Casing Burst Pressure Calculator: API Internal Yield Pressure via Barlow’s Equation

Solve the API internal yield pressure (burst pressure) of oilfield casing and tubing using Barlow’s equation: find the rated burst pressure from outside diameter, wall thickness, and minimum yield strength, compare burst pressure across API steel grades, or solve the wall thickness needed to hit a target burst rating. Everything updates live.

Barlow’s Equation and API Internal Yield Pressure

Casing and tubing fail in burst when internal pressure pushes the tangential stress in the pipe wall past the steel’s yield strength. Barlow’s equation models this directly: the theoretical burst pressure is 2 x Yp x t / OD, where Yp is minimum yield strength, t is nominal wall thickness, and OD is outside diameter. The American Petroleum Institute applies a 0.875 derating factor to this theoretical value to account for the -12.5% wall thickness manufacturing tolerance permitted under API 5CT, giving the rated internal yield pressure PB = 0.875 x 2 x Yp x t / OD.

This tool solves API burst pressure directly, compares burst pressure across common steel grades at a fixed pipe size, and rearranges the equation to solve for the wall thickness needed to reach a target burst rating.

Why the 0.875 Derating Factor Exists

Pipe manufacturing tolerances under API 5CT allow wall thickness to run as much as 12.5% below the nominal specified value at any single point around the pipe body. Barlow’s equation is sensitive to wall thickness, so a pipe rated using nominal thickness alone could, at its thinnest point, burst well below the stated rating. Multiplying by 0.875 builds that worst-case tolerance directly into the rating, so the published API burst pressure reflects the weakest point the pipe is allowed to have, not just its average wall thickness.

The quick solve tab reports both the derated API rating and the undiscounted theoretical value side by side, so the size of that safety margin is easy to see for any combination of pipe size and grade.

Comparing Burst Pressure Across Steel Grades

Because Barlow’s equation is linear in yield strength, moving from a lower grade like H-40 to a higher grade like N-80 raises burst pressure in direct proportion, with no change to pipe size or weight. The grade comparison tab holds OD and wall thickness fixed and varies only minimum yield strength across rows, isolating exactly how much grade selection alone changes the rated burst pressure of a given pipe body.

Solving for Required Wall Thickness

Casing design often works backward from a required burst rating, set by expected well control pressures, to the minimum wall thickness a given grade must carry. Rearranging Barlow’s equation makes that solve direct: t = PB x OD / (0.875 x 2 x Yp). The reverse tab defaults to the same 4.5in, J-55 example verified below, confirming the required thickness matches the published API casing table value exactly.

Common Mistakes

Forgetting the 0.875 derating factor and quoting the theoretical Barlow value as if it were the API-rated burst pressure, which overstates the true rating by about 14%.

Using burst pressure alone to qualify a casing string without also checking collapse resistance and axial (tension and compression) loads, since a full casing design must satisfy all three failure modes simultaneously.

Applying a safety design factor of exactly 1.0 against the API rating in practice, when actual well design typically requires a burst design factor above 1.0, commonly around 1.1 to 1.25, on top of the API-rated value itself.

Mixing up outside diameter and inside diameter when reading pipe specification tables, since Barlow’s equation as applied here uses nominal outside diameter, not inside diameter or average diameter.

Who Uses This

Drilling and completion engineers selecting casing and tubing grades and weights for a well design.

Well control specialists checking that a given string can hold anticipated surface and shut-in pressures.

Petroleum engineering students working through pipe body strength calculations.

Procurement and tubular sales teams cross-checking published API burst ratings against catalog data.

Try It Above

Use the quick solve tab to convert outside diameter, wall thickness, and minimum yield strength directly into API burst pressure, move to the grade comparison tab to see how much steel grade alone changes the rating, or use the solve tab when you need the wall thickness required to reach a target burst pressure.

Related Calculators and References

Continue building your workflow with the Fracture Gradient Calculator. For background and a verified worked example matching the defaults above, see DrillingFormulas.com: Burst (Internal Yield Pressure) Property of Tubular.