Fabricators and plumbers buying steel pipe hit the same confusion daily: a “1 inch pipe” doesn't measure one inch anywhere. The MS pipe size chart uses nominal bore (NB), outside diameter (OD) and wall thickness, and the class decides how thick the wall is. This guide untangles the terms and gives a chart buyers can carry to the dealer.
NB, OD and ID
- NB (nominal bore): a name for the size, like 25 NB or “1 inch”. It is roughly the bore of older pipes, not an exact measurement.
- OD (outside diameter): fixed for each NB size, whatever the class. That is what lets fittings and threads match.
- ID (inside diameter): OD minus twice the wall thickness, so it changes with the class.
ID = OD − 2 × wall thickness
Why is the OD fixed but the ID variable?
Threads and couplings are made to the outside diameter. Thicker classes add steel inward, so a heavy-class pipe has a smaller bore than a light-class pipe of the same NB.
MS pipe size chart with inside diameters
| NB | Inches | OD (mm) | Wall, medium (mm) | ID, medium (mm) |
|---|---|---|---|---|
| 15 | ½" | 21.3 | 2.6 | 16.1 |
| 20 | ¾" | 26.9 | 2.6 | 21.7 |
| 25 | 1" | 33.7 | 3.2 | 27.3 |
| 32 | 1¼" | 42.4 | 3.2 | 36.0 |
| 40 | 1½" | 48.3 | 3.2 | 41.9 |
| 50 | 2" | 60.3 | 3.6 | 53.1 |
| 80 | 3" | 88.9 | 4.0 | 80.9 |
| 100 | 4" | 114.3 | 4.5 | 105.3 |
Weights per metre for all three classes are on the MS pipe weight calculator page; they follow from the OD and wall with the formula (OD − t) × t × 0.02466.
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Light, medium and heavy classes
| Class | Colour band | Typical use |
|---|---|---|
| Light | Yellow | Railings, grills, furniture, light structures |
| Medium | Blue | Water lines, general plumbing, scaffolding |
| Heavy | Red | Higher-pressure lines, industrial and heavy structural use |
The colour band is painted near the pipe ends at the mill. It is a quick check that a “medium” pipe isn't a light one sold at medium rates.
Buying checks at the dealer
- Check the colour band and the ISI mark with the IS 1239 reference on the pipe.
- Measure the wall at the cut end with a vernier caliper.
- Weigh a sample length: a 6 m medium 25 NB pipe should weigh about 14.5 kg. A much lighter pipe means a thinner wall.
- Ask whether the price is per kg or per length, and get the weight on the bill.
GI (galvanised) pipe is MS pipe with a zinc coating; it weighs a few percent more than the same black pipe. For square and rectangular sections, the hollow-section calculators use the same steel density.
Frequently asked questions
What is the OD of a 1 inch MS pipe?
33.7 mm for a 25 NB pipe under IS 1239, whatever the class.
What is the difference between NB and OD?
NB is the nominal size name; OD is the actual outside diameter, fixed for each NB.
How do I find the inside diameter of a pipe?
ID = OD − 2 × wall thickness.
What do yellow, blue and red bands on MS pipes mean?
Under IS 1239 they mark light, medium and heavy class respectively.
How can I check if a pipe is the right class?
Check the colour band and marking, measure the wall thickness and weigh a sample length.
Keep reading
- MS Pipe Weight Calculator: the free tool this guide explains.
- All construction calculators
- M20 Concrete Ratio: Cement Bags, Sand and Aggregate per m³
Sources: IS 1239 (Part 1): mild steel tubes, dimensions and classes; steel density 7,850 kg/m³.