Outside Diameter (OD) vs Nominal Diameter (NB): Key Differences

What is the difference between Outside Diameter (OD) and Nominal Diameter (NB)? 

This guide explains the key distinctions between these two essential pipe size terms. You'll learn their definitions, why they are not the same, and how to use them correctly in engineering and procurement. We also cover related concepts including NPS (Nominal Pipe Size), ID (Inner Diameter), and WT (Wall Thickness) to give you a complete picture of pipe sizing.

Key Differences: NB vs OD vs ID vs WT

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.

Feature	Nominal Diameter (NB/DN)	Outside Diameter (OD)	Inner Diameter (ID)	Wall Thickness (WT)
Definition	Standardized size label	Actual outer measurement	Actual inner measurement	Thickness of pipe wall
Is it a measurement?	No (e.g., NB 4")	Yes (e.g., 114.3 mm)	Yes (e.g., 102.3 mm)	Yes (e.g., 6.02 mm)
Fixed or Variable?	Fixed for a given size	Fixed for a given NB/NPS	Varies with schedule	Varies with schedule
Formula	N/A	OD = ID + 2 × WT	ID = OD - 2 × WT	WT = (OD - ID) / 2

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.

Common Misconceptions Clarified:

1. NB is NOT the average of OD and ID. This is a common myth, especially for thin-walled pipes. NB is a standardized number, not a calculated average.
2. NB is NOT always equal to the inner diameter (ID). For thin-walled pipes, NB may be close to the ID, but for thick-walled pipes (e.g., SCH 160), the ID is significantly smaller than the NB.
3. A pipe with NB 4" does not have an OD of 4 inches. The OD for NB 4" (NPS 4") is 114.3 mm (4.5 inches), not 4 inches.

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.

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

Frequently Asked Questions:

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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