What Is Wall Thickness?

Quick Answer

Wall thickness (WT) is the distance between the outside diameter (OD) and inside diameter (ID) of a pipe. It determines the pipe’s pressure-carrying capacity, structural strength, and weight. WT is calculated using design formulas from ASME B31.3 (process piping) or B31.1 (power piping), then matched to the nearest standard schedule per ASME B36.10 or B36.19.

The basic wall thickness formula from ASME B31.3 for internal pressure is:

t = (P x D) / (2 x (S x E x W + P x Y))

Where:

• t = minimum required wall thickness (mm)
• P = internal design pressure (MPa)
• D = pipe outside diameter (mm)
• S = allowable stress at design temperature (MPa, from ASME B31.3 Table A-1)
• E = weld joint efficiency factor (1.0 for seamless, 0.85-1.0 for welded)
• W = weld strength reduction factor (1.0 below creep range)
• Y = coefficient (0.4 for temperatures below 482°C)

From Calculation to Schedule Selection

Step	Action
1	Calculate minimum wall thickness (t) from the design formula
2	Add corrosion/erosion allowance (c), typically 1.5-3.0 mm
3	Add mill tolerance (12.5% for seamless, per ASTM specs)
4	Select the next standard schedule that equals or exceeds the total

Example for NPS 6 pipe, P = 4.0 MPa, T = 300°C, A106 Gr. B:

Parameter	Value
Design pressure (P)	4.0 MPa
OD (D)	168.3 mm
Allowable stress (S) at 300°C	118 MPa
Joint factor (E)	1.0 (seamless)
Y coefficient	0.4
Calculated t	2.80 mm
Corrosion allowance	3.0 mm
Required before tolerance	5.80 mm
Mill tolerance (+12.5%)	6.63 mm
Selected schedule	SCH 40 (7.11 mm)

Mill Tolerance

Pipe manufacturing standards permit a negative deviation from the nominal wall thickness. This means the actual WT can be less than the specified nominal.

Pipe Type	Tolerance	Standard
Seamless	-12.5%	ASTM A106, A335, A312
ERW	-12.5%	ASTM A53, API 5L
LSAW/DSAW	-0.5 mm or -5% (larger of)	API 5L
EFW	Per plate tolerance	ASTM A671, A672

For seamless pipe, a nominal 7.11 mm wall (SCH 40, NPS 6) can be as thin as 6.22 mm at any point. The design calculation must account for this by dividing the required thickness by 0.875 (i.e., multiplying by 1/0.875 = 1.143).

Corrosion Allowance

Corrosion allowance is extra thickness added to compensate for material loss during the pipe’s design life.

Service	Typical Allowance
Clean, non-corrosive (stainless)	0 mm
Mild carbon steel service	1.5 mm
Standard process piping (CS)	3.0 mm
Erosive or highly corrosive service	4.5-6.0 mm
Sour service with H2S corrosion	Per corrosion study

Pro Tip

The corrosion allowance is not a safety factor; it is consumed during operation. A 3.0 mm corrosion allowance on a 25-year design life assumes a corrosion rate of 0.12 mm/year. If the actual corrosion rate exceeds this, the pipe will fall below minimum wall thickness before the end of its design life. Base the corrosion allowance on actual corrosion rate data from similar service, not on default values.

Minimum Wall by ASME B31.3

ASME B31.3 requires that the pressure design thickness, after subtracting corrosion allowance and mill tolerance, remains above the calculated minimum at all times during service. The retirement thickness is the calculated t value with no corrosion allowance remaining.

For the full range of pipe wall thickness charts per ASME B36.10 and B36.19, refer to the dimensional tables.

Read the full guide to pipe types