NB (Nominal Bore)
/glossary/nbNominal size label for selecting pipe, fittings and flanges; not the measured inside diameter.
Nominal Bore is the size label a Kanpur PWD or JJM BOQ uses to pick IS 1239 tube, fittings and flanges. It is not the measured inside diameter. A 50 NB line on the IS 1239 class chart is 60.3 mm OD; Medium wall is 3.6 mm, so the bore is 53.1 mm. RP Sales prices that line from the Bartan Bazar yard on an ex-warehouse Kanpur basis. Budget GST at 18% extra on the tube line; the tax invoice applies the rate in force.
In detail
Nominal Bore (NB) is the size label Indian BOQs use to pick steel tube, fittings and flanges. It is not a micrometer reading of the inside. NPS is the ASME B36.10M counterpart and DN is the ISO 6708 alphanumeric size. IS 1239 Part 1 lists 6 to 150 mm NB in the site's BIS preview record; the product manual's stocked series and this site's class chart run 15 to 150 mm NB. Once the drawing leaves that series it usually orders OD under IS 3589. Wall still comes from Light, Medium or Heavy class, or from a millimetre thickness.
What NB is, and what it is not
NB is a reference size. Indian water, gas and fire BOQs write 15 NB, 25 NB, 50 NB, 80 NB, 100 NB or 150 NB so that pipe, sockets, tees and flanges come from one series. The number is not a micrometer reading of the bore.
ASME B36.10M-2018 identifies pipe size by the dimensionless designator NPS, with DN shown in parallel. That catalogue page states that pipe NPS 12 (DN 300) and smaller have outside diameters numerically larger than the corresponding size. Tube sizes, by contrast, use an OD that matches the size number.
ISO 6708:1995 clause 2.1 defines DN as an alphanumeric designation comprising the letters DN followed by a dimensionless whole number. Note 2 on that page says the number following DN does not represent a measurable value and should not be used for calculation except where the relevant standard says so.
NB is not OD. It is not wall thickness. It is not Light, Medium or Heavy class. It is not a schedule number. It is not a pressure rating. A 50 NB Heavy fire riser and a 50 NB Light plumbing line share a label and share 60.3 mm OD; they do not share wall or duty.
Who should order by NB, and who should order by OD
Order by NB when the approved specification is IS 1239 Part 1, IS 1161 circular structural tube, or IS 4270 casing that follows the same NB series. Kanpur plumbing contractors, PWD and JJM bidders, fire-system installers, and sugar-mill utility crews write NB plus class on those lines.
Order by OD and wall when the item is a boiler tube, a honed barrel, or IS 3589 large-diameter water pipe. SA192 and SA210 heats are 50.8 mm by 3.6 mm, not 50 NB. A 168.3 mm IS 3589 trunk is an OD item, not a 150 NB leftover.
Do not use NB alone on a flange RFQ. The flange matches the pipe OD. A 2 inch ASME flange sits on 60.3 mm pipe. Writing 50 mm on that line produces a scrap pairing.
Do not pick class from the NB number. NBC 2016 Part 4, already cited on the IS 1239 page, requires Heavy class galvanised pipe for wet-riser and hydrant work. Light class on that service fails inspection.
NB to OD and class wall on the site chart
The IS 1239 class thickness chart and the pipe-weight table share one OD and one Light / Medium / Heavy wall for each NB. The table below is that chart. Mass still needs the formula in the worked example.
| NB (mm) | Inch label | OD (mm) | Light wall (mm) | Medium wall (mm) | Heavy wall (mm) |
|---|---|---|---|---|---|
| 15 | 1/2 inch | 21.3 | 2.0 | 2.6 | 3.2 |
| 25 | 1 inch | 33.7 | 2.6 | 3.2 | 4.0 |
| 40 | 1 1/2 inch | 48.3 | 2.9 | 3.2 | 4.0 |
| 50 | 2 inch | 60.3 | 2.9 | 3.6 | 4.5 |
| 80 | 3 inch | 88.9 | 3.2 | 4.0 | 4.8 |
| 100 | 4 inch | 114.3 | 3.6 | 4.5 | 5.4 |
| 150 | 6 inch | 165.1 | 4.5 | 4.8 | 5.4 |
150 NB on this chart is 165.1 mm OD. ASME B36.10M 6 inch NPS is 168.3 mm OD, and that figure is also the IS 3589 lower bound. Write the standard before you write 6 inch. A schedule wall from B36.10M is not an IS 1239 class wall.
How to write NB on an RFQ
A line that says only 50 NB MS pipe produces mixed Light and Medium offers. Put size, class, finish and documents on one line. Send that pack on the RFQ.
- Size: NB in mm (15 to 150) and the inch label if the BOQ uses it.
- Wall: Light / Medium / Heavy, or Class A / B / C, plus wall in mm from the class chart.
- Standard: IS 1239 Part 1:2004 for the tube. Part 2 is fittings, not a larger tube.
- Grade or material basis: YST 210, YST 240 or YST 310 when the drawing takes those IS 1161 yields; otherwise copy the IS 1239 wording.
- Length: 6 m standard; 4 m or 5 m when the site cannot handle 6 m. Cut-to-length with a tolerance.
- End finish: plain end or threaded and coupled.
- Coating: black, or hot-dip zinc to IS 4736 (amended minimum 360 g/m2).
- Documents: EN 10204 3.1 MTC, heat number, BIS licence number. TPI by SGS, Bureau Veritas, Lloyds or TUV when the tender names the agency.
- Count in metres or MT, the receiving PIN, and pickup at Bartan Bazar versus a FOR destination.
Worked mass example for 100 NB Medium
Take 100 NB Medium from the pipe-weight table: OD 114.3 mm, wall 4.5 mm, 12.18 kg/m, 73.1 kg per 6 m length. Use the site formula once to check that row: W = (OD − t) × t × 0.02466 kg/m.
W = (114.3 − 4.5) × 4.5 × 0.02466 = 109.8 × 4.5 × 0.02466 = 12.18 kg/m. Eighty lengths of 6 m are 480 m, so 480 × 12.18 = 5,846.4 kg, or 5.846 MT of plain-end steel.
The MS pipe price list band for 100 NB Medium is ₹51-57 per kg ex-warehouse Kanpur as of 24 May 2026. That steel line is 5,846.4 × 51 = ₹2,98,166 to 5,846.4 × 57 = ₹3,33,244 before GST and freight.
GI on the same NB and class sits on the GI pipe price list at ₹62-70 per kg as of 24 May 2026. Zinc typically adds roughly 3-6% mass over the plain-end MS weight, per the pipe-weight giNote. Do not mix the two bands on one comparison.
Common mistakes when a BOQ writes only NB
On Kanpur tenders, a contractor writes 2 inch and receives Sch 40 A106 against an IS 1239 Medium line. The OD matches at 60.3 mm. The wall does not: Sch 40 is 3.91 mm and Medium is 3.6 mm.
Class is omitted, so Light arrives on a fire riser. NBC 2016 Part 4 wants Heavy class GI on wet-riser and hydrant service. Colour bands (yellow Light, blue Medium, red Heavy) are trade practice on the class-chart page; read the stencil and the MTC.
150 NB is written on an RFQ line meant for a large-diameter IS 3589 trunk. The two items carry different class walls and different flange bores even where a buyer expects one continuous size ladder. Name the standard on the RFQ line, not just the number.
NB is used as a flow-area input. ISO 6708 Note 2 says the DN number is not a measurable value. Compute ID as OD minus twice the wall, then use that ID.
Fittings are ordered as IS 1239 without Part 2. Part 2:2011 (reaffirmed 2021) is the fittings document. The tube document is Part 1:2004 (reaffirmed 2019).
Standards that govern an NB call-out
IS 1239 : Part 1 : 2004, reaffirmed 2019, is the Active BIS listing for steel tubes, committee MTD 19. The BIS product manual PM/1239 (Part 1)/4 February 2022 states each steel tube is eddy-current or hydrostatically tested to 5 MPa. The site class table records that test as 50 kg/cm2.
IS 1239 : Part 2 : 2011 (reaffirmed 2021) covers steel pipe fittings. IS 3589:2001 covers water and sewage pipe from 168.3 mm to 2540 mm OD. IS 4270 casing follows the IS 1239 / IS 1161 dimensional series for 100, 125, 150, 200, 250 and 300 mm NB.
ASME B36.10M-2018 is the welded and seamless wrought-steel pipe dimensional standard behind the pipe schedule chart. NPS 2 equals DN 50 equals 60.3 mm OD. Schedule walls on that chart are not IS 1239 class walls.
ISO 6708:1995 is the DN definition. Preferred values on that page include DN 15, 25, 40, 50, 80, 100, 150 and 200, which is why a Kanpur BOQ can write 50 NB and a metric isometric can write DN 50 for the same 60.3 mm series.
Buying IS 1239 pipe by NB in Kanpur
Panki estate sits about 12 km from the Bartan Bazar stockyard. Plumbing and fire contractors there collect 25 NB to 80 NB bundles the same week. Dada Nagar, about 8 km west, rarely wants NB class pipe; those shops buy precision OD for barrels.
JJM and PWD tenders in Kanpur Dehat and Unnao still write NB and class on house connections and distribution. See steel pipes in Kanpur, GI pipes in Kanpur, MS ERW in Kanpur, and the Kanpur steel market.
Stocked IS 1239 on the glossary lead-time entry leaves in 3-7 days when the NB and class sit in the yard. A mill roll for a missing class follows that entry’s 14-21 day standard-grade window. RP Sales answers an NB-class RFQ inside 4 working hours.
Loads for Lucknow, Agra, Prayagraj, Varanasi, Gorakhpur, Jhansi, Bareilly and Patna should name the city page and the PIN. Steel pipes in Lucknow and steel pipes in Unnao describe those runs. The FAQ already lists daily coverage across Uttar Pradesh, Bihar, Madhya Pradesh, Delhi NCR and Uttarakhand.
NB versus NPS versus DN
NB is the Indian trade label. NPS is the ASME B36.10M label. DN is the ISO 6708 label. For the common water sizes they point at the same OD series: 50 NB, NPS 2 and DN 50 all sit on 60.3 mm OD.
The labels diverge at 150 / 6 inch. The class-chart 150 NB row is 165.1 mm OD. NPS 6 / DN 150 is 168.3 mm on the schedule chart. Write the standard on that boundary.
A schedule number travels with NPS, not with NB. Sch 40 at 2 inch is 3.91 mm. Medium class at 50 NB is 3.6 mm. See Sch 40 versus Sch 80 when the isometric uses schedule language.
Do not invent a 60 NB class row. If a drawing writes a DN value that is not on the class chart, ask the designer which OD series it belongs to.
Class, grade and wall on one BOQ line
Class is wall. Light is thinner than Medium, and Medium is thinner than Heavy, at the same NB. The IS 1239 page is explicit: a class name is not a working-pressure rating.
YST 210, YST 240 and YST 310 are yield grades taken from IS 1161. YST 210 is 210 MPa minimum yield. Writing Heavy grade on an RFQ mixes the two fields and produces a junk offer.
Finish is a third field. Black and GI share an NB and a class. GI follows IS 4736, whose amended minimum zinc mass is 360 g/m2. See galvanised and MS versus GI.
Ends are a fourth field. Threaded and coupled house connections on a JJM sheet are not plain-end weld pipe. State PE or T&C beside the NB.
Documents that travel with an NB bundle
The MTC must show NB, class or wall, heat number, and the hydrotest or eddy-current record. The BIS licence number belongs on the same sheet when the tender wants the ISI mark.
The dispatch challan lists piece count, lengths and heat stamps so the Panki or Lucknow store can tick the truck. It is not the tax invoice; the invoice is a separate commercial document from a separate desk.
Form III-C is not an NB-class certificate. IBR Form III-A is for steam pipes, Form III-B for tubes, and Form III-C for boiler mountings and fittings. An IS 1239 water line does not become boiler material by attaching a form.
TPI hold points, when named, sit on the RFQ before the truck is booked. SGS, Bureau Veritas, Lloyds and TUV are the agencies already listed on the site standards pages.
Where you'll meet this in procurement
You will meet NB (Nominal Bore) on size charts, mill quotes, and fitment checks. Cross-check NB (Nominal Bore) against OD, wall thickness or schedule, length, and end finish so the delivered pipe matches flanges, fittings, and pressure duty—not just a familiar nominal label.
Related terms
Related resources
- Pipe schedule chart
- Steel pipe weight calculator
- IS 1239 class thickness chart
- Pipe weight chart
- IS 1239 Part 1
- MS pipe price list
- GI pipe price list
- Steel pipes in Kanpur
- GI pipes in Kanpur
- MS ERW pipe supplier in Kanpur
- JJM contractor
- PWD tender
- Kanpur steel market
- Request a quotation (RFQ)
- Back to full glossary
Key points
- NB is a size label for pipe, fittings and flanges, not the measured inside diameter.
- 50 NB on the site class chart is 60.3 mm OD; Medium wall is 3.6 mm, so the bore is 53.1 mm.
- IS 1239 Part 1 lists 6 to 150 mm NB in the site's BIS preview record; the product manual's stocked series and this site's class chart run 15 to 150 mm NB. Part 2 is fittings. IS 3589 starts at 168.3 mm OD.
- 100 NB Medium is 12.18 kg/m on the pipe-weight table; 80 lengths of 6 m are 5.846 MT.
- Write NB, class, finish and documents on one RFQ line. Class is wall, not grade.
Frequently asked questions
Sources
- IS 1239 : Part 1 : 2004 (reaffirmed 2019). Steel Tubes, Tubulars and Other Wrought Steel Fittings : Part 1 Steel Tubes: Active listing, committee MTD 19 (Bureau of Indian Standards e-sale (BSB Edge))
- PM/1239 (Part 1)/4 Feb 2022. Product Manual for Steel Tubes, Tubulars and Other Wrought Fittings Part 1: Steel Tubes according to IS 1239 (Part 1):2004. NB 15-150 mm, class Light/Medium/Heavy, hydro/eddy-current 5 MPa (Bureau of Indian Standards)
- IS 1239 : Part 2 : 2011 (reaffirmed 2021). Steel Tubes, Tubulars and Other Steel Fittings Part 2 Steel Pipe Fittings (Bureau of Indian Standards e-sale (BSB Edge))
- PM/IS 3589:2001. Product Manual for Steel Pipes for Water and Sewage according to IS 3589:2001 (168.3 to 2540 mm Outside Diameter) (Bureau of Indian Standards)
- ASME B36.10M-2018. Welded and Seamless Wrought Steel Pipe: NPS 12 (DN 300) and smaller have OD numerically larger than the size number (American Society of Mechanical Engineers)
- ISO 6708:1995. Pipework components: Definition and selection of DN (nominal size), clause 2.1 and Note 2 (International Organization for Standardization)
- Indian Boiler Regulations, 1950. Forms index: III-A steam pipes, III-B tubes, III-C boiler mountings and fittings (Central Boilers Board, Government of India)