In the daily work of an installer, water-systems designer, or pump service technician, the same question comes up sooner or later: How many inches is DN? The question may seem trivial, but in practice the answer determines whether the ordered valves will fit the pump connection, whether the purchased fittings will seal on the steel pipe, and whether the flanges will meet without stress. At Dambat, a manufacturer and distributor of IBO and IPRO pumps, we deal with dimensions every day, advising customers on everything from small circulation pumps to large submersible pumps supplying industrial installations.
In this guide, we explain what the DN designation means, how it differs from the outside diameter (OD) and inside diameter (ID), why a pipe called a “half-inch” in a shop does not actually have 12.7 mm inside, and how to quickly convert pipe diameters between DN, inches, millimetres, and DIN, ISO, and ASTM standards. We also show where specific connection diameters appear on our pumps and how to use our company tool—the Dambat pipe diameter converter—to avoid costly mistakes.
What DN Means and Why It Is Not a Physical Pipe Dimension
The symbol DN (Diameter Nominal), meaning nominal diameter, is a conventional, standardised designation for the size of pipes, fittings, flanges, and valves used in European and international ISO and EN standards. Importantly, the nominal diameter is not the exact dimension of the pipe—it is neither its outside nor its inside diameter. It is a reference value that allows designers, installers, and manufacturers to use the same language when selecting installation components.
This is best illustrated by an example: a DN 50 pipe does not actually have 50 mm inside. In the case of a steel pipe complying with the EN 10220 standard, the outside diameter of a DN 50 pipe is 60.3 mm, while the actual inside 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 approximately 54.5 mm. In turn, for a PE 63 pipe (where 63 mm is the outside diameter, OD), the approximate inside diameters may differ by as much as several millimetres depending on the pressure class. This is why the nominal pipe diameter is so important—it helps avoid confusion when ordering components.
In plumbing systems, DN functions as a common denominator. Regardless of whether we are talking about submersible pumps with a 1¼" connection or surface pumps with a DN 32 connection, the designer will simply mark DN 32 on the technical drawing, and every specialist will know which components 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 work with pipe diameters effectively, an installer should understand four basic concepts:
- DN (Diameter Nominal) – a conventional value in millimeters describing the nominal size of a pipe and its fittings.
- Outside diameter (OD) – the actual measurable pipe dimension taken across its outer surface. This is a key parameter when selecting compression fittings and clamps.
- Inside diameter (ID) – the distance between the inner pipe walls, which determines flow capacity. The ID directly affects hydraulic resistance and water flow.
- Wall thickness – the value in millimeters that determines both pressure resistance and the actual inside diameter of the pipe.
The relationship is simple: ID = OD − 2 × wall thickness. Consequently, two pipes with the same outside diameter but different wall thicknesses will have different IDs and therefore different flow capacities. In practice, this is often seen with PE 32 pipes: the same actual outside diameter of 32 mm may correspond to an ID of 26 mm for PN10 and an ID of 23.2 mm for PN16. Here, the average outside diameter remains constant, while the pipe bore decreases as the pressure class increases.
The outside diameter is most often обозначed by the symbol “d” or “OD” and appears next to DN in supplier catalogs. Using the outside diameter as a technical dimension has a sound practical basis: pipe diameter is generally measured from the outside with calipers, and most fittings—especially compression and press fittings—are based on the actual outside diameter. The actual outside diameter may differ slightly from the tabulated value due to manufacturing tolerances and pipe ovality, so measurements at several points are recommended for critical connections, such as welded ones.
Pipe Diameter Table – DN, Millimeters, and Inches
The following pipe diameter table is an essential tool for every installer and designer. It shows how the nominal DN diameter corresponds to inches (imperial units) and the typical outside diameter of steel pipes according to European standards. It is a complete DN-to-inch conversion table and, at the same time, a DN-to-millimeter conversion table—all in one place.
| DN | Inches | Typical outside diameter of steel pipe (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 outside-diameter table is a starting point, but it does not replace the data sheet for a specific material. For PE, PVC, and copper pipes, as well as for steel pipes with different pressure classes, the actual pipe dimensions may differ from those shown here. Therefore, we recommend using the pipe dimension table provided by the material manufacturer alongside this one.
DN 15, DN 20, and DN 25—domestic water installations and connections to fixtures
In typical water and sewer installations in detached houses and apartment buildings, three sizes dominate: DN 15, DN 20, and DN 25. These serve connections to washbasin and kitchen faucets, washing machines, dishwashers, and risers distributing domestic water.
DN 15 is the classic “half-inch” size (1/2 inch), used for individual water outlets. DN 20 corresponds to a 3/4-inch thread—it is the most common size in residential risers and individual connections. DN 25 is 1 inch, commonly used for connections to detached houses and the main branches of distribution systems.
In practice at our company, Dambat, DN 20 and DN 25 outlets are the most common connections in circulation pumps, booster sets, and smaller IBO surface pumps. A domestic hot-water circulation pump typically has 1-inch or 1 1/2-inch outlets, corresponding to DN 25 and DN 40, respectively. Many central-heating circulation pumps, in turn, have 1 1/2-inch outlets (DN 40), while larger versions may even have DN 50.
It is worth remembering that selecting a pipe for a domestic installation is not solely a matter of matching the pump outlet. The designed water flow rate and pressure losses along the length also matter—which is why DN 20 is used for branch connections, while main distribution lines are increasingly made in DN 25 to ensure adequate pressure when several outlets are used simultaneously.
DN 32, DN 40, and DN 50—medium-sized installations, booster sets, and pumping stations
In the medium-diameter range, DN 32, DN 40, and DN 50 are the leading sizes. DN 32 (1 1/4 inches) is a common size for booster sets, short discharge sections, and connections to smaller boreholes. DN 40 (1 1/2 inches) appears in central-heating systems in larger homes, in higher-powered circulation pumps, and as the diameter of submersible pump outlets.
DN 50 plays a particularly important role—it is a diameter found in higher-powered submersible borehole pumps, domestic wastewater pumping stations, discharge sections of submersible pumps, and many group booster sets. For a DN 50 steel pipe, the nominal diameter does not match the outside diameter, which is 60.3 mm—an important consideration when selecting DN 50 clamps and flanges. PE 63, in turn, is often used as a “DN 50 equivalent,” although its actual dimensions are different.
In DN 50 pressure-rated PVC pipes, DN 50 most often denotes a pipe with an OD of 63 mm (here too, using the outside diameter as the basis for the designation is standard). PVC pipe sizes corresponding to DN 50 usually have wall thicknesses ranging from 2.4 mm to 4.7 mm, depending on the pressure class. This should be kept in mind when designing an installation with mixed materials—steel and plastic.
Medium-sized pumping stations and smaller waterworks facilities also use DN 50 in the discharge pipelines of grinder-equipped submersible pumps, auxiliary fire-protection systems, and connections to filter units. That is why our consultants who advise on pump selection use the term DN 50 as a natural reference point in almost every conversation.
DN 65, DN 80, DN 100, and larger – industrial and waterworks valves and fittings
Above DN 50, we enter the world of industrial, flanged, and waterworks valves and fittings. DN 65 (2 1/2 inches) is the classic diameter for check and shut-off valves in smaller water and wastewater pumping stations. DN 80 is used in medium-sized deep-well pumps and smaller municipal wastewater pumping stations. DN 100 (4 inches), in turn, is the standard diameter for discharge pipelines in medium-sized waterworks facilities.
DN 125, DN 150, and DN 200 serve larger deep-well pumps and high-capacity submersible pumps; at this scale, we already work with flange connections compliant with PN-EN 1092. For diameters from DN 250 to DN 1000, we begin designing dedicated pumping stations and water mains; dimensions such as DN 300, DN 350, DN 400, DN 450, DN 500, DN 600, DN 700, DN 800, DN 900, and DN 1000 appear here. The latter are found mainly in water transmission systems and industrial cooling installations.
For reference: DN 700 converts to 28 inches, DN 800 to 32 inches, DN 900 to 36 inches, and DN 1000 to 40 inches. For district heating system designers, DN 200 (8 inches), DN 250 (10 inches), and DN 300 (12 inches) are usually useful, as they form the basic grid of distribution pipelines. Project documentation for large facilities also includes the less commonly used DN 350, DN 450, and DN 700, as diameters tailored to the specific requirements of a given installation.
Steel and galvanized pipes – inch dimensions vs. OD
Steel pipes are the “classic” choice for water, gas, compressed-air, and industrial installations. Steel pipe dimensions are usually given in inches, while a steel pipe diameter table also includes the DN and the actual outside diameter in millimeters. This distinction is fundamental: inches (imperial units) historically refer to the nominal diameter, not the exact pipe diameter.
The best-known example is the 1/2-inch pipe – DN 15 – whose actual outside diameter is 21.3 mm, not 12.7 mm (that is, 1/2 inch = 12.7 mm when converted to millimeters). Similarly, a 3/4-inch pipe (DN 20) has an OD of 26.9 mm, while 1 inch (DN 25) corresponds to an OD of 33.7 mm. These actual pipe dimensions are specified in standards (EN 10255, EN 10220) and have not changed for decades.
Galvanized steel pipe is a popular material in drinking-water and industrial systems. In galvanized pipes, wall thickness matters not only for strength—it also determines the durability of the zinc coating. Standard wall thicknesses for galvanized steel pipe range from approximately 2.3 mm for smaller diameters (DN 15) to 4.5 mm and more for DN 50 and larger. This must be taken into account when calculating the actual pipe bore, because thicker walls in galvanized steel pipes result in a smaller ID at the same nominal diameter.
When measuring the diameter of a steel pipe in the field, it is worth using calipers accurate to 0.1 mm. Measuring the outside diameter makes it possible to compare the actual value with the tabulated OD, and subtracting twice the pipe-wall thickness gives the internal bore. This is a simple inspection method when technical data is missing from the documentation and fittings, flanges or reducers must be selected based on the existing installation.
PVC, PP, PE and copper pipes—dimensions specified by outside diameter
Unlike steel pipes, plastic and copper pipes are designated by their actual outside diameter. This is a key methodological difference—here, “PE 32” simply means a pipe with an OD of 32 mm, not DN 32. PVC pipe dimensions are available in the following size range: 16, 20, 25, 32, 40, 50, 63, 75, 90, 110, 125, 160, 200 mm and larger.
PP (polypropylene) pipes are used mainly in cold- and hot-water systems and central-heating systems. Here too, the size range corresponds to the outside 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 “15-gauge thin copper pipe” corresponds to DN 12, not DN 15. This is a common mistake among people starting work in the industry.
For surface pumps operating with PE pipes, it is worth remembering the following correlation: PE 25 works well with a 1-inch connection (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 connection (DN 50). However, due to differences in internal diameter, it is always worth checking the actual outside diameter and selecting a compression or electrofusion fitting suitable for the actual dimensions. Using outside diameter as the basis for selecting fittings is standard practice in the plastics industry.
Inch threads in IBO and IPRO pumps—what you should know
Pipe threads are a separate, fascinating subject. Our IBO and IPRO brand pumps predominantly use inch threads compliant with ISO 228 (parallel pipe threads, designated “G”) and ISO 7-1 (tapered pipe threads, designated “R” for external threads and “Rp” for internal threads). BSP threads—British Standard Pipe—are the family to which both systems belong, and their sizes are specified in inches. Importantly, inch threads do not correspond dimensionally to their names—the G 1/2 thread has an outside diameter of 20.955 mm, not 12.7 mm.
Why is this the case? Inch threads originated in an era when the name was based on the approximate internal diameter of an iron pipe. Over time, pipe walls became thinner and the ID increased, but the designation remained historical. Consequently, a G 3/4 thread (DN 20) has an OD of 26.441 mm, while a G 1 thread (DN 25) has an OD of 33.249 mm. Our pumps most commonly use G 1 (DN 25), G 1 1/4 (DN 32), G 1 1/2 (DN 40), and G 2 (DN 50) threads.
Practical tip: cylindrical (G) threads require a flat seal (gasket) or a mechanical seal (ring), because they do not provide a seal by themselves. Tapered threads (R, NPT) seal when tightened with PTFE tape or hemp. Mixing these systems is a common source of leaks—a tapered male thread will not seal in a cylindrical female thread without additional measures.
Our range includes pumps with inch threads, as well as flanged models. High-output deep-well pumps, industrial submersible pumps, and inline circulation pumps with large ports often already use DN 65, DN 80, or DN 100 flanged connections. Threads also appear in associated equipment, including pressure gauges (typically with G 1/4 or G 1/2 threads), ball valves, check valves, mesh filters, and the connection ports of liquid-cooled inverters. Threads also connect reducers, couplings, and nipples used when installing pumps, which is why equipment catalogues always specify both the DN size and the corresponding thread size in inches.
It should be remembered that thread designations may include the suffix “A” or “B”, indicating the tolerance class of the thread. For pressure-pump connections, we recommend using Class A threads. Fittings, reducers, and nipples purchased for IBO and IPRO pumps have the thread type specified in the catalogue data sheet, eliminating the risk of mistakes. G 1 1/4 and G 1 1/2 threads are absolute best-sellers in the domestic-pump segment, corresponding to the ports of typical pressure-booster sets.
Port diameters in Dambat pumps – from circulation pumps to deep-well pumps
Our range includes pumps with a wide range of port diameters. Smaller IBO circulation and domestic hot-water pumps typically have 1-inch or 1 1/2-inch male-threaded ports (DN 25 or DN 40), allowing them to be easily connected to standard central-heating and domestic hot-water systems. Higher-output inline circulation pumps are available with DN 25, DN 32, DN 40, and DN 50 ports, in threaded or flanged versions.
Submersible pumps (for clean water, dirty water, and wastewater) most commonly have discharge ports ranging from DN 25 to DN 80, depending on their capacity. Models with grinders usually have DN 32 or DN 40 ports, while large pumps for municipal pumping stations operate with DN 65, DN 80, and DN 100 ports. Deep-well pumps (4-, 6-, and 8-inch) generally have a 1 1/4-inch, 1 1/2-inch, or 2-inch male thread on the head, corresponding to DN 32, DN 40, and DN 50, respectively.
For surface pumps (self-priming, single-stage centrifugal, and multistage), typical connection diameters are 1 inch, 1 1/4 inches, and 1 1/2 inches. Larger surface pumps, such as inline pumps in group booster sets, use DN 50, DN 65, or DN 80 flanged connections. All of this affects the selection of suitable pipes and fittings – that is why, before purchasing a pump, we always verify with the customer the diameter of the installation connection.
The associated components are equally important. In booster sets, you will find pressure gauges with G 1/4 or G 1/2 threads, check and ball valves in DN 25 to DN 50 sizes, mesh and magnetic filters matched to the pipeline diameter, inverters controlling pump operation with various connection configurations, as well as pressure and flow controllers. All these components must be dimensionally compatible with the rest of the installation.
Dambat pipe diameter converter – all standards in one tool
To make life easier for our customers – installers, designers, and service specialists – we have made a dedicated pipe diameter converter available on the company website. This tool supports all the most important systems: DN, DIN, ISO, the metric system (METRIC), INCHES, and ASTM. This allows you to compare pipe diameters from different standards in a single table and avoid mistakes typical of working with mixed documentation.
The converter on the Dambat website is intuitive to use – you enter a known value (e.g. inches or DN), and the system displays the corresponding values in the other standards. This is convenient both when preparing quotations and during design, when a single set of documentation includes components marked according to DIN and ASTM. We particularly recommend it to employees of companies working with foreign partners – Dambat exports pumps to many countries, and customers outside Europe often use only ASTM or imperial units.
For a more comprehensive overview of pipe sizing, we also recommend our guide on the company blog: How to choose the right pipe diameter – a practical guide to DN, d, and inches. You will find additional tips there on selecting diameters based on flow, pressure, and the purpose of the installation.
The most common mistakes when selecting pipe diameter
Our consultants’ experience shows that typical mistakes when selecting pipe diameters recur regardless of the project’s scale:
- Confusing DN with OD – especially in mixed steel and plastic installations. PE 50 and DN 50 are completely different dimensions. That is why it is always worth accurately measuring the diameter of the existing pipe before ordering components.
- Ignoring pipe wall thickness – when calculating flow, the ID matters, not the OD. Two DN 50 pipes with different wall thicknesses have different cross-sectional areas.
- Mismatched threads – connecting parallel threads to tapered threads, or BSP threads to NPT threads, leads to leaks. The male thread must match the female thread with the same characteristics.
- A discharge pipe that is too small – the smaller the diameter, the greater the hydraulic losses and the higher the load on the pump. We most often see this with submersible pumps connected to undersized PE 32 pipes instead of PE 40 or PE 50.
- Failing to verify the actual pipe dimensions in old installations – after years of operation, corrosion inside steel pipes can drastically reduce the ID, so it is advisable to select a pump with a pressure reserve.
- Ignoring flanges and their pressure ratings – DN 50 PN10 and DN 50 PN16 flanges have different bolt dimensions and thicknesses, even though they accommodate the same nominal diameter.
To avoid these mistakes, we always recommend: accurately measuring existing pipes, verifying the design documentation, using our converter, and – if in doubt – contacting our technical consulting department. As a pump manufacturer, we know the specifics of the market and can help you select the pipe diameter, thread, inverter, pressure gauge, and other components that fit the entire system.
A brief summary – DN, inches, and millimeters at a glance
The most popular pipe diameters in domestic and industrial systems have clear equivalents in the imperial 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 inches, DN 40 is 1 1/2 inches, and DN 50 is 2 inches. Larger pipe diameters are standard sizes used in water supply systems, district 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 remain popular in international industry.
Remember that selecting a pipe and its diameter is not everything – in pumping systems, the material, pressure rating, threads, flanges, fittings, controllers, valves, filters, pressure gauges, and inverters also matter. We encourage you to use our online tools and contact our technical support team – we will be happy to help.
FAQ – frequently asked questions about the DN, inch, and millimeter converter
How many inches is DN 50?
DN 50 is 2 inches. The actual outside diameter of a steel pipe for DN 50 is 60.3 mm, while the most common equivalent for a PE pipe is PE 63. This is one of the most popular sizes in plumbing systems and medium-sized pumping stations.
How many inches is DN 20?
DN 20 corresponds to 3/4 inch. The actual outside diameter of a steel pipe for DN 20 is 26.9 mm. This is the most common diameter in residential risers and individual connections to buildings.
How many inches is DN 25?
DN 25 to 1 inch. The actual outside diameter of a steel pipe for DN 25 is 33.7 mm. It is used in domestic connections, as the connection size for circulation and surface pumps, and in the main branches of distribution systems.
How many inches is DN 40?
DN 40 corresponds to a 1 1/2-inch thread. The outside diameter of a steel pipe for DN 40 is 48.3 mm. This is a common size for central-heating circulation pumps in larger homes and medium-sized pressure-boosting sets.
How many inches is DN 80?
DN 80 is 3 inches. The outside diameter of a steel pipe for DN 80 is 88.9 mm. It is used in medium-sized submersible pumps, smaller-scale municipal wastewater pumping stations, and local water supply systems.
Is DN the inside or outside diameter?
DN (Diameter Nominal) is neither the inside nor the outside diameter. It is a conventional reference value used to classify pipes, fittings, and valves. The actual pipe dimensions (OD and ID) depend on the material, standard, and wall thickness.
How do I convert inches to millimeters?
One inch is exactly 25.4 mm. To convert inches to millimeters, multiply the value in inches by 25.4. However, remember that pipe markings in inches (e.g., 1/2", 3/4") do not correspond to their actual diameter in millimeters—they are nominal designations resulting from historical standards.
Where can I find the Dambat pipe diameter converter?
Our pipe diameter converter is available free of charge on the Dambat.pl website. It supports DN, DIN, ISO, METRIC, INCH, and ASTM standards, allowing you to quickly compare diameters across different systems.
What threads do IBO and IPRO pumps have?
IBO and IPRO pumps are standardly equipped with inch threads compliant with ISO 228 (“G” parallel 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, corresponding respectively to DN 25, DN 32, DN 40, and DN 50. Larger models are available with flanged connections in sizes DN 65, DN 80, and above.
Is DN 32 the same as PE 32?
No. DN 32 is a nominal diameter in the European system, whose equivalent for steel pipes is an OD of 42.4 mm. PE 32 is a polyethylene pipe with an outside diameter of 32 mm. These are completely different dimensions—confusing them causes problems with fitting connectors and valves.
How do I choose the pipe diameter for a submersible pump?
The discharge pipe diameter is selected based on the pump’s capacity and the length of the pipeline. For submersible pumps with capacities 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). A diameter that is too small creates high pressure losses and may overload the pump.
How do BSP and NPT threads differ?
BSP (British Standard Pipe) is a European pipe thread standard with a 55° angle, predominant 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. IBO and IPRO pumps use BSP threads exclusively.
