The Challenge
Pipe sizing is one of the most misunderstood measurement problems in construction. A 1-inch pipe is not 1 inch in diameter — it never was. NPS (Nominal Pipe Size) is a trade designation, not a measurement. A 1-inch NPS pipe has an outside diameter of 33.40mm. A 2-inch NPS pipe measures 60.33mm OD. The nominal size roughly corresponded to the inside diameter of early iron pipe, but wall thickness changes meant the relationship broke down over a century ago. This matters when buying fittings, threading pipe, or connecting US-spec pipe to European DN (Diameter Nominal) systems. DN25 is the metric equivalent of 1-inch NPS — both have a 33.40mm OD. Get this wrong and fittings either won't thread or will leak under pressure.
NPS Pipe Size Inches to mm: Actual Outside Diameter Chart
| NPS (inches) | DN (metric) | Outside Diameter (mm) | Outside Diameter (inches) |
|---|---|---|---|
| 1/8" | DN6 | 10.29 mm | 0.405 in |
| 1/4" | DN8 | 13.72 mm | 0.540 in |
| 3/8" | DN10 | 17.15 mm | 0.675 in |
| 1/2" | DN15 | 21.34 mm | 0.840 in |
| 3/4" | DN20 | 26.67 mm | 1.050 in |
| 1" | DN25 | 33.40 mm | 1.315 in |
| 1-1/4" | DN32 | 42.16 mm | 1.660 in |
| 1-1/2" | DN40 | 48.26 mm | 1.900 in |
| 2" | DN50 | 60.33 mm | 2.375 in |
| 2-1/2" | DN65 | 73.03 mm | 2.875 in |
| 3" | DN80 | 88.90 mm | 3.500 in |
| 3-1/2" | DN90 | 101.60 mm | 4.000 in |
| 4" | DN100 | 114.30 mm | 4.500 in |
| 5" | DN125 | 141.30 mm | 5.563 in |
| 6" | DN150 | 168.28 mm | 6.625 in |
| 8" | DN200 | 219.08 mm | 8.625 in |
| 10" | DN250 | 273.05 mm | 10.750 in |
| 12" | DN300 | 323.85 mm | 12.750 in |
Schedule 40 and Schedule 80 Wall Thickness and Bore: Common Sizes
| NPS | OD (mm) | Sch 40 Wall (mm) | Sch 40 ID (mm) | Sch 80 Wall (mm) | Sch 80 ID (mm) |
|---|---|---|---|---|---|
| 1/2" | 21.34 | 2.77 | 15.80 | 3.73 | 13.88 |
| 3/4" | 26.67 | 2.87 | 20.93 | 3.91 | 18.85 |
| 1" | 33.40 | 3.38 | 26.64 | 4.55 | 24.30 |
| 1-1/2" | 48.26 | 3.68 | 40.90 | 5.08 | 38.10 |
| 2" | 60.33 | 3.91 | 52.50 | 5.54 | 49.25 |
| 3" | 88.90 | 5.49 | 77.92 | 7.62 | 73.66 |
| 4" | 114.30 | 6.02 | 102.26 | 8.56 | 97.18 |
| 6" | 168.28 | 7.11 | 154.05 | 10.97 | 146.33 |
Why Nominal Pipe Size Has Nothing to Do With Actual Dimensions
NPS originated in the mid-1800s when wrought iron pipe was made with thick walls. The nominal size approximated the inside bore. As manufacturing evolved, thinner walls were possible, so the same fitting threads were kept but the bore got larger. By the time standards bodies formalized pipe dimensions in ASME B36.10M, the nominal size was purely a legacy designation — a name, not a number. The result: every NPS size below 14 inches has an OD that does not match the nominal size. At NPS 14 and above, the OD in inches does equal the nominal size — a 14-inch pipe is 355.60mm (14.000 inches) OD — but below that, you must use a table.
How to Identify Unknown Pipe Size from a Measured OD
- Measure the outside diameter with vernier calipers — wrap a flexible tape around the pipe and divide by π if calipers aren't available
- Convert your measurement to mm if measured in inches: multiply by 25.4
- Compare against the NPS OD table — find the closest standard OD value
- Confirm the nominal size by checking thread count with a thread gauge if the pipe is threaded
- For plastic pipe (PVC, CPVC, PEX): verify whether it follows NPS or CTS standards before ordering fittings
NPS vs CTS vs BSP: Three Systems, Three Different ODs
Gas Pipe and High-Pressure Systems: Verify Schedule Before Converting
- Gas line sizing requires schedule verification — the bore (ID), not OD, determines flow capacity and pressure drop
- Never substitute pipe schedule based on OD alone — a Schedule 10 and Schedule 80 pipe have the same OD but very different pressure ratings
- Black steel pipe for gas is typically Schedule 40; CSST flexible gas line uses its own sizing system not compatible with NPS tables
- Always confirm local building code pipe specifications — some jurisdictions require Schedule 80 minimum for certain gas applications
Step-by-Step Workflow
Identify whether you have nominal pipe size (NPS) or actual measured diameter
Enter the inch value — converter outputs actual OD in millimeters per ASME B36.10M
Cross-reference the DN equivalent if connecting to metric fittings
Specifications
- 1/2 inch NPS OD
- 21.34 mm
- 3/4 inch NPS OD
- 26.67 mm
- 1 inch NPS OD
- 33.40 mm
- 1-1/2 inch NPS OD
- 48.26 mm
- 2 inch NPS OD
- 60.33 mm
- 4 inch NPS OD
- 114.30 mm
- Rule
- Nominal size ≠ actual diameter
Best Practices
- NPS and DN share the same OD — DN25 and 1-inch NPS both have 33.40mm outside diameter
- Measure actual pipe OD with calipers if the nominal size is unknown — then look up the schedule
- Wall thickness (schedule) changes ID but never OD — Schedule 40 and Schedule 80 have the same OD
- Copper tube sizing (CTS) uses different OD than NPS — a 1/2-inch copper tube OD is 15.88mm, not 21.34mm
- PVC and CPVC follow NPS OD standards; PEX follows CTS OD standards
Frequently Asked Questions
Why doesn't a 1-inch pipe measure 1 inch?
NPS nominal sizes originated from the inside diameter of early wrought iron pipe. As pipe manufacturing improved and wall thicknesses standardized, the inside diameter changed but the thread and fitting dimensions were locked in. The nominal size became a trade name, not a measurement. A 1-inch NPS pipe has an OD of 33.40mm (1.315 inches) — the 'one inch' refers to nothing you can physically measure on the pipe today.
What is the actual outside diameter of 3/4 inch pipe in mm?
3/4 inch NPS pipe has an outside diameter of 26.67mm. This applies regardless of schedule — Schedule 40, Schedule 80, and Schedule 160 3/4-inch pipe all have the same 26.67mm OD. Wall thickness increases inward, reducing the bore, not outward.
What is DN pipe and how does it relate to NPS inches?
DN (Diameter Nominal) is the ISO metric pipe designation used in Europe and internationally. DN does not directly equal OD in mm either — it is also a nominal trade size. DN25 matches 1-inch NPS, DN50 matches 2-inch NPS, DN100 matches 4-inch NPS. Both systems share the same actual outside diameters per ISO 65 and ASME B36.10M, so fittings are interchangeable.
How do I convert pipe size when the actual OD is known?
Measure the OD with calipers, multiply by 25.4 to convert to mm if measured in inches, then look up the NPS schedule table to identify the nominal size. For example, a measured OD of 2.375 inches = 60.33mm = 2-inch NPS. The converter on this page handles nominal-to-mm; use calipers for unknown pipe.
Does pipe schedule affect the inch to mm conversion?
No. Schedule affects wall thickness and therefore inside diameter, but the outside diameter is fixed per nominal size. A 2-inch Schedule 40 pipe and a 2-inch Schedule 80 pipe both have an OD of 60.33mm. Schedule 40 has a 3.91mm wall (52.50mm ID); Schedule 80 has a 5.54mm wall (49.25mm ID).
How do copper pipe sizes convert to mm?
Copper tube follows CTS (Copper Tube Size) standards, not NPS. A 1/2-inch copper tube has an OD of 15.88mm (5/8 inch), not 21.34mm like 1/2-inch NPS. A 3/4-inch copper tube OD is 22.22mm. This is why copper fittings and iron pipe fittings are not interchangeable even at the same nominal size.
What pipe size in inches equals a 50mm metric pipe?
DN50 metric pipe corresponds to 2-inch NPS, with an OD of 60.33mm. If you have an actual OD of 50mm, that does not match any standard NPS size — closest is DN40 / 1.5-inch NPS at 48.26mm OD. Always verify with calipers and the schedule table rather than assuming a DN number equals the OD in mm.