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Reference chart

Pipe NB to OD Chart (IS 1239)

Nominal bore to outside diameter for IS 1239 steel tubes, and how NB, OD and class relate when ordering MS and GI pipes.

Pipes are ordered by nominal bore (NB), which is a size label, not a measured dimension. The actual outside diameter (OD) is fixed for each NB; the class — light, medium or heavy — changes wall thickness.

NB (mm)	NB (inch)	OD (mm)
15	½″	21.3
20	¾″	26.9
25	1″	33.7
32	1¼″	42.4
40	1½″	48.3
50	2″	60.3
65	2½″	76.1
80	3″	88.9
100	4″	114.3
125	5″	139.7
150	6″	165.1

Ordering

State NB and class: for example 50 NB Medium, GI, screwed & socketed.
For structural tubes on drawings, use OD × wall as written.
Wall thickness by class should be taken from the current IS 1239 edition or the producer's table.

Related: ERW pipes · Round pipes & tubes · IS 1239

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