The Challenge
Pipe sizing is one of the most misunderstood unit conversions in plumbing and construction. A '1-inch pipe' is not 1 inch in any actual dimension. NPS (Nominal Pipe Size) is a North American designation where the number is a historical reference — a 1-inch NPS pipe has an outside diameter of 33.4mm (1.315 inches), not 25.4mm. European DN (Diamètre Nominal) uses a different number again: DN25 is the metric equivalent of 1-inch NPS. For plain copper tube or metric pipe, the math is straightforward: divide mm by 25.4 to get inches. 15mm copper = 0.591 inches OD. But for threaded steel, iron, or PVC fittings sold as 'nominal' sizes, the number on the label does not correspond to any real measurement. You need the chart.
NPS Pipe Size Chart: Nominal vs Actual Outside Diameter
| NPS (inches) | DN (metric) | OD (mm) | OD (inches) | SCH 40 Wall (mm) | SCH 40 ID (mm) |
|---|---|---|---|---|---|
| ⅛ in | DN6 | 10.29 mm | 0.405 in | 1.73 mm | 6.83 mm |
| ¼ in | DN8 | 13.72 mm | 0.540 in | 2.24 mm | 9.24 mm |
| ⅜ in | DN10 | 17.15 mm | 0.675 in | 2.31 mm | 12.53 mm |
| ½ in | DN15 | 21.34 mm | 0.840 in | 2.77 mm | 15.80 mm |
| ¾ in | DN20 | 26.67 mm | 1.050 in | 2.87 mm | 20.93 mm |
| 1 in | DN25 | 33.40 mm | 1.315 in | 3.38 mm | 26.64 mm |
| 1¼ in | DN32 | 42.16 mm | 1.660 in | 3.56 mm | 35.04 mm |
| 1½ in | DN40 | 48.26 mm | 1.900 in | 3.68 mm | 40.90 mm |
| 2 in | DN50 | 60.33 mm | 2.375 in | 3.91 mm | 52.51 mm |
| 2½ in | DN65 | 73.03 mm | 2.875 in | 5.16 mm | 62.71 mm |
| 3 in | DN80 | 88.90 mm | 3.500 in | 5.49 mm | 77.92 mm |
| 3½ in | DN90 | 101.60 mm | 4.000 in | 5.74 mm | 90.12 mm |
| 4 in | DN100 | 114.30 mm | 4.500 in | 6.02 mm | 102.26 mm |
| 5 in | DN125 | 141.30 mm | 5.563 in | 6.55 mm | 128.20 mm |
| 6 in | DN150 | 168.28 mm | 6.625 in | 7.11 mm | 154.06 mm |
| 8 in | DN200 | 219.08 mm | 8.625 in | 8.18 mm | 202.72 mm |
| 10 in | DN250 | 273.05 mm | 10.750 in | 9.27 mm | 254.51 mm |
| 12 in | DN300 | 323.85 mm | 12.750 in | 9.53 mm | 304.79 mm |
Metric Copper Tube: Actual OD in Inches
| Metric OD (mm) | Actual OD (inches) | Nearest NPS | Compatible? |
|---|---|---|---|
| 6 mm | 0.236 in | — | No NPS match |
| 8 mm | 0.315 in | — | No NPS match |
| 10 mm | 0.394 in | ⅛ NPS (0.405 in) | No — 0.011 in gap |
| 12 mm | 0.472 in | — | No NPS match |
| 15 mm | 0.591 in | ½ NPS (0.840 in) | No — adapter needed |
| 18 mm | 0.709 in | ¾ NPS (1.050 in) | No — adapter needed |
| 22 mm | 0.866 in | ¾ NPS (1.050 in) | No — adapter needed |
| 28 mm | 1.102 in | 1 NPS (1.315 in) | No — adapter needed |
| 35 mm | 1.378 in | 1¼ NPS (1.660 in) | No — adapter needed |
| 42 mm | 1.654 in | 1¼ NPS (1.660 in) | Close — verify fitting |
| 54 mm | 2.126 in | 2 NPS (2.375 in) | No — adapter needed |
| 76.1 mm | 2.996 in | 3 NPS (3.500 in) | No — adapter needed |
Do Not Use the Formula for Nominal Pipe Sizes
- A ½-inch NPS pipe OD is 21.34mm — not 12.7mm (which is what ½ × 25.4 gives you)
- Using the direct conversion formula on nominal pipe sizes will give you a number that matches no real fitting
- Always use the NPS/DN chart for threaded steel, iron, or PVC fittings — formula applies only to actual OD measurements
How to Identify and Convert an Unknown Pipe Size
- Measure the outside diameter in mm using calipers — do not measure the bore
- If the OD matches a value in the NPS chart (e.g. 60.33mm = NPS 2), use the nominal designation for ordering fittings
- If the OD does not match any NPS value, divide by 25.4 to get inches — you likely have metric tube sized by actual OD
- For threaded connections, confirm thread standard: NPT (US taper), BSP (UK/EU parallel or taper), or metric ISO threads are not interchangeable
Nominal Pipe Size vs Actual Pipe Size: When Each Applies
Why Pipe Sizes Are Not What They Say: A Brief History
In the early 1800s, wrought iron pipe was sized by its approximate bore. A pipe with a 1-inch bore was called 1-inch pipe. As manufacturing improved and wall thicknesses standardized, the OD was fixed to allow interchangeable threading and fittings — but the bore shrank as walls got thicker. The nominal 'inch' label stayed. The American Standards Association formalized NPS in the 1920s, locking in OD values that no longer matched any actual inch measurement. The ISO DN system followed the same logic, just using approximate mm bore values. Today, both systems persist because the global installed base of fittings, valves, and threaded connections runs into the billions — changing the naming convention is not feasible.
Step-by-Step Workflow
Identify whether your pipe uses actual OD measurements (metric copper, plastic tube) or nominal sizing (NPS steel, iron, PVC fittings)
For actual OD: enter mm value and divide by 25.4 for exact inch equivalent
For nominal pipe: use the NPS/DN chart below — do not use the formula, the numbers will not match any fitting on the shelf
Specifications
- Formula (actual OD)
- inches = mm ÷ 25.4
- 1-inch NPS outside diameter
- 33.40 mm (1.315 in)
- DN25 equals
- NPS 1 inch (nominal match)
- 15mm copper tube OD
- 0.591 inches
- 22mm copper tube OD
- 0.866 inches
- ½-inch NPS outside diameter
- 21.34 mm (0.840 in)
Best Practices
- Outside diameter (OD) and nominal size are different — always confirm which one a spec sheet is listing
- Copper tube in the UK and EU is sold by actual OD: 15mm, 22mm, 28mm — these do not match NPS inch sizes
- PVC pressure pipe follows NPS OD standards in North America; metric PVC in Europe uses actual OD
- Thread size follows nominal pipe size — a ½-inch BSP fitting matches a ½-inch NPS fitting thread pitch
- When ordering fittings, match both nominal size AND pipe schedule (wall thickness) — a Schedule 40 and Schedule 80 pipe share the same OD but different IDs
Frequently Asked Questions
Why doesn't a 1-inch pipe measure 1 inch?
Nominal pipe sizes date to early 19th century iron pipe manufacturing when the bore (inside diameter) was approximately the nominal size. As manufacturing evolved, wall thicknesses changed but the naming convention didn't. Today, NPS 1 pipe has a 33.4mm (1.315-inch) outside diameter regardless of schedule. The nominal number is purely a size label, not a measurement.
What is DN pipe sizing and how does it convert to inches?
DN (Diamètre Nominal or Nominal Diameter) is the ISO/European equivalent of NPS. DN values are approximately 25× the NPS inch value: DN15 = ½ inch NPS, DN20 = ¾ inch NPS, DN25 = 1 inch NPS, DN40 = 1½ inch NPS, DN50 = 2 inch NPS. Above DN50, the relationship drifts — always use a DN-to-NPS table for sizes DN65 and larger.
How do I convert 15mm copper pipe to inches?
15mm copper tube has an actual outside diameter of 15mm. Dividing by 25.4 gives 0.591 inches OD. This does not correspond to any NPS nominal size — the closest NPS is ½ inch (21.34mm OD), which is physically larger. UK 15mm copper and US ½-inch copper are not interchangeable without adapters.
What is the outside diameter of common NPS pipe sizes in mm?
NPS ¼ = 13.72mm, NPS ⅜ = 17.15mm, NPS ½ = 21.34mm, NPS ¾ = 26.67mm, NPS 1 = 33.40mm, NPS 1¼ = 42.16mm, NPS 1½ = 48.26mm, NPS 2 = 60.33mm, NPS 2½ = 73.03mm, NPS 3 = 88.90mm, NPS 4 = 114.30mm. These OD values are fixed regardless of pipe schedule.
Can I connect metric pipe directly to inch pipe fittings?
Only if the outside diameters match closely enough for the fitting type. Compression fittings allow some size tolerance — a 15mm compression fitting will not accept 21.34mm NPS ½-inch pipe. Push-fit fittings are size-specific. For threaded connections, thread form matters: BSP (British Standard Pipe) and NPT (US National Pipe Taper) are not compatible even at the same nominal size.
What is pipe schedule and does it affect the mm to inch conversion?
Pipe schedule (SCH 40, SCH 80, SCH 160) refers to wall thickness, not outside diameter. All schedules of a given NPS share the same OD — what changes is the inside diameter (bore). SCH 40 NPS 1 pipe: OD 33.40mm, wall 3.38mm, ID 26.64mm. SCH 80 NPS 1 pipe: OD 33.40mm, wall 4.55mm, ID 24.30mm. The mm-to-inch OD conversion is the same for all schedules.
How do I identify an unmarked pipe's size?
Measure the outside diameter with calipers in mm, then compare to the NPS OD chart. If OD is 33.4mm, it's NPS 1. If OD is 48.3mm, it's NPS 1½. For metric pipe, the measured OD is the actual size designation. Never measure the bore to determine nominal size — bore varies by schedule.