Quickly convert a nominal pipe size DN (also called NB / NPS) to inches, millimeters (outside diameter) and the DIN, ISO, ASTM and metric designations. Enter a size in any system - the tool shows every equivalent at once.
Most common: DN15 = 1/2" • DN20 = 3/4" • DN25 = 1" • DN50 = 2"DN is a nominal value - it roughly matches the inside diameter (the bore). It is not the exact pipe dimension. When ordering fittings and threads, always compare against the actual outside diameter (OD ISO/EN) shown in the result.
If you are wondering “DN to inches”, below are ready answers for the most common threaded pipe sizes (BSP pipe thread per ISO 7 / EN 10226):
What is DN6 in inches? DN6 = 1/8".
What is DN8 in inches? DN8 = 1/4".
What is DN10 in inches? DN10 = 3/8".
What is DN15 in inches? DN15 = 1/2".
What is DN20 in inches? DN20 = 3/4".
What is DN25 in inches? DN25 = 1".
What is DN32 in inches? DN32 = 1 1/4" (5/4").
What is DN40 in inches? DN40 = 1 1/2" (6/4").
What is DN50 in inches? DN50 = 2".
What is DN65 in inches? DN65 = 2 1/2".
What is DN80 in inches? DN80 = 3".
What is DN100 in inches? DN100 = 4".
What is DN125 in inches? DN125 = 5".
What is DN150 in inches? DN150 = 6".
What is DN200 in inches? DN200 = 8".
What is DN250 in inches? DN250 = 10".
What is DN300 in inches? DN300 = 12".
What is DN350 in inches? DN350 = 14".
What is DN400 in inches? DN400 = 16".
What is DN500 in inches? DN500 = 20".
What is DN600 in inches? DN600 = 24".
What is DN800 in inches? DN800 = 32".
What is DN1000 in inches? DN1000 = 40".
Have a pipe or thread sized in inches and need the DN? Check below:
1/8 inch to DN? 1/8" = DN6.
1/4 inch to DN? 1/4" = DN8.
3/8 inch to DN? 3/8" = DN10.
1/2 inch to DN? 1/2" = DN15.
3/4 inch to DN? 3/4" = DN20.
1 inch to DN? 1" = DN25.
1 1/4 inch (5/4) to DN? = DN32.
1 1/2 inch (6/4) to DN? = DN40.
2 inch to DN? 2" = DN50.
2 1/2 inch to DN? = DN65.
3 inch to DN? 3" = DN80.
4 inch to DN? 4" = DN100.
5 inch to DN? 5" = DN125.
6 inch to DN? 6" = DN150.
8 inch to DN? 8" = DN200.
10 inch to DN? 10" = DN250.
12 inch to DN? 12" = DN300.
The question “how many mm is an X inch pipe” usually refers to the actual outside diameter (OD) of a steel pipe per ISO 4200 / EN 10220. The most common sizes:
1/4 inch pipe in mm? = DN8, OD 13.5 mm.
3/8 inch pipe in mm? = DN10, OD 17.2 mm.
1/2 inch pipe in mm? = DN15, OD 21.3 mm.
3/4 inch pipe in mm? = DN20, OD 26.9 mm.
1 inch pipe in mm? = DN25, OD 33.7 mm.
5/4 inch pipe (1 1/4") in mm? = DN32, OD 42.4 mm.
6/4 inch pipe (1 1/2") in mm? = DN40, OD 48.3 mm.
2 inch pipe in mm? = DN50, OD 60.3 mm.
2 1/2 inch pipe in mm? = DN65, OD 76.1 mm.
3 inch pipe in mm? = DN80, OD 88.9 mm.
Watch out for trade names: a so-called “half-inch” pipe (1/2") is not 12.7 mm inside. 1/2" is a thread with a bore of about 15 mm and a pipe OD of 21.3 mm - the inch value refers to the thread size, not the physical bore.
Plastic pipes (PP-R, PE, PEX) and copper pipes are sized by their outside diameter in mm. Approximate inch equivalents:
PEX/PP 16 mm to inches? ≈ 3/8" (DN10).
PP 20 mm to inches? ≈ 1/2" (DN15).
PP 25 mm to inches? ≈ 3/4" (DN20).
PP 32 mm to inches? ≈ 1" (DN25).
PP 40 mm to inches? ≈ 5/4" / 1 1/4" (DN32).
PP 50 mm to inches? ≈ 6/4" / 1 1/2" (DN40).
PP 63 mm to inches? ≈ 2" (DN50).
22 mm pipe to inches? ≈ 1/2"–3/4" (closest to DN15/DN20).
32 mm pipe to inches? ≈ 1" (DN25).
| DN | DIN | ISO | METRIC | INCH | ASTM |
|---|---|---|---|---|---|
| 6 | – | 10.2 | – | 1/8" | 10.29 |
| 8 | – | 13.5 | – | 1/4" | 13.72 |
| 10 | 14 | 17.2 | 12 | 3/8" | 17.15 |
| 15 | 20 | 21.3 | 18 | 1/2" | 21.34 |
| 20 | 25 | 26.9 | 23 | 3/4" | 26.67 |
| 25 | 30 | 33.7 | 28 | 1" | 33.4 |
| 32 | 38 | 42.4 | 35 | 1 1/4" | 42.16 |
| 40 | 44.5 | 48.3 | 43 | 1 1/2" | 48.26 |
| 50 | 57 | 60.3 | 54 | 2" | 60.33 |
| 65 | 76.1 | 76.1 | 69 | 2 1/2" | 73.03 |
| 80 | 88.9 | 88.9 | 84 | 3" | 88.9 |
| – | – | – | – | 3 1/2" | 101.6 |
| 100 | 108 | 114.3 | 104 | 4" | 114.3 |
| 125 | 133 | 139.7 | 129 | 5" | 141.3 |
| 150 | 159 | 168.3 | 154 | 6" | 168.3 |
| 200 | 216 | 219.1 | 204 | 8" | 219.1 |
| 250 | 267 | 273 | 254 | 10" | 273 |
| 300 | 318 | 323.9 | 304 | 12" | 323.9 |
| 350 | 368 | 355.6 | 354 | 14" | 355.6 |
| 400 | 419 | 406.4 | – | 16" | 406.4 |
| 450 | 459 | 457.2 | – | 18" | 457.2 |
| 500 | 521 | 508 | – | 20" | 508 |
| – | – | – | – | 22" | 558.8 |
| 600 | 622 | 609.6 | – | 24" | 609.6 |
| – | – | – | – | 26" | 660.4 |
| 700 | 720 | 711.2 | – | 28" | 711.2 |
| – | – | – | – | 30" | 762 |
| 800 | 820 | 812.8 | – | 32" | 812.8 |
| – | – | – | – | 34" | 863.6 |
| 900 | 920 | 914.4 | – | 36" | 914.4 |
| 1000 | 1020 | 1016 | – | 40" | 1016 |
Column ISO = steel pipe outside diameter per ISO 4200 / EN 10220. DIN = dimensions per German standards (incl. DIN 2448). ASTM = outside diameters per the US system (NPS, ASTM A53). METRIC = conventional, rounded metric designation (approximate).
DN (Diameter Nominal, nominal diameter) is a standardized, conventional size designation for pipes and fittings, given as a number close to the inside diameter in millimeters. DN is not an exact dimension - it is a “size label” that lets components from different manufacturers fit together. DN is defined in standard ISO 6708. It is the European equivalent of the US NPS (Nominal Pipe Size); in the UK it is also called NB (Nominal Bore).
OD vs DN: the diameter symbol Ø (“fi”) describes a specific, measured dimension in millimeters (usually the outside diameter), whereas DN is a nominal, conventional value. For example, a DN25 pipe has an outside diameter of about Ø 33.7 mm, even though its “1 inch” label would suggest 25.4 mm. So when selecting fittings and seals, always check the actual OD, not just the nominal size.
DIN - German standards (e.g. DIN 2448 for seamless steel pipe), used across Europe. ISO - international standards (e.g. ISO 4200), recognized worldwide. ASTM - US standards (e.g. ASTM A53), with dimensions in the imperial/inch system (NPS). METRIC - metric designations (mm), convenient and intuitive but approximate. In practice, for the same DN pipe the various standards give similar but not identical outside diameters.
It depends on the size. The most common: DN15 = 1/2 inch, DN20 = 3/4 inch, DN25 = 1 inch, DN32 = 1 1/4 inch (5/4), DN40 = 1 1/2 inch (6/4), DN50 = 2 inch. See the full list in the chart above. The equivalents refer to BSP pipe thread per ISO 7 / EN 10226.
DN20 is 3/4 inch. The outside diameter of a DN20 steel pipe is 26.9 mm (per ISO/EN), or 26.67 mm per ASTM.
DN25 is 1 inch. The outside diameter of a DN25 pipe is 33.7 mm (ISO/EN), i.e. more than 25.4 mm - the inch value describes the thread, not the actual bore.
DN50 is 2 inch. The outside diameter of a DN50 pipe is 60.3 mm (ISO/EN).
A 3/4 inch pipe corresponds to DN20. Its outside diameter is 26.9 mm (per ISO/EN) and its bore is about 20 mm.
A 1 inch pipe is DN25, with an outside diameter of 33.7 mm. The inch refers to the thread size, so the OD is larger than 25.4 mm.
Ø (the diameter symbol, “fi”) is a measured dimension in millimeters - usually the outside diameter. DN is a nominal, conventional value, close to the inside diameter. Example: DN25 has an OD of about 33.7 mm.
Because the 1/2 inch label describes the pipe thread size, not the physical inside diameter. A 1/2 inch pipe (DN15) has a bore of about 15 mm and an outside diameter of 21.3 mm.
A PP-R 32 mm pipe (outside diameter) corresponds roughly to a 1 inch thread (DN25). Plastic pipes are sized by their actual outside diameter in mm.
