In India and many export markets, the most common steel pipe enquiries are for GI pipe and MS pipe. Plumbers, contractors, fabricators and importers order them every day — usually by class and size under IS 1239. Yet confusion between classes, coatings and weights causes many disputes. This guide explains the difference and how to quote both correctly.
MS pipe and GI pipe — the basics
- MS pipe (mild steel pipe) — also called black pipe. Plain carbon steel, usually ERW welded, supplied black (uncoated) or varnished.
- GI pipe (galvanised iron pipe) — the same mild steel pipe, hot-dip galvanised with a zinc coating for corrosion protection. Despite the name "iron", it is steel.
So the steel is the same; the difference is the zinc layer — and that changes where the pipe can be used.
IS 1239 Part 1 — the main standard
IS 1239 (Part 1) covers welded and seamless mild steel tubes for water, gas, air and steam, from 15 NB to 150 NB. It defines three classes by wall thickness:
| Class | Also called | Colour band | Typical use |
|---|---|---|---|
| Light | A class | Yellow | Low-pressure, structural, light fabrication |
| Medium | B class | Blue | Water supply, plumbing, general use |
| Heavy | C class | Red | Higher pressure, gas, fire lines, threading |
Galvanising for GI pipe generally follows IS 4736.
Typical wall thickness (mm)
| Size (NB) | OD approx. (mm) | Light | Medium | Heavy |
|---|---|---|---|---|
| 15 | 21.3 | 2.0 | 2.6 | 3.2 |
| 25 | 33.7 | 2.6 | 3.2 | 4.0 |
| 50 | 60.3 | 2.9 | 3.6 | 4.5 |
Check the full table — OD, thickness and kg/m for every size and class — in our MS pipe chart (IS 1239).
Weight and price
MS pipes are sold by weight, often per metre or per tonne. A 50 NB medium class pipe weighs around 5 kg/m black (plain end). GI pipe weighs slightly more because of the zinc coating, and screwed-and-socketed pipes weigh a little more than plain-end pipes.
Learn the formula in how to calculate steel pipe weight, and get instant answers from the pipe weight calculator.
GI pipe vs MS pipe — where each is used
GI pipe
- Drinking water and plumbing lines (where local codes allow)
- Outdoor water lines and garden supply
- Fencing, railings and outdoor structures
- Agricultural and irrigation use
MS pipe
- Structural and fabrication work, which is later painted
- Fire-fighting lines (often painted red)
- Gas, air and oil lines
- Scaffolding and general engineering
- Lines that will be welded (galvanising burns off at weld zones)
Important differences
- Corrosion resistance — GI resists rust far better outdoors and in water. MS needs painting or coating.
- Welding — MS welds easily. Welding GI releases zinc fumes and damages the coating, so GI lines are usually threaded.
- Cost — GI costs more because of galvanising.
- Appearance — GI has a bright or dull silver finish; MS is black or varnished.
How to quote correctly
A complete description includes:
GI pipe, 50 NB Medium (B) class, IS 1239 Part 1, galvanised to IS 4736, screwed and socketed, 6 m length
- Size in NB (and OD if exporting — overseas buyers often think in inches or DN)
- Class: light, medium or heavy
- Black, varnished or galvanised
- Plain end, bevelled end or screwed and socketed
- Length and quantity in metres, pieces or tonnes
- BIS marking requirements for domestic supply
For export buyers
Overseas buyers may ask for "BS 1387" or "EN 10255" pipes, which are similar classes to IS 1239. Match the buyer's standard exactly and explain equivalents clearly. See ASTM vs EN vs IS standards.
Quality checks for MS and GI pipes
Before dispatch, check:
- Outside diameter and wall thickness against IS 1239 tolerances
- Straightness and squareness of ends
- Thread quality and socket fit for screwed pipes
- Zinc coating thickness and adhesion for GI pipes
- Colour band, BIS marking and size marking
- Hydrostatic test certificate where required
Frequently asked questions
Is GI pipe the same as galvanised steel pipe?
Yes. "GI" is the traditional name, but the base material is mild steel coated with zinc.
Which class is best for home plumbing?
Medium (B) class is the most common choice for water supply and plumbing. Follow local codes.
Why are colour bands painted on pipes?
The bands help identify the class quickly: yellow for light, blue for medium and red for heavy.
Can I weld GI pipe?
It is possible with precautions, but welding burns off zinc and produces harmful fumes. Threaded joints are preferred for GI.
Key takeaways
- GI is MS pipe with a hot-dip zinc coating.
- IS 1239 classes: light (yellow), medium (blue), heavy (red).
- Quote size, class, finish, ends and standard every time.
Build accurate MS and GI pipe quotations with weights and GST in QuoteMitra.
