Steel pipe sizing can be confusing, especially for buyers who are new to piping specifications. A pipe's designated size often differs from its actual dimensions, which can lead to mistakes during purchasing or design.
To avoid confusion, it's important to understand three key measurements: Nominal Pipe Size (NPS), Outside Diameter (OD), and Wall Thickness. This guide breaks down each term and explains how they work together when selecting steel pipe for industrial applications.This question applies to all types of pipes, including ERW pipes, LSAW pipes, and seamless steel pipes, although they all follow a standardized dimensional notation system.
Importantly, NPS does not always equal the actual pipe diameter. Instead, it serves as a standardized designation that allows manufacturers, engineers, and contractors to specify pipe dimensions consistently.
For example:
NPS 2 pipe has an actual outside diameter of 2.375 inches.
NPS 6 pipe has an actual outside diameter of 6.625 inches.
NPS 12 pipe has an actual outside diameter of 12.75 inches.
Therefore, NPS acts as a naming system rather than a direct measurement.
The OD does not change
Only the wall thickness increases
As a result, the inside diameter (ID) decreases
This is a fundamental design principle used in ASME pipe standards.
Historically, it was approximately derived from:
SCH ≈ 1000 × P / S
Where:
P = design pressure
S = allowable stress of material
However, in modern standards, Schedule numbers are fixed dimensional values and should not be directly interpreted as pressure ratings.
They are best understood as a standardized wall thickness series, not a direct engineering pressure value.
However, for NPS 10” and above:
SCH 80 and XS are not always identical
Dimensional differences may exist depending on ASME standards
Therefore, it is always necessary to verify dimensions using official tables.
Carbon steel pipes follow ASME B36.10M (no “S” suffix)
Stainless steel pipes follow ASME B36.19M (with “S” suffix)
Although SCH 80 and SCH 80S may share identical dimensions in many small sizes, they are governed by different standards and should not be assumed interchangeable without verification.
Common schedules include:
SCH 10
SCH 20
SCH 40
SCH 60
SCH 80
SCH 120
SCH 160
Schedules (SCH) – Wall Thickness Standard
Schedule (SCH) number defines wall thickness for a given NPS. Higher schedule = thicker wall = higher pressure rating.
Common schedule wall thickness (carbon steel, 6-inch NPS / 168.3mm OD):
To avoid confusion, it's important to understand three key measurements: Nominal Pipe Size (NPS), Outside Diameter (OD), and Wall Thickness. This guide breaks down each term and explains how they work together when selecting steel pipe for industrial applications.This question applies to all types of pipes, including ERW pipes, LSAW pipes, and seamless steel pipes, although they all follow a standardized dimensional notation system.
1.What Is NPS (Nominal Pipe Size)?
NPS stands for Nominal Pipe Size, a North American standard used to identify pipe sizes.Importantly, NPS does not always equal the actual pipe diameter. Instead, it serves as a standardized designation that allows manufacturers, engineers, and contractors to specify pipe dimensions consistently.
For example:
NPS 2 pipe has an actual outside diameter of 2.375 inches.
NPS 6 pipe has an actual outside diameter of 6.625 inches.
NPS 12 pipe has an actual outside diameter of 12.75 inches.
Therefore, NPS acts as a naming system rather than a direct measurement.
2. Constant Outside Diameter (OD) Principle
Outside Diameter (OD) refers to the actual diameter measured from one outer edge of the pipe to the opposite outer edge.Unlike NPS, OD is a physical measurement.For a given NPS, the outside diameter remains constant regardless of the pipe schedule.Whether the pipe is Schedule 40, Schedule 80, or Schedule 160:The OD does not change
Only the wall thickness increases
As a result, the inside diameter (ID) decreases
This is a fundamental design principle used in ASME pipe standards.
3. What is a Schedule Number?
A Schedule (SCH) number is a dimensionless designation used to represent the relative wall thickness of a pipe.Historically, it was approximately derived from:
SCH ≈ 1000 × P / S
Where:
P = design pressure
S = allowable stress of material
However, in modern standards, Schedule numbers are fixed dimensional values and should not be directly interpreted as pressure ratings.
They are best understood as a standardized wall thickness series, not a direct engineering pressure value.
4.Is SCH 80 Equivalent to XS (Extra Strong)?
For pipe sizes up to NPS 8”, Schedule 80 is generally equivalent to XS (Extra Strong).However, for NPS 10” and above:
SCH 80 and XS are not always identical
Dimensional differences may exist depending on ASME standards
Therefore, it is always necessary to verify dimensions using official tables.
5.SCH 80 vs SCH 80S
The suffix “S” indicates stainless steel pipe dimensions defined under ASME B36.19M.Carbon steel pipes follow ASME B36.10M (no “S” suffix)
Stainless steel pipes follow ASME B36.19M (with “S” suffix)
Although SCH 80 and SCH 80S may share identical dimensions in many small sizes, they are governed by different standards and should not be assumed interchangeable without verification.
6.How Pipe Schedule Relates to Wall Thickness ?
Instead of listing wall thickness directly, steel pipes often use a Schedule (SCH) number.Common schedules include:
SCH 10
SCH 20
SCH 40
SCH 60
SCH 80
SCH 120
SCH 160
Schedules (SCH) – Wall Thickness Standard
Schedule (SCH) number defines wall thickness for a given NPS. Higher schedule = thicker wall = higher pressure rating.
Common schedule wall thickness (carbon steel, 6-inch NPS / 168.3mm OD):
| Schedule | Wall Thickness (mm) | Wall Thickness (inches) | Pressure Rating (psi, Grade B) |
| SCH 10 | 3.4 | 0.134 | Low pressure |
| SCH 20 | 5.5 | 0.218 | Medium |
| SCH 30 | 7.1 | 0.28 | 250 psi |
| SCH 40 (standard) | 9.5 | 0.375 | 450 psi |
| SCH 80 (extra strong) | 11 | 0.432 | 580 psi |
| SCH 160 | 18.3 | 0.72 | High pressure |
