A buyer in Houston writes "4 inch", a buyer in Germany writes "DN 100" and a buyer in India writes "100 NB". They all mean the same pipe. This pipe size chart brings NPS, DN and NB together with outside diameters, so you can read any enquiry in seconds.
What NPS, DN and NB mean
- NPS (Nominal Pipe Size): the North American system in inches, used with ASME B36.10M and B36.19M.
- DN (Diamètre Nominal): the metric equivalent used in ISO and EN standards, roughly the size in millimetres.
- NB (Nominal Bore): the term used in India and the UK, usually written in mm (for example 50 NB) and matching DN.
None of these numbers is an actual measurement. For NPS 1/8" to 12", the outside diameter is larger than the nominal size. From NPS 14" up, the OD equals the NPS in inches.
NPS to DN pipe size chart (ASME B36.10M)
| NPS (inch) | DN / NB (mm) | OD (mm) | OD (inch) |
|---|---|---|---|
| 1/8 | 6 | 10.3 | 0.405 |
| 1/4 | 8 | 13.7 | 0.540 |
| 3/8 | 10 | 17.1 | 0.675 |
| 1/2 | 15 | 21.3 | 0.840 |
| 3/4 | 20 | 26.7 | 1.050 |
| 1 | 25 | 33.4 | 1.315 |
| 1-1/4 | 32 | 42.2 | 1.660 |
| 1-1/2 | 40 | 48.3 | 1.900 |
| 2 | 50 | 60.3 | 2.375 |
| 2-1/2 | 65 | 73.0 | 2.875 |
| 3 | 80 | 88.9 | 3.500 |
| 3-1/2 | 90 | 101.6 | 4.000 |
| 4 | 100 | 114.3 | 4.500 |
| 5 | 125 | 141.3 | 5.563 |
| 6 | 150 | 168.3 | 6.625 |
| 8 | 200 | 219.1 | 8.625 |
| 10 | 250 | 273.0 | 10.750 |
| 12 | 300 | 323.8 | 12.750 |
| 14 | 350 | 355.6 | 14.000 |
| 16 | 400 | 406.4 | 16.000 |
| 18 | 450 | 457.0 | 18.000 |
| 20 | 500 | 508.0 | 20.000 |
| 24 | 600 | 610.0 | 24.000 |
Need wall thickness and weight too? Use our free pipe schedule chart and pipe weight calculator.
Where EN sizes differ from ASME
European pipe to EN 10220 (and EN 10216/10217) uses the same DN numbers but some outside diameters differ slightly from ASME:
| DN | ASME OD (mm) | EN 10220 common OD (mm) |
|---|---|---|
| 20 | 26.7 | 26.9 |
| 25 | 33.4 | 33.7 |
| 32 | 42.2 | 42.4 |
| 65 | 73.0 | 76.1 |
| 125 | 141.3 | 139.7 |
| 300 | 323.8 | 323.9 |
The difference at DN 65 and DN 125 is big enough that fittings will not match. When a European buyer writes "DN 65", confirm whether they want 73.0 mm (ASME) or 76.1 mm (EN). Our ASTM vs EN vs IS standards guide covers other traps between standards.
Indian IS 1239 sizes
IS 1239 Part 1 covers MS and GI pipes in Light, Medium and Heavy classes from 6 NB to 150 NB. The outside diameters are close to ISO/EN values — for example 15 NB has an OD of about 21.3 mm and 25 NB about 33.7 mm — but wall thickness is defined by class, not schedule. See GI pipe vs MS pipe and our MS pipe weight chart (IS 1239).
Schedule vs nominal size
The NPS fixes the OD. The schedule fixes the wall thickness. So a 2" SCH 40 and a 2" SCH 80 pipe have the same 60.3 mm OD, but SCH 80 is thicker and has a smaller bore. For details, read pipe schedules explained.
Quick tips for reading enquiries
- "2 inch", "2"", "NPS 2", "DN 50" and "50 NB" are the same size.
- If OD in mm is given (e.g., "60.3 × 3.91"), it is a 2" SCH 40 pipe.
- If the size has an odd OD (e.g., 50.8 mm), it is probably a tube, not a pipe. See pipe vs tube difference.
- For large diameters above 24", check whether the buyer uses ASME B36.10M or API 5L sizes.
Why a size chart matters for sales
Size confusion is one of the most common causes of delays in export orders. A buyer sends a drawing in DN, the mill works in NPS and the stockist thinks in NB. Every conversion is a chance for error. Exporters who reply with the size in all three systems — for example "4" NPS / DN 100 / 100 NB, OD 114.3 mm, SCH 40 (6.02 mm)" — remove that risk and look far more professional than competitors who just repeat the buyer's words.
Put a size chart like this on your own website as well. Pages with useful reference tables attract searches from engineers and buyers every day, and they keep visitors on your site longer. Our guide to product page SEO for pipes and fittings shows how to build these pages.
Frequently asked questions
What is the OD of a 4 inch pipe?
A 4" NPS (DN 100) pipe has an outside diameter of 114.3 mm (4.500 inch).
Is DN the inside diameter?
No. DN is a nominal designation. The actual inside diameter depends on wall thickness and changes with schedule.
Why is 1 inch pipe 33.4 mm OD?
Pipe sizes come from historical iron pipe sizes where the nominal size roughly matched the bore. The OD was fixed and has been kept for compatibility with fittings.
What does NB mean in pipe size?
NB means nominal bore. It is used in India and the UK and is equivalent to DN, for example 100 NB = DN 100 = 4" NPS.
Key takeaways
- NPS, DN and NB are names, not measurements.
- OD is fixed per size; schedule changes wall thickness.
- Some EN outside diameters differ from ASME — confirm with European buyers.
- Keep this chart handy and use our free tools for weights.
Bookmark this page, and when you need to turn an enquiry into a priced offer, QuoteMitra does the weights and totals for you.
