In the everyday work of an installer, water system designer or pump service technician, the same question arises sooner or later: DN how many inches is that? The question seems trivial, but in practice the answer to it determines whether the ordered fittings will fit into the pump stub, whether the purchased connectors will seal on the steel pipe, and whether the flanges will adhere to each other without stress. In our company, Dambat is a manufacturer and distributor of IBO and pumps IPRO – we touch on the topic of dimensions every day, advising customers both on small circulation pumps and on large submersible pumps supplying industrial installations.
In this guide, we explain what the DN marking is, how it differs from the external diameter OD and internal ID, why a pipe called a "half-inch" in the store is not 12.7 mm inside, and how to quickly convert pipe diameters between DN systems, inches, millimeters, DIN, ISO and ASTM standards. We also show where specific connector diameters appear in our pumps, and how to use our proprietary tool - Dambat pipe diameter converter – to avoid costly mistakes.
What does DN mean and why is it not the physical dimension of the pipe
Symbol DN (Diameter Nominal), i.e. nominal diameter, is a conventional, standardized designation of the size of pipes, fittings, flanges and fittings, used in European and international ISO and EN standards. Importantly, the nominal diameter is not the exact dimension of the pipe - neither its external nor internal diameter. This is a reference value thanks to which designers, installers and manufacturers use the same language when selecting installation elements.
This is best illustrated by an example: a DN 50 pipe does not have 50 mm inside. In the case of a steel pipe according to EN 10220, the external diameter of the pipe for DN 50 is 60.3 mm, and the actual internal diameter depends on the wall thickness - for a typical pipe wall thickness of 2.9 mm, the internal cross-sectional area corresponds to a diameter of approx. 54.5 mm. In turn, for a PE 63 pipe (where 63 mm is the external diameter OD), the approximate internal diameters may differ by up to several millimeters depending on the pressure series. This is why the nominal pipe diameter is so important - it helps avoid chaos when ordering components.
In hydraulic installations, DN functions as a common denominator. Regardless of whether we are talking about deep well pumps with a 1¼" connector or surface pumps with a DN 32 connector, the designer will simply mark DN 32 on the technical projection and every specialist knows what elements to order. This mechanism saves thousands of installations every day, but it requires knowledge of how to read the markings.
DN, OD, ID and wall thickness – four dimensions you need to know
To consciously operate pipe diameters, the installer should understand four basic concepts:
- DN (Diameter Nominal) – conventional value in millimeters, describing the nominal size of the pipe and fittings.
- Outer diameter (OD) – real, measurable dimension of the pipe calculated on the outer surface. This is a key parameter when selecting compression fittings and clamps.
- Inner diameter (ID) – the distance between the internal walls of the pipe, determining the capacity. It is ID that directly affects hydraulic resistance and water flow.
- Wall thickness – value in millimeters, which determines both the pressure resistance and the actual internal diameter of the pipe.
The relationship is simple: ID = OD − 2 × wall thickness. For this reason, two pipes with the same outer diameter but different wall thicknesses will have different IDs and, therefore, different throughputs. In practice, we often see this with the example of PE 32 pipes: the same actual external diameter of 32 mm can mean an ID of 26 mm for PN10 and an ID of 23.2 mm for PN16. Here the average outside diameter remains constant, but the tube diameter decreases with increasing pressure class.
The outer diameter is most often marked with the symbol "d" or "OD", and in suppliers' catalogs it appears next to DN. The use of the external diameter as a technical dimension has a deep justification - the measurement of the pipe diameter with a caliper is usually performed from the outside, and most fittings (especially clamping and pressed ones) are based on the actual external diameter. The actual outer diameter may differ slightly from the tabulated value (production tolerances, pipe ovality), therefore in critical connections, e.g. welded ones, it is recommended to measure in several places.
Table of pipe diameters – DN, millimeters and inches
The table of pipe diameters below is an essential tool for every installer and designer. Shows how nominal DN translates into inches (imperial units) and the typical outside diameter of steel pipe according to European standards. This is a complete DN to inches converter and DN to millimeters converter - in one place.
| DN | Inches | Typical steel pipe outside diameter (mm) |
|---|---|---|
| DN 6 | 1/4" | 10,2 |
| DN 8 | 5/16" | 13,5 |
| DN 10 | 3/8" | 17,2 |
| DN 15 | 1/2 inch | 21,3 |
| DN 20 | 3/4 inch | 26,9 |
| DN 25 | 1 inch | 33,7 |
| DN 32 | 1 1/4 inches | 42,4 |
| DN 40 | 1 1/2 inches | 48,3 |
| DN 50 | 2 inches | 60,3 |
| DN 65 | 2 1/2 inches | 76,1 |
| DN 80 | 3 inches | 88,9 |
| DN 100 | 4 inches | 114,3 |
| DN 125 | 5 inches | 139,7 |
| DN 150 | 6 inches | 168,3 |
| DN 200 | 8 inches | 219,1 |
| DN 250 | 10 inches | 273,0 |
| DN 300 | 12 inches | 323,9 |
| DN 350 | 14 inches | 355,6 |
| DN 400 | 16 inches | 406,4 |
| DN 450 | 18 inches | 457,0 |
| DN 500 | 20 inches | 508,0 |
| DN 600 | 24 inches | 610,0 |
| DN 700 | 28 inches | 711,0 |
| DN 800 | 32 inches | 813,0 |
| DN 900 | 36 inches | 914,0 |
| DN 1000 | 40 inches | 1016,0 |
This table of external diameters is a starting point, but does not replace the data sheet for a specific material. For PE, PVC and copper pipes, as well as for different pressure classes of steel pipes, the actual dimensions of the pipes may differ. Therefore, we encourage you to use the pipe dimensions table provided by the material manufacturer.
DN 15, DN 20 and DN 25 – domestic water installations and fittings connections
Typical water and sewage installations in single-family and residential buildings are dominated by three sizes: DN 15, DN 20 and DN 25. They serve the connections to washbasin and kitchen faucets, washing machines, dishwashers and risers distributing utility water.
DN 15 is a classic "half-inch" (1/2 inch), used for single water intake points. DN 20 corresponds to a 3/4 inch thread - this is the most common diameter in residential risers and individual connections. DN 25 is 1 inch, popular in connections to single-family buildings and in the main branches of the distribution system.
In the practice of our Dambat company, DN 20 and DN 25 connections are the most common connections in circulation pumps, pressure booster sets and smaller IBO surface pumps. The circulation pump for hot utility water usually has 1 inch or 1 1/2 inch connections, which correspond to DN 25 and DN 40, respectively. Many central heating circulation pumps have 1 1/2 inch (DN 40) connections, and even DN 50 in larger versions.
It is worth remembering that the selection of a pipe for a home installation is not only a matter of matching it to the pump stub. The designed water flow and pressure losses along the length are also important - hence DN 20 is used for approaches, but main distribution is increasingly made in DN 25 to ensure adequate pressure while drawing from several points at the same time.
DN 32, DN 40 and DN 50 – medium-sized installations, hydrophores and pumping stations
In terms of medium diameters, DN 32, DN 40 and DN 50 are the leading ones. DN 32 (1 1/4 inch) is a common dimension for pressure booster sets, short delivery sections and connections to smaller deep wells. DN 40 (1 1/2 inch) appears in central heating installations of larger houses, in higher power circulation pumps and as the diameter of the connections of submersible pumps.
DN 50 plays a special role - it is a diameter found in higher-power submersible pumps, in domestic sewage pumping stations, in the discharge sections of submersible pumps and in many group hydrophore sets. For a DN 50 steel pipe, the nominal diameter does not coincide with the external diameter - it is 60.3 mm, which is important when selecting DN 50 clamps and flanges. The PE 63 pipe, in turn, is often used as an "equivalent" of DN 50, although its actual dimensions are different.
In pressure PVC pipes, DN 50 most often means a pipe with an OD of 63 mm (here, the use of the outer diameter as the basis for the marking is also standard). PVC pipe diameters corresponding to DN 50 usually have a wall thickness of 2.4 mm to 4.7 mm depending on the pressure class. This should be remembered when designing an installation with mixed materials - steel and plastic.
In medium-sized pumping stations and smaller water supply facilities, we also find DN 50 in the discharge pipelines of submersible pumps with a grinder, in auxiliary fire protection installations and at the connections to filter sets. This is why our consultants advising on the selection of pumps use the phrase DN 50 as a natural point of reference in almost every conversation.
DN 65, DN 80, DN 100 and larger - industrial and water supply fittings
Above DN 50 we enter the world of industrial, flanged and water supply fittings. DN 65 (2 1/2 inches) is the classic diameter of non-return and shut-off valves in smaller water and sewage pumping stations. We use DN 80 in medium-sized deep well pumps and smaller-scale municipal sewage pumping stations. DN 100 (4 inches) is the standard diameter of pressure pipelines in medium-sized water supply facilities.
DN 125, DN 150 and DN 200 support larger submersible pumps and Submersible pumps high efficiency - at this scale we already work on flange connections in accordance with PN-EN 1092. For diameters from DN 250 to DN 1000, we begin to design dedicated pumping stations and water mains; dimensions include DN 300, DN 350, DN 400, DN 450, DN 500, DN 600, DN 700, DN 800, DN 900 and DN 1000. The latter are found mainly in water supply systems and industrial cooling installations.
For the record: DN 700 is 28 inches, DN 800 is 32 inches, DN 900 is 36 inches, and DN 1000 is 40 inches. For district heating designers, DN 200 (8 inch), DN 250 (10 inch) and DN 300 (12 inch) are usually useful and form the basic distribution pipeline network. In the design documentation of large facilities, we also encounter less frequently used DN 350, DN 450 and DN 700 as diameters adapted to the specificity of a given installation.
Steel and galvanized pipes - dimensions inch and OD
Steel pipes are the "classic" of water, gas, compressed air and industrial installations. The dimensions of steel pipes are usually given in inches - at the same time, the steel pipe diameter table contains DN and the actual external diameter in millimeters. This distinction is fundamental: inches (imperial units) historically refer to the nominal diameter, not the exact diameter of the pipe.
The most famous example is a 1/2 inch pipe - DN 15 - whose actual outer diameter is 21.3 mm, not 12.7 mm (i.e. 1/2 inch = 12.7 mm in millimeters). Similarly, 3/4 inch (DN 20) pipe has an OD of 26.9 mm and 1 inch (DN 25) pipe has an OD of 33.7 mm. These actual pipe dimensions are included in the standards (EN 10255, EN 10220) and have not changed for decades.
Galvanized steel pipe is a popular material in drinking water and industrial installations. In galvanized pipes, wall thickness is not only important for strength - it also determines the durability of the zinc layer. Standard wall thicknesses for galvanized steel pipes range from approximately 2.3 mm for smaller diameters (DN 15) to 4.5 mm and more for DN 50 and higher. This must be taken into account when calculating the actual diameter of the pipe, because thicker walls of galvanized steel pipes provide lower ID for the same nominal diameter.
It is worth measuring the diameter of a steel pipe in the field using a caliper with an accuracy of 0.1 mm. Measurement on the outside allows you to compare the actual value with the tabulated OD, and after subtracting twice the pipe wall thickness, you get the inside light. This is a simple control method when the documentation lacks technical data and you need to select connectors, flanges or reductions based on the existing installation.
PVC, PP, PE and copper pipes – dimensions given in external diameter
Unlike steel pipes, plastic pipes and copper pipes are marked with their actual outside diameter. This is a key methodological difference - here the entry "PE 32" simply means a pipe with an OD of 32 mm, not DN 32. The dimensions of PVC pipes are available in the series of types 16, 20, 25, 32, 40, 50, 63, 75, 90, 110, 125, 160, 200 mm and higher.
PP (polypropylene) pipes are mainly used in cold and hot water and central heating installations. Here, too, the series corresponds to the outer diameter: 16, 20, 25, 32, 40, 50, 63, 75 mm. Copper pipes typically come in diameters of 12, 15, 18, 22, 28, 35, 42, 54 mm - which means that the classic "thin copper 15" corresponds to DN 12, not DN 15. This is a common mistake of people starting work in the industry.
For surface pumps working with PE pipes, it is worth remembering the correlation: PE 25 works well with a 1-inch connector (DN 25), PE 32 with a 1 1/4-inch thread (DN 32), PE 40 with a 1 1/2-inch thread (DN 40), and PE 50 or PE 63 with a 2-inch connector (DN 50). However, due to differences in internal diameter, it is always worth checking the actual external diameter and selecting a compression or electrofusion fitting appropriate to the actual dimensions. This use of outer diameter as the basis for fitting selection is standard in the plastics industry.
Inch threads in IBO i pumps IPRO – what is worth knowing
Pipe threads are a separate, fascinating story. In our IBO and brand pumps IPRO inch threads prevail in accordance with ISO 228 (parallel pipe threads, marked "G") and ISO 7-1 (conical pipe threads, marked "R" for external and "Rp" for internal). BSP threads – British Standard Pipe – are the family to which both of these systems belong, and their sizes are given in inches. Importantly, inch threads do not correspond to their name - the G 1/2 thread has an external diameter of 20.955 mm, not 12.7 mm.
Why is this so? Because inch threads originated in an era when nomenclature resulted from the approximate internal diameter of an iron pipe. Over time, the pipe walls decreased, the ID increased, but the designation remained historical. Hence the G 3/4 thread (DN 20) has an OD of 26.441 mm and the G 1 thread (DN 25) has an OD of 33.249 mm. The most common threads in our pumps are G 1 (DN 25), G 1 1/4 (DN 32), G 1 1/2 (DN 40) and G 2 (DN 50).
Practical tip: cylindrical threads (G) require flat sealing (gasket) or mechanical sealing (ring), as they do not ensure tightness on their own. Tapered threads (R, NPT) seal by tightening with Teflon tape or tow. Mixing these systems is a common source of leaks - a conical male thread will not seal into a cylindrical female thread without additional work.
Our offer includes pumps with inch threads, but also flange models - high-power submersible pumps, Submersible pumps industrial and inline circulation pumps with large connectors often have DN 65, DN 80 or DN 100 flanged connections. Threads also appear in accompanying fittings - in pressure gauges (typically G 1/4 or G 1/2 thread), in ball valves, check valves, in mesh filters, as well as on the connection stubs of inverters with liquid cooling. Threads also connect reducing connectors, sleeves and nipples used in pump assembly, which is why fittings catalogs always provide both DN and the corresponding thread in inches.
Please remember that thread markings appear with the note "A" or "B" - they indicate the tolerance class of the thread. For pressure pump connections, we recommend using class A threads. Fittings, reductions and nipples purchased for IBO and IPRO have the thread type described in the catalog card, which eliminates the risk of confusion. G 1 1/4 and G 1 1/2 threads are absolute sales record holders in the home pump segment - they correspond to the connections of typical hydrophore sets.
Connection diameters in Dambat pumps - from circulation to submersible
Our offer includes pumps with a wide range of connection diameters. Smaller IBO circulation pumps and circulation pumps usually have connectors with a 1 inch or 1 1/2 inch external thread (DN 25 or DN 40), which allows them to be easily connected to typical central heating and hot water installations. Higher power inline circulation pumps are available with DN 25, DN 32, DN 40 and DN 50 connections - in threaded or flange versions.
Submersible pumps (for clean water, dirty water and sewage) most often have a discharge port in the range of DN 25 to DN 80, depending on the capacity. Models with a grinder usually have a DN 32 or DN 40 connection, while large pumps for municipal pumping stations work with DN 65, DN 80 and DN 100 connections. Submersible pumps (4-, 6- and 8-inch) usually have an external thread of 1 1/4 inch, 1 1/2 inch or 2 inches at the head - which translates accordingly on DN 32, DN 40 and DN 50.
For surface pumps (self-priming, single-stage and multi-stage centrifugal pumps), typical port diameters are 1 inch, 1 1/4 inch and 1 1/2 inch. Larger surface pumps, e.g. inline pumps in group hydrophore sets, operate on DN 50, DN 65 or DN 80 flanged connections. All this affects the selection of appropriate pipes and fittings - therefore, before purchasing the pump, we always verify its diameter with the customer. Connection installation.
The accompanying elements are equally important. Hydrophore sets include pressure gauges with a G 1/4 or G 1/2 thread, check valves and ball valves in diameters DN 25 to DN 50, mesh and magnetic filters adapted to the pipeline diameter, inverters controlling the operation of pumps with various connection configurations, and pressure and flow controllers. All these components must be dimensionally consistent with the rest of the installation.
Dambat pipe diameter converter – all standards in one tool
To make life easier for our clients - installers, designers and service specialists - we have made available a dedicated one on the company website pipe diameter converter. This tool supports all major systems: DN, DIN, ISO, metric system (METR), CALE and ASTM. Thanks to this, you can compare pipe diameters from different standards in one table and avoid errors typical of working with mixed documentation.
The converter on the Dambat website works intuitively - you enter a known value (e.g. inches or DN), and the system shows equivalents in other standards. It is convenient both for valuation and design when elements marked in DIN and ASTM appear simultaneously in one documentation. We especially recommend it to employees of companies cooperating with foreign partners - Dambat exports pumps to many countries, and customers from outside Europe often use only ASTM or imperial units.
For a more complete insight into the selection of dimensions, we also recommend our guide on the company blog: How to choose the right pipe diameter - a practical guide DN, d and inches. There you will find additional tips on selecting diameters depending on the flow, pressure and purpose of the installation.
The most common mistakes when selecting the pipe diameter
The experience of our consultants shows that typical mistakes when selecting pipe diameters occur regardless of the scale of the project:
- Confusing DN with OD – especially with mixed steel and plastic installations. PE 50 and DN 50 are completely different dimensions. Therefore, it is always worth carefully measuring the diameter of the existing pipe before ordering the elements.
- Ignoring the pipe wall thickness – when calculating the flow, it is ID that counts, not OD. Two DN 50 pipes with different wall thicknesses give different cross-sectional areas.
- Mismatched threads – connecting cylindrical threads with conical threads, or BSP threads with NPT, leads to leaks. The external thread must match an internal thread with the same characteristics.
- Discharge pipe diameter too small – the smaller the diameter, the greater the hydraulic losses and the greater the pump load. We most often see this with submersible pumps connected to too thin PE 32 pipes instead of PE 40 or PE 50.
- No verification of actual pipe dimensions in old installations - after years of operation, a layer of corrosion in steel pipes can drastically reduce ID, so it is a good idea to choose a pump with a pressure reserve.
- Ignoring flanges and their pressure classes – DN 50 PN10 and DN 50 PN16 flanges have different bolt dimensions and thicknesses even though they support the same nominal diameter.
To avoid these errors, we always recommend: accurate measurement of existing pipes, verification of design documentation, use of our converter and - in case of doubt - contact our technical advisory department. As a pump manufacturer, we know the specifics of the market and will help you select the pipe diameter, thread, inverter, pressure gauge and other components that fit the entire system.
A short summary – DN, inches and millimeters in one thought
The most popular pipe diameters in domestic and industrial installations have clear equivalents in the inch and metric systems. DN 15 is 1/2 inch, DN 20 is 3/4 inch, DN 25 is 1 inch, DN 32 is 1 1/4 inch, DN 40 is 1 1/2 inch and DN 50 is 2 inch. Larger pipe diameters are standard pipe diameters used in water supply, heating and industry - DN 80 (3 inches), DN 100 (4 inches), DN 150 (6 inches), DN 200 (8 inches). The metric system is used in Europe, but imperial units and ASTM standards are still popular in international industry.
Remember that the selection of the pipe and its diameter is not everything - in pump installations, the material, pressure class, threads, flanges, connectors, controllers, valves, filters, pressure gauges and inverters also matter. We encourage you to use our online tools and support our technical department - we will be happy to help.
FAQ – most frequently asked questions about DN, inches and millimeters
DN 50 how many inches is it?
DN 50 is 2 inches. The actual external diameter of a steel pipe for DN 50 is 60.3 mm, and for a PE pipe the equivalent is most often PE 63. This is one of the most popular dimensions in hydraulic installations and medium-sized pumping stations.
DN 20 how many inches is it?
DN 20 corresponds to 3/4 inch. The actual outer diameter of the steel pipe for DN 20 is 26.9 mm. This is the most common diameter in residential risers and individual connections to buildings.
DN 25 how many inches is it?
DN 25 is 1 inch. The outer diameter of the steel pipe for DN 25 is 33.7 mm. It is used in house connections, as a connector diameter in circulation and surface pumps and in the main branches of the distribution system.
DN 40 how many inches is it?
DN 40 corresponds to a 1 1/2 inch thread. The outer diameter of the steel pipe for DN 40 is 48.3 mm. This is a common dimension for central heating circulation pumps in larger houses and for medium-sized hydrophore sets.
DN 80 how many inches is it?
DN 80 is 3 inches. The outer diameter of the steel pipe for DN 80 is 88.9 mm. It is used in medium-sized deep well pumps, smaller-scale municipal sewage pumping stations and in local waterworks.
Is DN the inside or outside diameter?
DN (Diameter Nominal) is neither the inside nor the outside diameter. This is a conventional reference value used to classify pipes, fittings and fittings. Actual pipe dimensions (OD and ID) depend on the material, standard and wall thickness.
How to convert inches to millimeters?
1 inch is exactly 25.4 mm. To convert inches to millimeters, multiply the value in inches by 25.4. Remember, however, that pipe markings in inches (e.g. 1/2", 3/4") do not correspond to their actual diameter in millimeters - these are nominal markings resulting from historical standards.
Where can I find a Dambat pipe diameter converter?
Ours pipe diameter converter is available free of charge at Dambat.pl. Supports DN, DIN, ISO, METR, INCH and ASTM standards, allowing you to quickly compare diameters in different systems.
What threads do IBO i pumps have? IPRO?
Pumps of the IBO and brand brands IPRO They are standardly equipped with inch threads in accordance with ISO 228 ("G" cylindrical threads) or ISO 7-1 ("R"/"Rp" tapered threads). The most common threads are G 1, G 1 1/4, G 1 1/2 and G 2, i.e. DN 25, DN 32, DN 40 and DN 50, respectively. Larger models are available with flange connections in the DN 65, DN 80 and higher series.
Is DN 32 the same as PE 32?
NO. DN 32 is the nominal diameter in the European system, the equivalent of which in steel pipes is OD 42.4 mm. PE 32 is a polyethylene pipe with an external diameter of 32 mm. These are completely different dimensions - a mistake leads to problems with matching the connectors and fittings.
How to choose the pipe diameter for a submersible pump?
The diameter of the discharge pipe is selected based on the pump capacity and the length of the pipeline. For deep well pumps up to 3 m³/h, PE 32 (DN 25) is usually sufficient, for capacities of 3–6 m³/h we recommend PE 40 (DN 32), and above 6 m³/h – PE 50 or PE 63 (DN 40 or DN 50). Too small a diameter generates high pressure losses and may lead to overload of the pump.
What is the difference between BSP and NPT threads?
BSP (British Standard Pipe) is the European standard for 55° pipe threads, dominant in Poland, Germany and most of Europe. NPT (National Pipe Thread) is an American standard with a 60° angle. BSP and NPT threads are not compatible - connecting them causes leaks and damage. In IBO i pumps IPRO we only use BSP threads.
