Pipe Size Converter
NPS, DN & actual OD.
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About Pipe Size Converter
Convert pipe sizes between NPS (nominal pipe size, inches), DN (nominal diameter, metric) and the actual outside diameter in millimetres and inches. Enter any one and see the rest, with a note on where nominal stops matching the real size.
A '2-inch' pipe doesn't measure 2 inches across — it's the classic trap a generic converter falls into — so this is built on the real ASME dimensions. It is completely free with no sign-up, and it runs entirely in your browser.
Frequently asked questions
- What is DN50 in inches?
- DN50 is NPS 2, and its actual outside diameter is 60.3 mm (2.375 inches). Below NPS 14 the nominal number is a label, not the measured size.
- How do I convert NPS to DN?
- It's a fixed nominal mapping, not a multiplication — NPS 2 is DN 50 (not DN 51), NPS 4 is DN 100, and so on. The tool has the full table.
- What is the actual outside diameter of a 2-inch pipe?
- 60.3 mm, or 2.375 inches. The nominal '2 inch' is a size name; the outside diameter is fixed while the wall thickness changes with the schedule.
- Why isn't the nominal pipe size the real diameter?
- Nominal sizes are historical. For small pipe (below NPS 14) the number roughly relates to the old bore, so the outside diameter is always a bit larger; from NPS 14 up, the nominal number equals the outside diameter in inches.
- Does the outside diameter change with the schedule?
- No — the outside diameter stays the same for a given NPS across all schedules (40, 80, etc.); only the wall thickness and inside diameter change.