Pipe diameter and thread dimensions: how to correctly read the markings ½", ¾", 1"
Pipe Diameter and Thread Dimensions: How to Correctly Read the Markings ½", ¾", 1"
When you first stand in front of the counter at a plumbing store or browse an e-shop looking for a reducing valve, filter, or other fitting, you will encounter seemingly simple numbers: ½", ¾", 1", 1¼", 2". It seems simple – a larger number means a larger pipe. But the reality is a bit more complicated, and this "simplicity" is the cause of dozens of unnecessary returns, product returns, and re-installs each year.
Over twenty years in the industry, I have seen many situations where an experienced craftsman came with a complaint about a valve "that doesn't fit," only to find the problem was simply confusing the pipe's outer diameter with the thread marking. Or he bought a part with an inch thread and tried to screw it onto a metric thread neck. This article is therefore devoted precisely to this – a systematic explanation of what these numbers actually mean, why they are as they are, and how to use them correctly when selecting fittings for your family home, apartment building, or industrial facility.
Historical context: Why inches and not millimeters?
We start from the basics. The inch-based pipe thread marking comes from the 19th century, when Britain dominated the industrial revolution and British standards spread around the world. The system we know today was defined in 1841 by British engineer Joseph Whitworth – hence the abbreviation BSP (British Standard Pipe), or Whitworth pipe thread.
Although Europe has long switched to the metric system for most technical applications, the pipe thread remains in inches to this day. The reason is practical: there are hundreds of millions of meters of pipe and billions of installed fittings around the world that use this system. Changing it would mean replacing the entire infrastructure – which is economically unthinkable. Therefore, inches remain the standard for water, gas, and central heating systems across all of Europe in the 21st century.
Basic misunderstanding: Inch markings DO NOT CORRESPOND to the actual pipe diameter
This is a key point that even experienced people find confusing. When we say "½ inch pipe," it does not mean that the outer diameter of the pipe is 0.5 inches (12.7 mm). The actual outer diameter of a pipe with the marking G½" (BSP thread) is 20.955 mm – that is, more than an inch!
Where does this seemingly irrational marking come from? In the early days of the industrial revolution, pipes were marked according to the inner diameter – that is, the pipe's bore. A pipe with an inner diameter of half an inch was a "half-inch pipe." The wall thickness was added to the outer diameter, so the actual dimension was always larger than the nominal value. When wall thicknesses later changed (due to higher pressure requirements), the marking system remained historically rooted, but the direct relationship between the name and the actual dimension no longer applied.
Result: today, inch markings are purely conventional – they are the nominal dimension (DN – Diameter Nominal), not an actual measurement. Look at the specific values:
| Thread marking | Outer diameter of the thread (mm) | Thread pitch (mm) | Threads per inch (TPI) | Common use |
|---|---|---|---|---|
| G ⅛" | 9,728 mm | 0,907 mm | 28 | pressure gauges, air vents |
| G ¼" | 13,157 mm | 1,337 mm | 19 | small valves, measuring instruments |
| G ⅜" | 16,662 mm | 1,337 mm | 19 | corner valves, WC inlets |
| G ½" | 20,955 mm | 1,814 mm | 14 | domestic water supply, radiators |
| G ¾" | 26,441 mm | 1,814 mm | 14 | main supply in the apartment, boilers |
| G 1" | 33,249 mm | 2,309 mm | 11 | main house supply, reducing valves |
| G 1¼" | 41,910 mm | 2,309 mm | 11 | apartment buildings, industry |
| G 1½" | 47,803 mm | 2,309 mm | 11 | larger industrial lines |
| G 2" | 59,614 mm | 2,309 mm | 11 | industry, pumps, filters |
Types of BSP threads: BSPP vs. BSPT – cylindrical and tapered
BSP threads are not all the same. There are two basic families that look similar from the outside but differ in a crucial detail – the shape of the thread profile:
BSPP – cylindrical (parallel) thread
Designation G (e.g. G½"). The thread has the same diameter along its entire length – that is, it is cylindrical. Sealing is not achieved by the thread itself, but by a compression seal – a flat rubber gasket (fiber, EPDM) on the face of the fitting, or a sealing insert in the thread. Precisely because it depends on the gasket or sealing ring, you must always check during installation that the gasket is correctly positioned, not slipped, and not displaced.
Cylindrical thread G½" can be found on the vast majority of domestic fittings – angle valves, pressure-reducing valves, taps, filters, and water taps.
BSPT – tapered thread
Designation R (external tapered) or Rc (internal tapered). The thread tapers inward – that is, it is conical, with a taper of 1:16 (1.7891°). Sealing is achieved directly metal-to-metal when the taper seats into the mating thread. Despite this, in most applications, sealing tape (Teflon) or paste is added to eliminate leaks due to minor inaccuracies. Tapered thread is standard in gas systems and some industrial applications.
For the average customer in plumbing: if you are purchasing fittings for domestic water supply, in most cases you are working with cylindrical thread G (BSPP). Tapered thread R/Rc is encountered mainly in gas lines and pressure vessels.
How to measure a thread correctly when you don't know its designation
Practical situation: you have an old fitting that needs to be replaced, but there is no marking anywhere. What do you do? You need a caliper and a minute of patience.
Procedure for external thread (on the fitting's end): Measure the external diameter of the thread – that is, the diameter of the thread peaks. Then compare the measured value with the table above. For example: you measure 20.8 mm – this is G½" (20.955 mm, the difference is due to wear and tolerances).
Procedure for internal thread (in the fitting's neck): Measuring the internal diameter of the thread (that is, the diameter of the thread grooves) is more difficult – here a thread gauge is useful, or simply try to screw in a reference rod with a known thread. Easier method: measure the diameter of the hole (the diameter of the neck from groove to groove) and subtract twice the thread depth from the table (about 1.1 mm for G½").
Measuring pitch: If you have doubts, place a ruler against the thread and count the number of threads in 25.4 mm (one inch). G½" and G¾" have 14 threads per inch, G1" and larger have 11 threads per inch.
Difference between BSP and NPT thread: be careful with American imports
If you are purchasing valves from the American market or certain industrial components, you may encounter NPT (National Pipe Thread) – an American conical thread. At first glance, it looks similar to BSP, but they are not mutually compatible, even though they have the same nominal designation in inches.
The difference lies in the thread profile angle: BSP threads have a 55° profile angle, while NPT threads have a 60° angle. Moreover, the conicity of NPT is 1:16, the same as BSPT, but the diameters for the same nominal size differ slightly. Result: a ½" NPT and a G½" BSP can be "screwed" together for about two turns, then they jam. The resulting connection is mechanically unreliable and will leak. Conclusion: always verify whether it is BSP or NPT when purchasing valves outside the European market.
Metric threads vs. BSP threads: when are they used?
In hydraulics and pneumatics, you also commonly encounter metric threads (marked with M: M10, M12, M16, M22...). These are governed by the ISO 261 standard and have a 60° profile angle with metric pitch in millimeters. Metric threads are common on hydraulic fittings, hose coupling nuts, boiler air vent valves, and similar applications. For standard household plumbing and fixtures, metric threads are not used – BSP dominates there.
Mixing a metric thread M22×1.5 (common on taps for hose connections) with G¾" BSP is a fairly common mistake when buying shower hoses and adapters. The outer diameter of M22 is 22 mm, while G¾" BSP has an outer diameter of 26.44 mm – so you cannot even physically interchange them. However, M24 (diameter 24 mm) can be confused with ½" BSP, and this is where problems arise.
Practical scenarios from installation practice
Scenario 1: Replacing a pressure-reducing valve at the building entrance
The customer calls saying their pressure-reducing valve is leaking. You arrive for inspection: the old valve is brass with cast iron hexagonal nuts, the hexagonal nut diameter is about 46 mm. You measure the outer diameter of the thread – you get 33.1 mm. Conclusion: G1" BSP. Then you check the thread direction: internal thread on both sides (IN and OUT). The correct choice is a valve with G1" IG/IG (internal/internal thread). A good example for a larger size is also a pressure-reducing valve with a separate replaceable insert 1¼" with a 1" reduced insert, which is suitable where the pipe is 1¼", but you want to reduce the flow to 1".
Scenario 2: Installation of a water inlet filter in the apartment
The cold water supply pipe in the apartment is DN 25 (which corresponds to G1"). The customer wants to install a Y-filter to catch impurities before the pressure-reducing valve. They measure the cap diameter on the existing tap: 33.2 mm → G1". Larger facilities and industrial systems work with sizes G2" and above – for example, a Y-type flanged brass filter with a stainless steel screen 2" or for even higher flow rates a Y-type flanged brass filter with a stainless steel screen 4".
Scenario 3: The customer buys the wrong size pressure-reducing valve
The customer comes in with a request for a ½" pressure-reducing valve. We ask: "Where will it be installed?" The answer: "At the main inlet to a family house." We immediately stop – for a main inlet to a house with 4-6 people, the pipe is typically G¾" or G1". A G½" valve would be undersized, causing pressure loss and noise. The correct choice is a pressure-reducing valve ½" only for local connections – for example, after the water meter in an apartment, before a washing machine or dishwasher. More about sizing and pressure setting can be found in the article How to choose a pressure-reducing valve: pressure, diameter, material and in the article What is the correct water pressure in a household and how to set it.
Scenario 4: Confusion with sewage pipes
Another category is sewage pipes and hoses. Sewage pipes are marked with an outer diameter in millimeters – typically DN 50, DN 75, DN 110. Here, inch markings are not primary, although you may encounter old designations such as "2 inches" for DN 50 and "4 inches" for DN 110. These are only colloquial simplifications, not standardized thread dimensions – sewage pipes are not threaded but have flanged connections with rubber seals. For cleaning sewage pipes regardless of their diameter, mechanical tools are used – for example, a sewage snake PROFI 10 meters, thickness 12 mm – which you can insert through existing inspection openings without the need to disassemble the fittings.
Nominal diameter DN vs. inch designation: conversion and relationship
In technical documentation and project practice, you often encounter the designation DN (Diameter Nominal – nominal diameter according to ISO 6708 standard). This is a globally unified numerical designation that does not directly correspond to either the outer or inner diameter – it is just a number for identifying size groups. The relationship between DN and inch BSP designation:
| DN (nominal) | BSP designation | Actual outer diameter (mm) | Typical use |
|---|---|---|---|
| DN 8 | G ¼" | 13.16 mm | measuring instruments |
| DN 10 | G ⅜" | 16.66 mm | toilets, corner valves |
| DN 15 | G ½" | 20.96 mm | domestic distribution, taps |
| DN 20 | G ¾" | 26.44 mm | domestic main distribution |
| DN 25 | G 1" | 33.25 mm | house entrance, house shut-off |
| DN 32 | G 1¼" | 41.91 mm | apartment buildings, commercial facilities |
| DN 40 | G 1½" | 47.80 mm | industrial distribution |
| DN 50 | G 2" | 59.61 mm | industry, pumps, large filters |
External (AG) and internal (IG) thread: how to distinguish them when ordering
When selecting fittings, you will encounter the terms AG (Außengewinde – external thread, male) and IG (Innengewinde – internal thread, female). In Slovak, they are also referred to as "samček" (external) and "samica" (internal). These terms describe the type of thread on a given fitting of the valve.
Practical example: a corner valve under a sink usually has an internal thread G½" (IG ½") on the pipe side and an external thread G⅜" (AG ⅜") on the hose connector side. Therefore, when you buy a replacement, you must know both dimensions and both types of threads.
The marking on valves is read, for example, as G½" IG × G⅜" AG or abbreviated as ½" F × ⅜" M (F = female = female, M = male = male). This marking can be found in the technical specification of each valve.
Sealing of threads: when to use tape, when to use paste, and when to use no sealing?
Every proper plumber knows that a proper thread is only half the job – the other half is proper sealing. Here are clear rules:
- Cylindrical thread G (BSPP) with flat face sealing – do not use Teflon tape or thread paste! Sealing is ensured by a rubber or fiber gasket on the face (sealing surface). Tape on the thread would cause the gasket to not sit properly and the valve would leak.
- Cylindrical thread G without face sealing (e.g., some plumbing fittings) – either Teflon tape wound around the thread (3-5 turns in the tightening direction) or hemp thread with sealing paste is used. Tape is preferred today because it is non-organic and does not mold.
- Conical thread R (BSPT) – always Teflon tape or paste. Since the sealing is metal to metal, every micrometer of imprecision must be compensated by sealing material.
- Metric thread M in hydraulics – usually sealed with a metallic conical seat or an O-ring (O-ring) – no tape is used.
Want to know more about the selection and installation of filters and valves? A detailed procedure can be found in the articles Installation of a pressure reducing valve: step by step and Installation of a check valve and corner valve: where and how to install them.
Sizing: how to choose the correct size for a given flow
The nominal diameter is not just a label – it significantly affects the hydraulic capacity (flow) of the installation. A simplified reference table of flows at a normal operating pressure of 3 bar:
| Thread (BSP) | Estimated max. flow (l/min) | Typical application |
|---|---|---|
| G ⅜" | up to 15 l/min | supply to WC, sink |
| G ½" | up to 30 l/min | 1 bathroom, bathtub, shower |
| G ¾" | up to 60 l/min | larger apartment, water heater, washing machine |
| G 1" | up to 120 l/min | family house, house inlet |
| G 1¼" | up to 200 l/min | apartment building, commercial operation |
| G 2" | up to 500 l/min | industry, storage pumps |
These values are approximate – the actual flow also depends on the length of the pipe, the number of elbows, pressure, and the Kvs value (hydraulic flow coefficient) of the valve. For precise sizing, consult a designer. More about filters and their selection according to diameter can be found in the article Water filters: Y-filter, corner filter or filter with reduction – which one to choose.
Special cases: fractional sizes 1¼", 1½", 2½"
Sizes such as 1¼" and 1½" are common in practice for inputs to family houses and apartment buildings. Customers often confuse or misread them. It is important to remember that 1¼" is not "one and a half inches" – "1¼" is read as "one and a quarter" (i.e., 1.25 inches). Similarly, 1½" is "one and a half inches" (1.5 inches). If these sizes are mixed up when ordering, the valve simply will not fit.
Sizes 2½" and 3" occur in industrial installations and in large apartment buildings. For a regular plumber working in a family house, these sizes are rather exceptions. The issue of larger sizes, pressure losses, and the sizing of industrial piping is covered by separate technical standards (STN EN 806, STN 73 6655).
How to read valve markings in the online shop: real-life examples
When browsing products in the plumbing category, you will encounter various formats of notation. Here is an overview of how to correctly interpret them:
- „G½" IG" = cylindrical BSP thread, half inch, internal (female)
- „G¾" AG" = cylindrical BSP thread, three quarters of an inch, external (male)
- „DN15 / G½"" = nominal diameter 15 mm, BSP half inch – the same in two systems
- „1/2" F × 3/8" M" = inlet internal ½", outlet external ⅜" (typical angle valve)
- „1" × 1¼" reduced body" – valve with a larger body of 1¼", but the internal hydraulic part is dimensioned for a 1" flow
- „PN25" = maximum operating pressure 25 bar (Pressure Nominal) – not related to diameter, but to the pressure class of the valve
Do you have a question about this topic?
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