Casing Burst Pressure Calculator

Estimate internal-yield pressure for oil and gas well casing using Barlow's formula with the API 5CT wall-tolerance factor. Enter OD, wall thickness, yield strength or grade — returns burst pressure, D/t ratio, and an approximate collapse indicator.

Wall tolerance factor

0.875

N-80 yield

80,000psi

P-110 yield

110,000psi

Q-125 yield

125,000psi

Units Switches inputs and outputs

Outside diameter OD (in) Nominal casing OD

Wall thickness t (in) Nominal body wall thickness

API 5CT grade Sets minimum yield strength

Yield strength Y (psi) Edit to override the grade default

Burst pressure (internal yield)

7,927psi

N-80, 80,000 psi

Collapse pressure (approximate)

11,970psi

D/t ratio

17.66

Grade & yield used

N-80 (80,000 psi)

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Tip The 0.875 factor is the API 5C3 / 5CT wall-tolerance allowance for 12.5% minimum wall. Published API Bulletin 5C2 tabulated burst values use the same factor — this calculator matches those tables to ±1%.

Worked example

9-5/8 in, 0.545 in wall, API 5CT N-80 (Y = 80,000 psi).

1. Barlow with API wall factor P_b = (2 × Y × t × 0.875) / OD P_b = (2 × 80,000 × 0.545 × 0.875) / 9.625 P_b = 76,300 / 9.625 P_b ≈ 7,927 psi 2. D/t ratio D/t = 9.625 / 0.545 = 17.66 3. Approximate elastic collapse (screening only) P_c ≈ 2E / (1 − ν²) × (t/OD)³ with E = 30 × 10⁶ psi, ν = 0.3 P_c ≈ 11,970 psi

API 5CT casing grade reference

Grade	Min yield (psi)	Min tensile (psi)	Typical use
H-40	40,000	60,000	Legacy low-strength applications and shallow service.
J-55	55,000	75,000	General-purpose casing and tubing in moderate-depth wells.
K-55	55,000	95,000	Similar yield to J-55 with higher tensile requirements.
N-80	80,000	100,000	Common higher-strength casing for deeper, higher-pressure service.
L-80	80,000	95,000	Sour-service compatible, 23 HRC hardness cap.
C-90	90,000	100,000	Sour-service capable under tighter metallurgical control.
T-95	95,000	105,000	Sour-service and high-strength service where SSC resistance is required.
P-110	110,000	125,000	High-pressure and deeper well designs, sweet service.
Q-125	125,000	135,000	Ultra-deep HPHT sweet service.

Full properties, hardness limits, and NACE sour-service status in the API 5CT casing grades reference.

Common mistakes

Treating burst as working pressure. Barlow gives yield, not allowable operating pressure. Apply the company design factor on top.
Forgetting the 0.875 factor. Without wall tolerance the result is ~14% high — and won't match API 5C2 tables.
Mixing OD and ID. Barlow uses OD. Using ID drops burst by roughly 10–12% on typical casing.
Skipping collapse. Deep sweet-gas and HPHT designs are often collapse-limited, not burst-limited. Check both.

Warn The collapse result here is a simplified elastic-shell estimate, not an API-rated collapse value. Production design must use the governing API collapse regime (elastic / transition / plastic / yield), connection limits, and biaxial-load interaction per company standards.

FAQ

What is casing burst pressure?

Casing burst pressure is the internal pressure level that causes the casing body to reach yield in hoop stress. It is a pipe-body pressure rating estimate, not automatically the allowable operating pressure for the full string.

Why 0.875 in the Barlow formula?

The 0.875 factor is a conservative API wall tolerance allowance. It reduces the effective contribution of nominal wall thickness so the burst estimate reflects the 12.5% minimum wall tolerance instead of assuming the pipe wall is at nominal thickness everywhere.

What is the difference between burst and collapse pressure?

Burst pressure is resistance to internal pressure pushing the pipe outward. Collapse pressure is resistance to external pressure squeezing the pipe inward. Burst is largely a hoop-yield problem, while collapse is strongly influenced by D/t ratio, ovality, and collapse regime.

Which API 5CT grade should I choose?

Choose the grade that satisfies burst, collapse, tensile, environment, and cost requirements for the actual load cases in your well. N-80 is common for stronger general service; L-80, C-90, and T-95 are typical when sour-service or corrosion constraints apply.

How much safety factor should I apply?

There is no single universal factor. Operators set design factors based on company standards, uncertainty in predicted loads, and the consequence of failure. Theoretical burst pressure should be treated as a capacity estimate, then reduced by the required design margin.

Sources

API Specification 5CT API Bulletin 5C2 — performance properties AMPP/NACE MR0175 / ISO 15156 SLB Oilfield Glossary

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