Converting inches to millimetres is useful, but in beverage dispensing there is a key difference: an inch measurement applied to tubing is not the same as an inch measurement applied to a BSP thread. Confusing them can cause leaks, incompatibilities, loss of flow or incorrect purchases.
Quick answer: inches in tubes and inches in threads are not the same thing
One inch equals 25.4 mm, but that direct conversion is not always enough to choose a part. For a tube, the inch size usually refers to the tube diameter, normally the outside diameter in push-fit fittings. For a BSP thread, the inch size identifies a thread standard and should not be interpreted as the actual tube diameter.
For example, a 3/8" tube may correspond to an approximate outside diameter of 9.5 mm, while a 3/8 BSP thread may show an approximate thread diameter of 16.5 mm. These are different concepts and should not be compared as if they were the same measurement.
Looking for tubing, a fitting or an adapter and not sure what size to choose?
At Install Beer we work with food-grade tubing, John Guest fittings, BSP adapters, connectors, couplers and components for beer, water, wine, vermouth, kombucha, cocktail and other on-tap beverage systems.
See John Guest View food-grade tubingThe most important difference: 3/8" tube is not 3/8 BSP thread
This is the point that causes the most mistakes. When you see a size like 3/8", you must first ask: does it refer to tubing, a thread, a barb, an internal bore or a push-fit connection?
In tap beer and beverage installations, the same number can appear in different contexts. That’s why, before buying a fitting, adapter, or tube, you must identify the part’s function within the system.
| Example | What it means | Approximate size | How to use |
|---|---|---|---|
| 3/8" tube | Tube size, usually related to the outer diameter in quick-connect fittings. | Approx. 9.5 mm outer diameter. | It is inserted into compatible push-fit fittings, connectors or outlets. |
| Lupulus II 3/8" tube | Technical beverage tubing with inner x outer dimensions. | 6.7 x 9.5 mm | Beer, soft drink, gas, wine or other compatible beverage line. |
| 3/8 BSP thread | Thread standard, not a direct tube size. | Approx. 16.5 mm in the product reference. | It screws into compatible BSP components. |
| 3/8 BSP adapter to 3/8" tube | Transition piece between a thread and a tube. | Thread and tube are different measurements. | Allows you to go from a BSP threaded connection to a quick connection for tubing. |
Inch to millimeter conversion table for pipes
In dispense tubing, the inch size is usually related to the tube diameter. In technical tubing such as Lupulus II, it is especially important to check the full size, because it may appear as inside x outside.
| Measured in inches | Typical outside diameter equivalent | Example of inside x outside measurement | Frequently used in dispensing |
|---|---|---|---|
| 3/16" | 4.8 mm | 3 x 4.8 mm | Small lines, restriction, testing or specific applications. |
| 1/4" | 6.35 mm | 4.3 x 6.35 mm | Water, filtration, gas, auxiliary lines and compact equipment. |
| 5/16" | 8 mm | 4.8 x 8 mm | Beverage, coils, compact units and push-fit connections. |
| 3/8" | 9.5 mm | 6.7 x 9.5 mm | One of the most common sizes for beer, water and beverages on tap. |
| 1/2" | 12.7 mm | Variable depending on tubing type | Higher flow, water, recirculation, return or specific installations. |
Reference table of BSP threads used in adapters
In BSP threads, the inch size identifies the thread standard. You must not convert the fraction directly as if it were a tube. That’s why, in a female BSP to John Guest adapter, the thread size and the tube size are selected separately.
| Thread | Approximate size indicated on product | Does not equal... | Usual use |
|---|---|---|---|
| 1/4 BSPT | 13.2 mm | 1/4" (6.35 mm) tubing | Adapters, small inlets, water or gas equipment. |
| 3/8 BSP | 16.5 mm | 3/8" tubing, 9.5 mm | Threaded adapters, equipment connections, and technical components. |
| 1/2 BSP | 21 mm | 1/2" tube with 12.7 mm | Couplers, taps, adapters, product or gas outlets. |
| 5/8 BSP | 23 mm | 5/8" tube, 15.9 mm | Professional dispensing connections, couplers and accessories. |
| 3/4 BSP | 26.5 mm | 3/4" (19 mm) tubing | Larger threads, drains, cleaning or auxiliary connections. |
Real‑world example: female BSP thread to John Guest adapter
A female BSP-to-John Guest adapter is used to go from a threaded connection to a push-fit connection for tubing. That’s why it involves two separate decisions:
1. Thread size
This is the part that screws into the equipment, coupler, tap, metal fitting, or compatible component. It can be 1/4 BSPT, 3/8 BSP, 1/2 BSP, 5/8 BSP, 3/4 BSP, or another size.
2. Tube measurement
This is the part where the tube enters the quick connector. It can be 5/16" (8 mm), 3/8" (9.5 mm), 1/2" (12.7 mm), 15 mm or another size depending on the variant.
| You need to connect... | You must check... | Related product |
|---|---|---|
| A threaded part on a food-grade tube | BSP thread type + outer diameter of the tubing. | Female BSP Thread to John Guest Adapter |
| One Lupulus II 3/8" tube | That the fitting accepts 9.5 mm outside diameter tubing. | Lupulus II tubing for beer and beverages |
| One Lupulus II 5/16" tube | That the fitting accepts 8 mm outside diameter tubing. | Lupulus II 5/16" tube – 4.8 x 8 mm |
| A connection with coupler or tap | Actual thread on the coupler/tap + required tubing outlet. | View beer couplers |
Quick inch-to-millimetre conversion
This table serves as a general mathematical reference. It is useful for understanding equivalences, but in real installations it must be applied correctly depending on whether we are talking about tube, thread, barb or push-fit connection.
| Inches | Exact millimetres | Typical commercial equivalence | Typical application |
|---|---|---|---|
| 1/8" | 3.18 mm | 3.2 mm | Small connections or reduced passages. |
| 3/16" | 4.76 mm | 4.8 mm | Small‑bore or restriction tubing. |
| 1/4" | 6.35 mm | 6.35 mm | Tubing, water, gas or auxiliary lines. |
| 5/16" | 7.94 mm | 8 mm | Food-grade tubing and quick-connect fittings. |
| 3/8" | 9.53 mm | 9.5 mm | Food-grade tube, beverage, water and gas. |
| 1/2" | 12.70 mm | 12.7 mm | Higher-flow tubes or auxiliary connections. |
| 5/8" | 15.88 mm | 15.9 mm | Mathematical reference; it is not interpreted the same way in BSP threads. |
| 3/4" | 19.05 mm | 19 mm | Mathematical reference; check whether it’s tube or thread. |
| 1" | 25.40 mm | 25.4 mm | Large connections, fixings or associated plumbing. |
How to know whether you need to measure tube, thread, or barb
To choose correctly, first identify the type of connection. Don’t start with the chart: start with the part in front of you.
| Connection type | What you need to measure | How to choose the part | Typical mistake |
|---|---|---|---|
| Tubing for push-fit fitting | Outer tube diameter. | Fitting with the same outer diameter as the tube. | Measuring the inside and buying a fitting that is too small. |
| Flexible tube over barb fitting | Inner diameter of the tube and diameter of the barb. | Tubing that fits firmly over the barb, usually with a clamp. | Buy by the outer diameter of the tube. |
| BSP thread | Thread type, male/female and nominal size. | Adapter with the same thread and compatible seal. | Comparing it with a tube size. |
| Reducer | Inlet and outlet size. | Reducer suited to the connection direction and flow rate. | Reducing without considering foam, flow loss or restriction. |
| Keg coupler | Product outlet, gas inlet and available threads. | Adapter compatible with coupler, tubing and beverage. | Choosing only by keg type without checking threads. |
Why it matters so much in tap beer and beverages
In a dispensing installation, the correct size is not just a matter of assembly. It affects flow, pressure, foam, cleaning, safety against leaks and maintenance.
Prevents beverage or gas leaks
A tube that doesn’t match the fitting may look inserted but not seal properly. This can cause product loss, CO₂ leaks, or air ingress.
Keeps the correct flow rate
The internal diameter of the tube affects the flow. An incorrectly sized line can cause slow service, foam, or difficulty balancing the system.
Makes cleaning easier
A system with consistent dimensions allows you to clean lines, disassemble components and replace parts more safely and quickly.
Reduce purchasing errors
Being able to distinguish between thread, tubing, barb and push-fit prevents buying parts that look similar but are not compatible.
Shop by connection type, not just by inches
Before choosing, confirm whether you need food‑grade tubing, a push‑fit fitting, BSP adapter, barb, reducer, coupler or gas connection. The same inch fraction can mean different things.
View connectors and fittings See BSP to John Guest adapterCommon sizes of food-grade tubing for dispensing
In beer, water, wine, vermouth, kombucha, soft drink, syrup, cocktail or nitro coffee systems, the most common tubing sizes are usually between 3/16", 1/4", 5/16", 3/8" and 1/2". The choice depends on beverage, pressure, distance, temperature, flow and type of fitting.
3/16" — approx. 4.8 mm outer
Used in small lines, restriction, or specific applications where a reduced passage is needed.
1/4" — approx. 6.35 mm outer diameter.
Common in water, gas, filtration, compact equipment and auxiliary connections.
5/16" — approx. 8 mm outer
Very common in beverage systems, coils, compact equipment and quick-connect fittings.
3/8" — approx. 9.5 mm outer diameter.
One of the most common sizes in beer, water, chilled drinks, gas and push-fit fittings.
1/2" — approx. 12.7 mm outside
Used on higher-flow lines, recirculation, return, coolant water or specific applications.
Metric sizes
Some systems can work directly in millimetres, such as 8 mm, 10 mm, 12 mm or 15 mm. They must not be mixed without checking compatibility.
Common mistakes when converting inches and millimetres
- Confusing 3/8" tubing with 3/8 BSP thread.
- Confusing 5/16" with 3/8" because they look similar to the eye.
- Buy a push-fit connector by measuring the tube’s inner diameter.
- Buying a tube for a barb by measuring only the outside diameter.
- Choosing a BSP thread only by mathematical conversion to millimetres.
- Not checking whether the thread is male or female.
- Not checking whether the thread is parallel, tapered or needs a gasket.
- Mixing metric and imperial tubing without the correct adapter.
- Not considering pressure, temperature, beverage and food-grade use.
- Not checking for leaks after assembling the connection.
Checklist before buying tubing, fittings or adapters
Before buying a part, check these points. They’ll help you avoid common mistakes and unnecessary returns.
- Identify whether you need to connect tubing, thread, barb or quick fitting.
- Check whether the inch measurement refers to the tube or to the thread.
- For push-fit fittings, measure the outside diameter of the tube.
- For barbs, check the internal diameter of the tube and whether you need a clamp.
- For threads, confirm BSP, BSPT, male, female and gasket.
- Check whether the tube is marked as inside x outside.
- Check whether the part is suitable for beverage, gas, water, or cleaning.
- Check working pressure and temperature.
- Cut the tube straight and clean before inserting it into a push-fit fitting.
- Perform a leak test before putting the installation into service.
Related products to help you choose the right size
Lupulus II tubing for beer and beverages
Premium dual-layer beverage dispense tube, available in several sizes such as 3/8", 5/16", 1/4" and 3/16". Ideal when you need a technical line for beer, gas, wine, soft drinks, or BiB.
Female BSP Thread to John Guest Adapter
Adapter to go from a BSP threaded connection to a push-fit connection for tubing. It lets you choose thread size and tube size separately.
John Guest fittings
Push‑fit connectors, elbows, tees, reducers and adapters for tubing in beverage, water, gas and dispense installations.
Dispensing connectors
Fittings, adapters, reducers, valves, connections, and accessories for tap beer and beverage systems.
Not sure whether you need 3/8 tubing or 3/8 BSP thread?
Send us a photo of the part, tube, coupler, tap or equipment you want to connect. We’ll help you identify whether you need 3/8" tube, 3/8 BSP thread, adapter, reducer, barb or compatible push-fit fitting.
Check compatibility View food-grade tubingFrequently asked questions about inches, millimetres, tubing and threads
Is 3/8" tubing the same as a 3/8 BSP thread?
No. A 3/8" tube usually refers to a tube size, typically 9.5 mm outside diameter in push-fit fittings. A 3/8 BSP thread is a thread standard and is not equivalent to a 9.5 mm tube.
How many millimetres is 3/8" in a tube?
3/8" is mathematically equivalent to 9.53 mm. In dispensing tubes it is usually used as approx. 9.5 mm outside. In a technical tube it may appear as inside x outside size, for example 6.7 x 9.5 mm.
What does 6.7 x 9.5 mm mean in a tube?
It usually indicates inner diameter x outer diameter. In this example, the inner bore would be approximately 6.7 mm and the outer 9.5 mm. For push-fit fittings, the outer diameter is usually the key.
What does 1/2 BSP mean?
1/2 BSP is a thread size, not a direct flexible tube size. You must choose an adapter or component with a compatible thread and, if applicable, separately select the size of the tube to be connected.
How do I know whether I should measure the inside or the outside of the tube?
For push-fit fittings, you normally need to measure the outer diameter of the tube. For barbed connections, the inner diameter of the tube and clamping with a hose clamp are usually what matter.
Why does an adapter have a thread size and a tube size?
Because it joins two different worlds: one side screws into a BSP component and the other receives tubing via a push-fit connection. The thread and the tubing may share a similar fraction, but they are not the same size.
Can I mix metric and imperial tubing?
Only if the fitting or adapter is designed for that specific size. It’s not advisable to force a metric tube into an imperial fitting or vice versa, because this can cause leaks or poor retention.
What should I do if I have leaks after installing a fitting?
Check that the tube is cut straight, that it is fully inserted, that the size is correct, that the seal is in good condition and that you are not mixing incompatible tubing, threads or adapters.
Tubing, fittings and adapters for beverage systems
Install Beer supplies technical equipment for draft beer and beverage systems: food-grade tubing, John Guest fittings, connectors, BSP adapters, reducers, couplers, taps, installation and technical support.
See John Guest Request technical helpTechnical note: the equivalences in this guide are for general reference. In real installations, always check the product spec sheet, the tube’s internal and external diameter, thread type, connection type, working pressure, food-grade suitability and compatibility of the complete system.