📚 Theory, formulae and code references for this calculator — sourced directly from ASME B31.3-2020, B31.4-2019, and B31.8-2003. For full course content and worked examples, visit PipingPro Academy courses.
ASME B31.3-2020 · Process Piping — Clause 304.1.2 Straight Pipe Under Internal Pressure
tm = t + c [Eq. (2)]
t = P·D / [ 2·(S·E·W + P·Y) ] [Eq. (3a)]
c = sum of mechanical allowances (thread/groove depth) plus corrosion and erosion allowances (mm)
D = outside diameter of pipe, mm (as listed in standards or as measured)
E = quality factor from Table A-1A or A-1B — 1.00 for seamless
P = internal design gage pressure, MPa
S = stress value for material from Table A-1 or A-1M at design temperature, MPa
t = pressure design thickness (calculated), mm
tm = minimum required thickness including mechanical, corrosion and erosion allowances, mm
T = pipe wall thickness (measured or minimum per purchase specification), mm — must be ≥ tm
W = weld joint strength reduction factor per para. 302.3.5(e) — see table below
Y = coefficient from Table 304.1.1, valid for t < D/6 — see table below
Mill tolerance: tordered = (t + c) / (1 − mill_tol/100) — round up to next commercial schedule
For t ≥ D/6 or P/SE > 0.385, special consideration of factors such as theory of failure, fatigue, and thermal stress is required per para. 304.1.2(b). This calculator covers only the t < D/6 case.
ASME B31.3-2020 · Table 304.1.1 — Values of Coefficient Y for t < D/6
| Material | ≤482°C (900°F) | 510°C (950°F) | 538°C (1000°F) | 566°C (1050°F) | 593°C (1100°F) | ≥621°C (1150°F) |
|---|
| Ferritic steels | 0.4 | 0.5 | 0.7 | 0.7 | 0.7 | 0.7 |
| Austenitic steels | 0.4 | 0.4 | 0.4 | 0.4 | 0.5 | 0.7 |
| Nickel alloys (N06617, N08800, N08810, N08825) | 0.4 | 0.4 | 0.4 | 0.4 | 0.4 | 0.4–0.7 |
| Gray iron | 0.0 | — | — | — | — | — |
| Other ductile metals | 0.4 | 0.4 | 0.4 | 0.4 | 0.4 | 0.4 |
For t ≥ D/6, Y = (d + 2c) / (D + d + 2c) where d = inside diameter. The value of Y may be interpolated for intermediate temperatures.
ASME B31.3-2020 · Table 302.3.5 — Weld Joint Strength Reduction Factor W
W applies to longitudinal welds at elevated temperature to account for creep strength reduction of the weld joint relative to the base metal.
Carbon Steel: W = 1.0 for all temperatures up to 760°C (1,400°F)
CrMo steels (½Cr–½Mo through 9Cr–1Mo):
W = 1.00 at ≤427°C (800°F) · 0.95 at 454°C · 0.91 at 482°C · 0.86 at 510°C · 0.82 at 538°C · 0.77 at 566°C · 0.73 at 593°C
CSEF steels (N+T) — grades 91, 92, 911, 122, 23:
W = 1.00 at ≤510°C · 0.95 at 538°C · 0.91 at 566°C · 0.86 at 593°C · 0.82 at 621°C
Austenitic SS grade 3xx and N088xx nickel alloys:
W = 1.00 at ≤510°C · 0.95 at 538°C · 0.91 at 566°C · 0.86 at 593°C · 0.82 at 621°C · 0.77 at 649°C
W = 1.0 for seamless pipe at all temperatures (no longitudinal weld). Linear interpolation is permitted for intermediate temperatures. See full Table 302.3.5 for CSEF subcritical PWHT and autogenous weld cases.
ASME B31.4-2019 · Liquid Pipelines — Clause 403.2.1 Criteria
tn ≥ t + A
US Customary: t = Pi·D / (2·S)
SI Units: t = Pi·D / (20·S)
A = sum of allowances for threading, grooving, corrosion and erosion (para. 403.2.2–403.2.4), mm (in.)
D = outside diameter of pipe, in. (mm)
Pi = internal design gage pressure, psi (bar)
S = applicable allowable stress value, psi (MPa), determined by: S = F × E × Sy
E = weld joint factor per Table 403.2.1-1 — 1.00 for seamless and SAW
F = design factor — shall not be greater than 0.72 per this Code
Sy = specified minimum yield strength, psi (MPa)
t = pressure design wall thickness, in. (mm)
tn = nominal wall thickness satisfying requirements for pressure and allowances, in. (mm)
⚠️ No temperature derating factor in B31.4 Eq. 403.2.1. The SI factor of 20 (vs. 2 in US Customary) accounts for the unit conversion: 1 bar = 0.1 MPa, hence 2S × 10 = 20S.
Note: B31.4 para. 403.2.3 states corrosion allowance is not required if pipe is protected per Chapter VIII. Where required, it is included in A.
B31.4 design factors reflect the underthickness tolerances and maximum allowable imperfection depths in the applicable pipe specifications — no additional mill tolerance deduction is required beyond selecting a nominal schedule wall ≥ tn.
ASME B31.4-2019 · Table 403.2.1-1 — Weld Joint Factors E (selected values)
| Specification | Pipe Type | Weld Joint Factor E |
|---|
| API 5L / ASTM A53 / A106 / A333 | Seamless | 1.00 |
| API 5L / ASTM A53 / A135 / A333 | Electric Resistance Welded (ERW) | 1.00 |
| API 5L / ASTM A381 | Submerged Arc Welded (SAW) | 1.00 |
| ASTM A671 / A672 (Class X2, post-PWHT RT) | Electric Fusion Welded | 1.00 |
| ASTM A671 / A672 (Class X3 or pre-PWHT RT) | Electric Fusion Welded | 0.70 / 0.80 |
| ASTM A53 Type F / API 5L A25 | Furnace Butt Welded (continuous) | 0.60 |
ASME B31.8-2003 · Gas Transmission — Para. 841.11 Steel Pipe Design Formula
P = 2·S·t·F·E·T / D (design pressure form)
t = P·D / (2·S·F·E·T) (wall thickness form)
D = nominal outside diameter of pipe, in.
E = longitudinal joint factor from Table 841.115A
F = design factor from Table 841.114A (see table below)
P = design pressure, psig — see also para. 841.111 for limitations
S = specified minimum yield strength (SMYS), psi — per pipe purchase specification or paras. 817.13(h) and 841.112
T = temperature derating factor from Table 841.116A (see table below)
t = nominal wall thickness, in.
Add corrosion allowance separately after t is calculated. Apply mill tolerance (12.5% under per API 5L) to arrive at specified nominal wall.
B31.8 uses the hoop stress design formula in US Customary units natively. S is SMYS — not a temperature-derated allowable stress. The design factor F already incorporates underthickness tolerance allowances from the approved pipe specifications.
ASME B31.8-2003 · Table 841.114A — Basic Design Factor F
| Location Class | Design Factor F |
|---|
| Location Class 1, Division 1 (offshore / remote) | 0.80 |
| Location Class 1, Division 2 (open country) | 0.72 |
| Location Class 2 (semi-rural) | 0.60 |
| Location Class 3 (suburban / residential) | 0.50 |
| Location Class 4 (urban / dense) | 0.40 |
Aerial crossings in Location Class 1 require F = 0.60 per para. 841.122. See Table 841.114B for facility-specific design factors (fabricated assemblies, bridge crossings, compressor station piping etc.).
ASME B31.8-2003 · Table 841.115A — Longitudinal Joint Factor E (selected values)
| Specification | Pipe Class | E Factor |
|---|
| API 5L | Seamless | 1.00 |
| API 5L | Electric Resistance Welded / Electric Flash Welded | 1.00 |
| API 5L | Submerged Arc Welded (SAW / DSAW) | 1.00 |
| API 5L | Furnace Butt Welded | 0.60 |
| ASTM A 53 | Seamless / ERW | 1.00 |
| ASTM A 53 | Furnace Butt Welded (Continuous Weld) | 0.60 |
| ASTM A 106 | Seamless | 1.00 |
| ASTM A 134 / A 139 | Electric Fusion Arc / Fusion Welded | 0.80 |
| ASTM A 135 | Electric Resistance Welded | 1.00 |
| ASTM A 381 | Double Submerged Arc Welded | 1.00 |
| ASTM A 671 / A 672 (Classes 12, 22, 32, 42, 52) | Electric Fusion Welded | 1.00 |
| ASTM A 671 / A 672 (Classes 13, 23, 33, 43, 53) | Electric Fusion Welded | 0.80 |
ASME B31.8-2003 · Table 841.116A — Temperature Derating Factor T for Steel Pipe
| Temperature, °F | Temperature, °C (approx.) | Derating Factor T |
|---|
| 250 or less | 121 or less | 1.000 |
| 300 | 149 | 0.967 |
| 350 | 177 | 0.933 |
| 400 | 204 | 0.900 |
| 450 | 232 | 0.867 |
For intermediate temperatures, interpolate the derating factor (para. 841.116). B31.8 is a gas transmission code; temperatures above 250°F are uncommon in normal gas pipeline service but may apply in specific process gas or geothermal applications.
Reference Standards
| Standard | Edition | Clause | Application |
|---|
| ASME B31.3 | 2020 | 304.1.1, 304.1.2 | Process piping — pressure design wall thickness (Eq. 3a) |
| ASME B31.3 | 2020 | 302.3.5, Table 302.3.5 | Weld joint strength reduction factor W |
| ASME B31.3 | 2020 | Table 304.1.1 | Temperature coefficient Y for ferritic, austenitic, and other materials |
| ASME B31.4 | 2019 | 403.2.1 | Liquid pipeline — t = PiD/(20S) SI, S = F·E·Sy — P in bar, D in mm. No T factor. |
| ASME B31.4 | 2019 | Table 403.2.1-1 | Weld joint factors E for liquid pipeline materials |
| ASME B31.8 | 2003 | 841.11 | Gas transmission — t = PD/(2SFET) — P psig, D & t inches, S psi |
| ASME B31.8 | 2003 | Table 841.114A | Basic design factor F by location class (0.40–0.80) |
| ASME B31.8 | 2003 | Table 841.115A | Longitudinal joint factor E by pipe specification and class |
| ASME B31.8 | 2003 | Table 841.116A | Temperature derating factor T — 250°F to 450°F |
| ASME B36.10M | 2018 | — | Standard pipe schedules and OD — carbon & alloy steel |
| ASME B36.19M | 2018 | — | Stainless steel pipe schedules 5S / 10S / 40S / 80S (NPS ⅛"–12") |
| API Spec 5L | — | — | Line pipe specification — mill tolerance 12.5% under nominal wall |
Allowable stress values (S) for B31.3 materials in this calculator are approximate representations of ASME B31.3 Table A-1. Always verify against the current code edition for final design. SMYS values for B31.4/B31.8 materials are per API 5L.