold the body of the fitting (so you do not twist the entire hose and strain it), and the other to tighten the thread. Never tighten the hose by gripping the braid – this can scratch or deform it. Tightening torques are usually stated in the manufacturer's installation instructions. For a G 1/2" thread, a typical torque is 15–20 Nm. Do not over-tighten – cracking the sealing washer or damaging the thread can cause a leak.

Can I route a flexible gas hose through a hole in the wall of a kitchen cabinet?

Yes, but with a condition: the hole in the wall of the cabinet must be large enough (at least 30 mm diameter for a standard hose DN 13), the edges of the hole must not be sharp – they must be rounded or protected with a rubber grommet to prevent cutting the hose. The hose must not be permanently fixed in the hole – it must be easily removable for inspection and replacement. The hose must not be routed through an enclosed space that is not ventilated.

Conclusion: three basic rules when choosing a flexible gas hose

If you remember only three things from this article, let them be these: First – always verify the type and size of the thread on both sides of the connection before you buy the hose. The dependency R1/2" vs. G1/2", conical vs. cylindrical, internal vs. external – these details are crucial. Second – measure the route length with a 10–20 % reserve and consider the direction of bends. A longer hose is safer than a short one. Third – never buy a hose without CE certification and without verifying which type of gas it is intended for.

A correctly chosen and installed flexible gas hose is a quiet and reliable part of the installation that you will not even think about for years. An incorrectly chosen one is a silent threat that can manifest at any time. Invest a little time in the correct selection – it is worth it.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.