GI Pipe Size Chart — BS 1387 / EN 10255 Medium Class
Updated 4 min readBy OR Steels
Short answer
GI pipe is ordered by nominal bore in inches, which is not its real outside diameter: a ½-inch pipe is about 21.3 mm OD and a 2-inch pipe about 60.3 mm OD. In BS 1387 / EN 10255 medium class the wall thickness runs from 2.6 mm (½ and ¾ inch) to 5.0 mm (5 and 6 inch).
GI pipe size chart (medium class)
| Nominal size | DN | Outside diameter (mm) | Wall thickness (mm) |
|---|---|---|---|
| ½ inch | 15 | 21.3 | 2.6 |
| ¾ inch | 20 | 26.9 | 2.6 |
| 1 inch | 25 | 33.7 | 3.2 |
| 1¼ inch | 32 | 42.4 | 3.2 |
| 1½ inch | 40 | 48.3 | 3.2 |
| 2 inch | 50 | 60.3 | 3.6 |
| 2½ inch | 65 | 76.1 | 3.6 |
| 3 inch | 80 | 88.9 | 4.0 |
| 4 inch | 100 | 114.3 | 4.5 |
| 5 inch | 125 | 139.7 | 5.0 |
| 6 inch | 150 | 165.1 | 5.0 |
Light, medium and heavy class
BS 1387 tube comes in three wall classes with the same outside diameter: light, medium and heavy. Heavier classes have thicker walls, weigh more per metre and handle higher pressure and threading better. Medium is the common general-purpose class; use the class your designer specifies.
Black, galvanized, ERW and seamless
- Black pipe is uncoated carbon steel, often painted or used where it will be protected.
- GI (galvanized iron) pipe is zinc-coated for corrosion resistance — the usual choice for water lines and outdoor work.
- ERW pipe is made from strip welded along its length; seamless pipe has no weld and is used for higher-pressure process work (for example to ASTM A53 or API 5L).
- Square and rectangular hollow sections (SHS/RHS) are ordered by outside dimensions and wall thickness, for example 50 × 50 × 3 mm.
How to order pipe
- Nominal size (inches or DN) and class or schedule.
- Black or galvanized, ERW or seamless, and the standard (BS 1387 / EN 10255, ASTM A53, API 5L).
- Ends: plain, bevelled or threaded and socketed.
- Quantity in pieces (standard length is 6 m) and the delivery location.
Frequently asked questions
Is a 1-inch pipe really 1 inch in diameter?
No. “1 inch” is the nominal bore. A 1-inch medium-class pipe has an outside diameter of about 33.7 mm and a wall of 3.2 mm.
What does DN mean?
DN (diamètre nominal) is the metric nominal size. DN 25 corresponds to 1-inch nominal pipe, DN 50 to 2-inch and so on.
