How to Calculate Pipe Wall Thickness per ASME B31.3

Wall thickness is the first calculation for any process line: it defines the schedule, the weight of the pipe, and a large share of the material cost. ASME B31.3 (Process Piping) solves it with a short equation, but each term has conditions worth reviewing. In this guide you will find the equation, how to go from design thickness to the schedule you purchase, a worked example, and the errors that come up most often.

The references in this article correspond to ASME B31.3-2024. If your project is contracted to a different edition, verify paragraphs and values in that edition.

The 3a equation

For straight pipe under internal pressure, paragraph 304.1.2 gives the pressure design thickness with equation 3a:

t = P · D / [ 2 · (S · E · W + P · Y) ]

The equation works with any consistent unit system: MPa and mm, or psi and inches.

When the equation applies

Equation 3a is valid for thin-walled pipe. The same paragraph 304.1.2 requires special consideration (failure theory, fatigue, and thermal stresses) in two cases:

• When t is equal to or greater than D/6.
• When the ratio P / (S · E) is greater than 0.385.

In most moderate-pressure process lines neither condition is met, but this must be checked and documented in the calculation record. In high-pressure services, where these cases tend to appear, chapter IX of the code also applies.

From design thickness to nominal thickness

The value t is not what you buy. Three steps get you to the nominal thickness:

1. Corrosion allowance (c): sum of the corrosion and erosion allowance, plus the depth of threads or grooves if present. The minimum required thickness is tm = t + c (equation 2).
2. Manufacturing tolerance: the material specification allows the wall to come out thinner than nominal. For ASTM A106 and A53 the tolerance is 12.5%, so the nominal thickness must be at least tm / 0.875. For other materials, check the tolerance in their specification.
3. Schedule: select from ASME B36.10M (carbon and alloy steel) or B36.19M (stainless) the first nominal thickness that covers the value above.

Worked example

NPS 6 carbon steel line, ASTM A106 grade B, seamless, at ambient temperature:

• P = 5.0 MPa
• D = 168.3 mm
• S = 137.9 MPa (example value: verify it in Table A-1M of the edition that applies to your project)
• E = 1.0 · W = 1.0 · Y = 0.4
• c = 1.5 mm for corrosion

Step 1: design thickness

t = (5,0 × 168,3) / [2 × (137,9 × 1,0 × 1,0 + 5,0 × 0,4)]
t = 841,5 / 279,8
t = 3,01 mm

Step 2: check the limits

D / 6 = 168,3 / 6 = 28,1 mm      →  t = 3,01 mm < 28,1 mm   ✓
P / (S · E) = 5,0 / 137,9 = 0,036 →  0,036 < 0,385          ✓

Equation 3a applies.

Step 3: corrosion allowance and tolerance

tm = 3,01 + 1,5 = 4,51 mm
espesor nominal mínimo = 4,51 / 0,875 = 5,15 mm

Step 4: selection

For NPS 6, Schedule 40 (STD) has 7.11 mm of wall. It is the first numbered schedule that covers 5.15 mm, so NPS 6 Sch 40 is specified.

ASME B36.10M also lists wall thicknesses for NPS 6 without a schedule number, such as 6.35 mm. If the project's piping specification allows them, they may be a lighter option.

The same calculation in US units

With P = 725 psi, D = 6.625 in, and S = 20,000 psi:

t = (725 × 6,625) / [2 × (20 000 + 725 × 0,4)]
t = 4 803,1 / 40 580 = 0,118 in  (3,01 mm)

The result matches, because the equation does not depend on the unit system.

What if the pipe has a weld seam?

If the same line were ASTM A53 type E (ERW), the joint factor drops to E = 0.85:

t  = 841,5 / [2 × (137,9 × 0,85 + 2,0)] = 841,5 / 238,4 = 3,53 mm
tm = 3,53 + 1,5 = 5,03 mm
espesor nominal mínimo = 5,03 / 0,875 = 5,75 mm

Sch 40 still covers it, but with less margin. At larger diameters or pressures, switching from seamless to ERW can raise the schedule.

Common errors

• Using the nominal diameter instead of the outside diameter. For NPS 6, D is 168.3 mm, not 152.4 mm (6").
• Taking S at ambient temperature when the line operates hot. Allowable stress decreases with temperature, and for carbon steel the drop is significant above roughly 340 °C.
• Forgetting the depth of a thread or groove within the corrosion allowance c.
• Applying Y = 0.4 outside its range. Above 482 °C the coefficient depends on the material, and when t ≥ D/6 it is calculated using the formula in the note to Table 304.1.1.
• Comparing t directly against the nominal thickness, without adding c or deducting the manufacturing tolerance.
• Not keeping traceability. A correct calculation without the code edition, the paragraphs, and the source of each value is hard to approve in a review.

Frequently asked questions

How much corrosion allowance is used?

It is defined by the project's or client's materials specification, depending on the fluid and service life. For carbon steel in process services, 1.5 to 3 mm is common. For stainless steel with minimally corrosive fluids it is usually zero.

Can this equation be used for elbows and reducers?

Not directly. Elbows and bends use paragraph 304.2.1, which adds the factor I for the intrados and extrados. Branch connections also require reinforcement verification under paragraph 304.3.

What if the pipe operates under external pressure or vacuum?

Equation 3a is only for internal pressure. For external pressure, paragraph 304.1.3 refers to the procedure in ASME BPVC Section VIII, Division 1, which also depends on the length between stiffeners.

Does B31.1 use the same formula?

The structure is similar, but the notation, factors, and stress tables differ. Values from one code should not be mixed with the equation of the other.

Do it in minutes, with traceability

This calculation is simple once. Repeating it across hundreds of lines, with different materials and temperatures, and documenting it for the project file is what takes days. PipeStamp solves it with the formula, the substitution, and the code reference at each step, SI/US conversion, and PDF export with the project letterhead.

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