What Does NB Mean in Pipe?
NB stands for Nominal Bore. It is a standardized pipe size designation used to identify compatible pipes, fittings, flanges and valves. NB is a nominal name rather than an exact measured dimension, so it should not be treated as the pipe’s actual inside diameter or outside diameter.
The term “bore” comes from earlier pipe-sizing systems in which the nominal size was approximately related to the internal opening of a standard-wall pipe. As additional wall thicknesses and pipe schedules were introduced, pipes with the same NB size could have different inside diameters.
For example, both 2-inch NB SCH 40 and 2-inch NB SCH 80 pipes have the same outside diameter of 2.375 inches (60.3 mm). However, the SCH 80 pipe has a thicker wall and therefore a smaller inside diameter.
In steel pipe specifications, NB is often used interchangeably with NPS (Nominal Pipe Size). The corresponding metric designation is generally expressed as DN (Nominal Diameter). For example:
- 2-inch NB = NPS 2 = DN 50
- 4-inch NB = NPS 4 = DN 100
- 6-inch NB = NPS 6 = DN 150
These designations identify corresponding nominal pipe sizes; they are not direct measurements or simple unit conversions. Therefore, a buyer should specify the NB, NPS or DN size together with the pipe schedule or wall thickness when requesting a quotation.
Key point: NB identifies a nominal pipe size. It does not tell you the exact OD, ID or wall thickness by itself.
Is NB the Same as NPS or DN
| Term | Full Form | Typical System | Example |
|---|---|---|---|
| NB | Nominal Bore | Common British/international usage | 2-inch NB |
| NPS | Nominal Pipe Size | North American/inch designation | NPS 2 |
| DN | Diamètre Nominal / Nominal Diameter | Metric designation | DN 50 |
| OD | Outside Diameter | Actual measured dimension | 60.3 mm |
| ID | Inside Diameter | Calculated/measured opening | Changes with schedule |
NB Pipe Size Chart
| NB | NPS | DN | Actual OD (inch) | Actual OD (mm) |
|---|---|---|---|---|
| 1/2 inch | 1/2 | DN 15 | 0.840 | 21.3 |
| 3/4 inch | 3/4 | DN 20 | 1.050 | 26.7 |
| 1 inch | 1 | DN 25 | 1.315 | 33.4 |
| 1 1/4 inch | 1 1/4 | DN 32 | 1.660 | 42.2 |
| 1 1/2 inch | 1 1/2 | DN 40 | 1.900 | 48.3 |
| 2 inch | 2 | DN 50 | 2.375 | 60.3 |
| 2 1/2 inch | 2 1/2 | DN 65 | 2.875 | 73.0 |
| 3 inch | 3 | DN 80 | 3.500 | 88.9 |
| 4 inch | 4 | DN 100 | 4.500 | 114.3 |
| 5 inch | 5 | DN 125 | 5.563 | 141.3 |
| 6 inch | 6 | DN 150 | 6.625 | 168.3 |
| 8 inch | 8 | DN 200 | 8.625 | 219.1 |
| 10 inch | 10 | DN 250 | 10.750 | 273.0 |
| 12 inch | 12 | DN 300 | 12.750 | 323.9 |
OD values should be verified against the applicable dimensional standard and project specification before ordering.
See our complete Nominal Pipe Size chart for additional sizes and schedules.
Why Does Pipe Schedule Change ID but Not OD?
For a given NB or NPS size, the pipe’s outside diameter remains fixed. The pipe schedule, such as SCH 40 or SCH 80, determines the wall thickness rather than the nominal size.
When the schedule increases, the pipe wall becomes thicker and the inside diameter becomes smaller. The outside diameter does not change. This allows pipes, fittings and flanges of the same nominal size to remain dimensionally compatible.
The inside diameter can be estimated using:
ID = OD − 2 × Wall Thickness
For example, both NPS 2 SCH 40 and NPS 2 SCH 80 steel pipes have an outside diameter of 2.375 inches (60.3 mm). However, their wall thicknesses and inside diameters differ:
| Pipe Size | Outside Diameter | Wall Thickness | Approximate Inside Diameter |
|---|---|---|---|
| NPS 2 SCH 40 | 2.375 in (60.3 mm) | 0.154 in (3.91 mm) | 2.067 in (52.5 mm) |
| NPS 2 SCH 80 | 2.375 in (60.3 mm) | 0.218 in (5.54 mm) | 1.939 in (49.2 mm) |
This is why a 2-inch NB pipe should not be interpreted as having an exact 2-inch inside or outside diameter. Always specify both the nominal size and the required schedule or wall thickness.
Buyer’s note: A higher schedule does not mean a larger pipe diameter. It means a thicker pipe wall and a smaller internal opening for the same NB or NPS.
How to Specify NB Pipe in an RFQ
Writing only “2-inch NB pipe” is not enough for a supplier to prepare an accurate quotation. Pipes with the same nominal size may have different standards, grades, wall thicknesses, manufacturing methods and finishes.
A quote-ready steel pipe inquiry should include:
- Size: NB, NPS or DN designation
- Standard: ASTM, API, EN, JIS or another applicable specification
- Material or grade: such as ASTM A106 Grade B
- Wall thickness: schedule number or wall thickness in millimetres
- Manufacturing method: seamless, ERW, LSAW or SSAW
- Pipe ends: plain, bevelled, threaded or grooved
- Surface finish: black, galvanized, painted or externally coated
- Length: fixed, random or double-random length
- Quantity: pieces, metres or total weight
- Application: service medium and relevant design requirements
Instead of writing:
Need 2-inch NB steel pipe.
Use a specification such as:
NPS 2 (DN 50) seamless carbon steel pipe, ASTM A106 Grade B, SCH 40, plain ends, black finish, 6 m fixed lengths, 500 pieces, for a water pipeline.
If the pipe is intended for pressure service, also provide the design pressure, design temperature and any testing, inspection or certification requirements. This helps the supplier confirm whether the requested standard, grade and wall thickness are suitable.
FAQ
1. Is NB the same as pipe inside diameter?
No. NB is a nominal designation. The actual inside diameter depends on the pipe’s outside diameter and wall thickness.
2. Is NB the same as NPS?
They are commonly used interchangeably in steel pipe purchasing, although NPS is the formal North American nominal size designation.
3. Is NB the same as DN?
They identify corresponding nominal pipe sizes in different designation systems. For example, 2-inch NB or NPS 2 generally corresponds to DN 50.
4. What is the outside diameter of a 2-inch NB pipe?
A standard NPS 2/DN 50 steel pipe has an outside diameter of 2.375 inches or 60.3 mm.
5. Does pipe schedule change the NB size?
No. Schedule changes the wall thickness and inside diameter, while the nominal size and outside diameter remain unchanged.
6. Can NB be used for steel tubes?
Normally, tubes are ordered by actual outside diameter and wall thickness rather than NB or NPS. Do not assume a 2-inch tube has the same dimensions as an NPS 2 pipe.
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