What formula does this collapse calculator use?

It applies the API Bulletin 5C3 / ISO 10400 four-regime collapse model. From the pipe D/t ratio it selects the governing regime — yield-strength, plastic, transition, or elastic collapse — and applies that regime's equation. The A/B/C and F/G coefficients are computed as functions of the grade minimum yield strength.

Does it handle combined axial load?

Yes. Enter an axial tension and the calculator applies the API 5C3 biaxial correction (Hencky–von Mises, formula 8): it derates the yield strength to Ypa before evaluating the collapse regime, which lowers the collapse rating under tension.

Which grades are supported?

All API 5CT casing and tubing grades (H40 through Q125) and all API 5DP drill pipe grades (E75, X95, G105, S135, V150). Collapse depends only on the minimum yield strength, so any grade with a known yield can be evaluated.

Are results suitable for final well design?

No. Results are reference values for planning and feasibility. Final casing or drill-string design requires full API TR 5C3 / ISO 10400 analysis including design factors, ovality, wear, temperature derating and connection ratings.

Collapse Pressure Calculator

Calculate the minimum collapse pressure of casing, tubing or drill pipe per the API Bulletin 5C3 / ISO 10400 four-regime model. Enter OD, wall thickness and grade — the calculator computes D/t, selects the governing collapse regime (yield-strength, plastic, transition or elastic), and returns the rated collapse pressure. An optional axial tension applies the biaxial derating.

API 5C3 / ISO 10400 — casing, tubing and drill pipe. Results are reference values only — verify against full well design before use.

Collapse Pressure (API 5C3 four-regime)

Selects Yield / Plastic / Transition / Elastic regime from D/t | API Bull. 5C3 / ISO 10400

Enter OD and wall in mm and select the grade. Optional axial tension applies the biaxial (Ypa) derating.

OD (mm) Wall thickness (mm) Grade

Axial tension (kN) — optional

Engineering Disclaimer: Results are reference collapse values for the nominal pipe body only. Full casing and drill-string design must apply design factors and account for ovality, eccentricity, wear, temperature derating of yield, and connection collapse ratings per API TR 5C3 / ISO 10400. Always verify with qualified design software before procurement or running.

How the Four-Regime Model Works

Collapse resistance is not a single formula — it depends on how thick-walled the pipe is. API 5C3 divides the D/t range into four regimes and applies a different equation in each. The calculator computes the D/t breakpoints for the grade and picks the correct one automatically.

Regime	Applies for	Formula
Yield strength	low D/t (thick wall)	Pyp = 2·Yp·[(D/t−1)/(D/t)²]
Plastic	(D/t)_YP … (D/t)_PT	Pp = Yp·[A/(D/t) − B] − C
Transition	(D/t)_PT … (D/t)_TE	Pt = Yp·[F/(D/t) − G]
Elastic	high D/t (thin wall)	Pe = 46.95×10⁶/[(D/t)·(D/t−1)²]

A, B, C (plastic) and F, G (transition) are grade coefficients derived from the minimum yield strength. Under axial tension, the yield strength is first derated by the biaxial formula Ypa = Yp·[√(1 − 0.75·(Sa/Yp)²) − 0.5·(Sa/Yp)] before the regime is evaluated. See the API 5C3 Performance Properties page for the full coefficient tables.

Worked Example — 7″ 26 lb/ft L80 Casing

Inputs: OD = 177.8 mm, wall = 9.19 mm, L80 (min yield 552 MPa / 80 ksi), no axial load.

D/t = 177.8 / 9.19 = 19.35, which falls in the plastic regime for L80 (between (D/t)_YP ≈ 13.4 and (D/t)_PT ≈ 22.5).
Pp = 80,000 × [3.071/19.35 − 0.0667] − 1955 = ≈ 5,410 psi (37.3 MPa / 373 bar) — matching the API 5C3 tabulated collapse rating for 7″ 26# L80.

Which collapse regime governs my pipe?

It depends on D/t. Thick-wall pipe (low D/t) fails in the yield-strength or plastic regime; thin-wall pipe (high D/t) fails in the transition or elastic regime. The calculator computes the grade-specific D/t breakpoints and reports which regime governs your result.

Why does axial tension reduce collapse resistance?

Combined axial tension and external pressure is a biaxial stress state. Per the Hencky–von Mises criterion (API 5C3 formula 8), axial tension lowers the effective yield strength available to resist collapse, so the rated collapse pressure drops. Enter the axial tension to see the derated value.

Does this apply to drill pipe as well as casing?

Yes. API 5C3 / ISO 10400 is titled for casing, tubing and drill pipe, and the same collapse model applies. Select an API 5DP grade (E75–V150) to rate drill pipe collapse; the biaxial correction is especially relevant for drill pipe, which is typically in high axial tension.