Is NB the Same as OD? Nominal Diameter vs Outside Diameter

This is the most common confusion in pipe sizing. Here is the exact difference between Nominal Diameter (NB) and Outside Diameter (OD), along with how to use them correctly in engineering.

They are completely different. NB is a standardized label used for identification and fitting compatibility—it is not an actual measurement. OD is the real physical measurement of the pipe’s outer wall.

For example, a 4-inch NB pipe has an actual OD of 114.3 mm (4.5 inches), not 4 inches. This mismatch is exactly why so many purchase orders go wrong.

Why People Confuse NB with OD (and What to Remember)

Look at the diagram above. Notice the question mark next to the inner arrow: "Nominal Bore?" This is exactly where most pipe sizing errors begin. Many people assume the inner arrow is the NB, or they assume a 4-inch NB pipe must have a 4-inch OD.

They are wrong on both counts.That inner arrow is the Actual Inner Diameter (ID). And the OD is never the same as the nominal label (except for sizes 14" and above).

Here are the three biggest myths that cause sizing mistakes:

❌ Myth 1: Nominal Bore (NB) is the same as Inner Diameter (ID).

Reality: For thin-walled pipes, NB might look close to the ID. But for thick-walled pipes (like SCH 160), the actual inner diameter is much smaller than the nominal label.

❌ Myth 2: A 4-inch pipe has an outside diameter (OD) of 4 inches.

Reality: Look at the Outside Diameter line in the diagram. For an NB 4" (NPS 4") pipe, the actual OD is fixed at 114.3 mm (4.5 inches). For sizes NPS 12 and below, the actual outer diameter is always larger than the nominal number.

❌ Myth 3: NB is the average of OD and ID.
Reality: NB is simply a standardized label used for interchangeability (so the same flanges and fittings will fit). It is not a calculated average.

(Note: The diagram also points to the pipe wall and labels it Schedule. Technically, that physical thickness is the Wall Thickness (WT). "Schedule" is just the code (like SCH 40)—that dictates how thick the wall should be.)

NB to OD: Standard Reference Table

The table below shows the relationship between NB and OD for common pipe sizes (based on ASME B36.10 standard). The OD is fixed for each NB/NPS.

NB (Nominal Bore, inch)	DN (Nominal Diameter)	Outside Diameter (OD) mm
1/2"	15	21.3
3/4"	20	26.7
1"	25	33.4
1 1/4"	32	42.2
1 1/2"	40	48.3
2"	50	60.3
2 1/2"	65	73.0
3"	80	88.9
4"	100	114.3
5"	125	141.3
6"	150	168.3
8"	200	219.1
10"	250	273.0
12"	300	323.8

NB vs OD vs ID vs WT: Key Differences

The table below compares the four key terms you'll encounter in pipe sizing: Nominal Diameter (NB/DN) as the standard label, Outside Diameter (OD) as the fixed outer measurement, and Inner Diameter (ID) and Wall Thickness (WT) which vary with the pipe schedule.

What is Outside Diameter (OD)?

Outside Diameter (OD) is the actual physical measurement of the pipe's outer wall, expressed in millimeters (mm) or inches.

● It is a fixed, precise value for a given pipe size and schedule. For example, an NPS 4" pipe has an OD of 114.3 mm regardless of whether it is Schedule 40 or Schedule 80.
● Key point: The OD is constant for a given nominal pipe size (NPS). Only the wall thickness and inner diameter (ID) change with different schedules.
● Usage: OD is used for precise engineering calculations, such as pipe strength, flow area, and compatibility with flanges and fittings.

What is Nominal Diameter (NB)?

Nominal Diameter (NB - Nominal Bore) is a dimensionless, standardized number used for identification purposes. It is not an actual physical measurement.

● NB does not equal OD or ID. It is a convenient "label" that specifies the pipe size for the purpose of fitting, valve, and flange compatibility.
● Based on imperial units: NB values are expressed in inches (e.g., NB 2", NB 4"). This is the standard designation in many industries, especially in North America.
● Metric Equivalent: NB has a direct metric equivalent called DN (Diameter Nominal) , where DN ≈ NB × 25 (e.g., NB 2" ≈ DN 50).

Common Misconception: NB is not the "average outer diameter" or the "inner diameter." It is a nominal (in-name-only) value.

What is Nominal Pipe Size (NPS), Inner Diameter (ID) and Wall Thickness (WT)?

To fully understand pipe sizing, it helps to see all key terms together:

Nominal Pipe Size (NPS): A North American standard set of sizes (in inches) used to identify pipe. For NPS 12" and below, NPS does not equal the actual OD. For NPS 14" and above, NPS equals the actual OD.
Inner Diameter (ID): The actual physical measurement of the pipe's inner wall. It determines the flow area and is calculated as: ID = OD - 2 × WT.
Wall Thickness (WT): The thickness of the pipe wall. It is determined by the schedule (SCH). A higher schedule means a thicker wall, which reduces the ID.

The relationship: OD = ID + 2 × WT. For a given NPS, the OD is fixed, but WT and ID vary with the schedule.

Why Do We Use NB Instead of Just OD?

The reason is interchangeability. The same NB can accommodate pipes with different wall thicknesses (schedules) for different pressure requirements.

● Example: An NB 4" (DN100) pipe can be specified as:
SCH 40: OD 114.3 mm, Wall 6.02 mm, ID ~102.3 mm
SCH 80: OD 114.3 mm, Wall 8.56 mm, ID ~97.2 mm

● The OD and NB remain the same for both, allowing the same flanges, valves, and fittings to be used, even though the wall thickness and ID differ.

FAQs

Q1: Is nominal diameter the same as outside diameter?
A: No. Nominal diameter (NB) is a standardized label for size identification. Outside diameter (OD) is the actual physical measurement of the pipe's outer wall. They are different concepts.

Q2: Is nominal diameter the same as inner diameter?
A: Not exactly. For thin-walled pipes, NB may be close to the ID, but for thick-walled pipes, the ID is significantly smaller than the NB. NB is not a direct measurement of the ID.

Q3: Why is NB used if it's not an actual measurement?
A: NB ensures interchangeability. The same NB (e.g., NB 4") can be used for pipes with different wall thicknesses (schedules) while still fitting the same flanges, valves, and fittings.

Q4: What is the OD of an NB 4" pipe?
A: The OD of an NB 4" (NPS 4") pipe is 114.3 mm, based on ASME B36.10 standards. This OD is fixed regardless of the wall thickness schedule.

Q5: Where is NB used vs. OD?
A: NB is used in design drawings, specifications, and procurement to identify pipe size for fitting compatibility. OD is used for precise engineering calculations (strength, flow, pressure drop) and is often specified alongside wall thickness (e.g., OD 114.3mm × WT 6.02mm).

Related Resources:
● Schedule 40 vs. Schedule 80 Steel Pipe: A Selection Guide
● ASME B36.10 / B36.19 Steel Pipe Dimensions & Sizes Chart

● Difference between the Heat No. and Lot No.

● Schedule 80 Pipe Thickness in mm

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