Pipe Sizes, NB, DN and NPS - Are they all the same?

Pipe Sizes, NB, DN and NPS - Are they all the same?

Pipe Sizes, NB, DN and NPS - Are they all the same?

First some common definitions of the two words in the title:

Nominal: as expected or planned; normal

Bore: diameter of a tube / pipe

So, putting those together we would expect “Nominal Bore” to mean the expected diameter of a tube - but within the realms of pipe support systems, it is often a cause for errors.

U-bolts, pipe clips, pipe clamps, hangers and of course our modular pipe support system - the 1205 series, all have one thing in common - you need to know the size of fitting to fit your pipe. And this is where the “Nominal Bore” phrase comes into play, and where it sometimes goes awry.

For pipes, the bore is the hollow centre, and Nominal is used in the sense of “in name only” or approximate. Nominal bore is actually the approximate internal measurement across the diameter of the mouth of a pipe - NOT necessarily the expected diameter.

Nominal Bore Image

As we know, pipes are for carrying fluids and gasses and therefore the internal diameter is critical for allowing us to calculate capacity, flow rates etc. However, regulations govern the size of a pipe’s outside dimensions to ensure it is manufactured in standard sizes for easy fitting. But because pipes vary in material, weight, and the way they are formed, internal dimensions of pipes that share the same outer dimension may vary. For this reason, any measurement of that internal dimension can only be “nominal.”

It is worth remembering also that nominal bore can have different names. In the inch system, nominal bore is abbreviated as NB; in metric systems, it is “diameter nominal” (DN), or Nominal Pipe Size (NPS) in North America.

So, how does all this help us when purchasing a pipe support system? Quite often people assume that the nominal bore relates to the inside diameter of a pipe support, or perhaps the outside diameter of the pipe being supported, but it is neither - it is the approximate inside diameter of the pipe.

For example, a support that is quoted as 3” nb means it is for a PIPE with a 3″ nominal bore, and that PIPE has a fixed outside diameter of 3.5”. The wall thickness (Schedule) can vary hence the inside diameter is 3” nominal (around 3” up or down dependent upon the wall thickness)

There is an added complication. because of how pipes have traditionally been made, the fact that the Nominal Bore size is less than the pipe outside diameter only holds up to a 12" pipe (300 NB). Above that size, the NB is the SAME as the outside diameter!

So, in summary, watch out for nominal bore, it is NOT the expected bore of the pipe, but the approximate inside diameter based on a standard outside diameter. You should be fully aware of the outside diameter of the pipe to ensure you get the correct supports


Nominal Bore (metric / NB / DN) Nominal Bore (Imperial) Pipe Outside Diameter (metric)
10 mm 3/8 " 17.2 mm
15 mm 1/2 " 21.3 mm
20 mm 3/4 " 26.9 mm
25 mm 1 " 33.7 mm
32 mm 1.25 " 42.4 mm
40 mm 1.5 " 48.3 mm
50 mm 2 " 60.3 mm
65 mm 2.5 " 76.1 mm
80 mm 3 " 88.9 mm
100 4 " 114.3 mm
125 5 " 139.7 mm
150 6" 168.3 mm
175 7" 193.7 mm
200 8" 219.1 mm
225 9" 244.5 mm
250 10" 273 mm
300 12" 323.9 mm
350 14" 355.6 mm
400 16" 406.4 mm
450 18" 457.0 mm
500 20"  508.0mm
550 22" 559.0 mm
600 24" 610.0 mm
650 26" 660.0 mm
700 28" 711.0 mm
750 30" 762.0 mm
800 32" 813.0 mm
Posted on 04/11/20 Pipe Support Systems, General Engineering 0 16017

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