Stainless steel pipe OD and wall thickness explained

The national standard thickness of stainless steel pipes is determined primarily by the nominal diameter (DN) or outer diameter (OD). In Chinese GB standards, thin-wall stainless steel pipes are classified into Ⅰ series and Ⅲ series. Even with the same nominal diameter, the actual outer diameter may vary slightly between different series.

“Ф” vs “DN”: Key Diameter Markings in Stainless Steel Pipes

When describing stainless steel pipe sizes, two common identifiers are used:

1. “Ф” — Actual Outer Diameter (OD)

“Ф” represents the actual measured outer diameter of the stainless steel pipe. It is often written with wall thickness:
Example: Ф25×3 → 25 mm OD, 3 mm wall thickness.

2. “DN” — Nominal Diameter (Nominal Pipe Size)

“DN” refers to the nominal diameter used for standardized pipe and valve systems. DN values are close to the inner diameter and ensure compatibility during engineering installation.

Conversion Rule（OD to DN Conversion Formula）

Ф (OD) = actual outer diameter

DN = nominal diameter (approx. inner diameter)

Approximate formula:
DN = Ф − 2 × wall thickness

However, in engineering practice, DN does not correspond exactly to OD. The relationship relies on experience or a DN–OD–WT pipe size chart.
Examples:

Ф108×5 → DN100

Ф32×3 → DN25

To ensure accuracy, engineers typically refer to standardized conversion tables when selecting stainless steel pipe sizes.

Seamless Steel Pipe Wall Thickness Calculation

The theoretical wall thickness of seamless steel pipes is the minimum thickness required to withstand internal pressure.

But actual engineering calculations must also include:

design safety factors

manufacturing tolerance

corrosion allowance

pressure fluctuation impact

Therefore, the engineering wall thickness is always greater than the theoretical value.

Basic Pipe Size Concepts（OD, WT, DN Explained）

1. OD — Outer Diameter

The actual measured diameter of the pipe’s exterior (mm).
OD directly affects installation, pipe fitting compatibility, and layout spacing.

2. WT — Wall Thickness

Determines the pipe’s:

pressure resistance

structural strength

corrosion resistance

3. DN — Nominal Diameter

DN is a specification identifier, not a real measurement.
Different combinations of OD and WT can correspond to the same DN, so size charts must be consulted during design or procurement.

Commonly Used DNs and Actual OD References

When selecting the appropriate wall thickness for stainless steel pipes, several key factors must be considered:

Thicker wall thickness is not always better.
The optimal wall thickness depends on working pressure, medium characteristics, operating temperature, and the welding or installation process.

Increasing wall thickness improves strength but raises cost.
While a thicker wall can enhance pressure resistance and corrosion resistance, it also increases the pipe’s weight and overall project cost. Therefore, engineers must balance performance requirements with economic considerations.

Standard pipe dimensions are expressed as “OD × WT.”
For example:
DN50 → OD 60.3 mm × WT 20 mm
In this case, 60.3 mm is the actual outer diameter corresponding to DN50, and 20 mm represents the wall thickness.