ASME B31.3 Pipe Wall Thickness Calculator
Returns the pressure design thickness t, the minimum required thickness tm, and the ordering thickness grossed up for mill under-tolerance, per ASME B31.3 ¶304.1.2 Eq. (3a). Allowable stress S, factors E and W, and coefficient Y stay user inputs from the governing Code edition.
Computes the pressure design thickness t, the minimum required thickness tm, and the ordering thickness for straight pipe under internal pressure per ASME B31.3 ¶304.1.2, Eq. (3a). Allowable stress S, quality factor E, weld joint strength reduction factor W, and coefficient Y are entered by you from the Code tables — no ASME table values are embedded.
Method
B31.3 Eq. (3a) for straight pipe under internal pressure, thin-wall form:
t = P·D / ( 2 (S·E·W + P·Y) )
tm = t + c
torder = tm / (1 − mill tolerance)
where P is the internal design gauge pressure, D the outside diameter, and c the sum of the corrosion/erosion allowance and any mechanical allowances (threading, grooving). The ordering thickness grosses up tm for the seamless-pipe mill under-tolerance (typically 12.5%).
| Inputs | ||
|---|---|---|
| P | Internal design gauge pressure | psi |
| D | Pipe outside diameter | in |
| S | Allowable stress at design temperature (Code Table A-1 — user-supplied) | psi |
| E | Quality factor (Table A-1A/A-1B — user-supplied) | — |
| W | Weld joint strength reduction factor (¶302.3.5(e) — user-supplied) | — |
| Y | Wall-thickness coefficient (Table 304.1.1 — user-supplied) | — |
| c | Corrosion/erosion + mechanical allowances | in |
| mill tol | Mill under-tolerance fraction (e.g. 0.125) | — |
| Outputs | ||
| t | Pressure design thickness | in |
| tm | Minimum required thickness t + c | in |
| t_order | Ordering thickness after mill tolerance | in |
Limitations — what this calculator is not
- Straight pipe under internal pressure only — bends are covered by the Bend Thickness calculator (¶304.2.1), miters by ¶304.2.3, and external pressure by BPVC Section VIII UG-28.
- Eq. (3a) is the thin-wall form, valid for t < D/6; the calculator warns when the result leaves that range (¶304.1.2(b) then requires special consideration).
- S, E, W and Y are user-supplied from the governing Code edition — selecting them (including weld joint factors for longitudinally welded pipe) is the engineer's responsibility.
- No structural, sustained, occasional, or displacement loading is evaluated here — pressure only. Use the stress-range calculators for ¶302.3.5 / ¶319.4.4 checks.
Worked example — fixture-verified
NPS 6 (6.625 in OD) carbon-steel line at 2,000 psi design pressure. Allowable stress 20,000 psi, seamless (E = 1.0, W = 1.0), Y = 0.4, corrosion allowance 1/16 in, mill tolerance 12.5%.
| Given | ||
|---|---|---|
| Design pressure P | 2,000 | psi |
| Outside diameter D | 6.625 | in |
| Allowable stress S | 20,000 | psi |
| Quality factor E | 1.0 | — |
| Weld factor W | 1.0 | — |
| Coefficient Y | 0.4 | — |
| Allowance c | 0.0625 | in |
| Mill tolerance | 12.5 | % |
Step by step
- Denominator: 2·(S·E·W + P·Y) = 2·(20,000·1.0·1.0 + 2,000·0.4) = 2·20,800 = 41,600 psi.
- Pressure design thickness: t = P·D / 41,600 = 2,000·6.625 / 41,600 = 13,250 / 41,600 = 0.31851 in.
- Minimum required: tm = 0.31851 + 0.0625 = 0.38101 in.
- Ordering thickness: 0.38101 / (1 − 0.125) = 0.38101 / 0.875 = 0.43544 in → order Sch 80 (0.432 in) only after checking the actual purchase tolerance, or the next heavier wall.
| Result COMPUTED | ||
|---|---|---|
| t — pressure design thickness | 0.31851 | in |
| tm — minimum required | 0.38101 | in |
| Ordering thickness | 0.43544 | in |
t = 0.31851 in is well under D/6 = 1.10 in, so the thin-wall Eq. (3a) form applies.
b313-wall-thickness.json — case “NPS 6 @ 2000 psi, S=20000, Y=0.4, c=1/16, 12.5% mill tol” (tolerance 0.00001) — in the
calc-core release gate. It re-runs on every commit; a red fixture blocks deployment.
See the validation methodology.Sources & citations
- ASME B31.3, Process Piping — ¶304.1.2 Eq. (3a), pressure design of straight pipe.
- ASME B31.3 — Table 304.1.1 (coefficient Y), Table A-1 (allowable stress), Tables A-1A/A-1B (quality factors) — values user-supplied, not embedded.
Per the source & citation policy, allowable-stress and factor table values are user-supplied — this page and the app cite paragraph numbers and never reproduce ASME table data.
FAQ
What is the difference between t, tm, and the ordering thickness?
t is the thickness needed to resist pressure alone (Eq. 3a). tm adds the corrosion, erosion and mechanical allowances. The ordering thickness additionally grosses tm up for the mill under-tolerance (12.5% for most seamless pipe), because the wall actually delivered may be that much thinner than nominal.
Where do S, E, W and Y come from?
From the ASME B31.3 tables for your material, temperature and weld type: S from Table A-1, E from Tables A-1A/A-1B, W per ¶302.3.5(e), Y from Table 304.1.1. This tool keeps them as inputs — it never embeds ASME table values.
Does this calculator handle external pressure or vacuum?
No. B31.3 ¶304.1.3 points external-pressure design to BPVC Section VIII UG-28 — use the External Pressure calculator on the Equipment line.
Related calculators
- ASME B31.3 Allowable Working Pressure Calculator — Inverse check: rate the wall you actually ordered
- Pipe Bend Minimum Wall Thickness Calculator (B31.3 ¶304.2.1) — Bends are excluded here — use the ¶304.2.1 I-factor
- Mitered Elbow Allowable Pressure Calculator (ASME B31.3 ¶304.2.3) — Miters are excluded here — rate them per ¶304.2.3
- ASME B31.1 Power Piping Wall Thickness Calculator — Same check under B31.1 Power Piping