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How It Works
Barlow's formula calculates the maximum internal pressure a cylindrical pipe can withstand based on its wall thickness, outside diameter, and the allowable stress of the material. For aluminum pipe the typical allowable stress is 7,500 psi. You can rearrange the formula to solve for any of the four variables.
Example Problem
An aluminum pipe has an outside diameter of 6 in and a wall thickness of 0.25 in. Using S = 7,500 psi, what is the pressure rating?
- PR = 2 × 7,500 × 0.25 / 6
- PR = 3,750 / 6 = 625 psi
Frequently Asked Questions
What is the allowable stress for aluminum pipe?
Under most conditions S = 7,500 psi is used for aluminum irrigation and process piping. Higher-alloy aluminum may have different values specified by the manufacturer.
How does wall thickness affect pressure rating?
Pressure rating is directly proportional to wall thickness. Doubling the wall thickness doubles the allowable pressure, assuming the same outside diameter and material.
When should I use aluminum pipe instead of steel?
Aluminum is preferred when corrosion resistance and light weight matter, such as in agricultural irrigation, portable piping, and marine applications. Steel is stronger per unit cost and better for high-pressure industrial use.
Related Calculators
- Steel Pipe Design Calculator — Barlow's formula for steel pipe.
- Plastic Pipe Design Calculator — pressure class equations for plastic pipe.
- Ductile Iron Pipe Calculator — wall thickness and pressure for ductile iron.
- Pipe Hydrostatic Pressure Calculator — external water pressure on buried pipes.
- Pressure Converter — convert between psi, kPa, bar, and other pressure units.
National Resources Conservation Service. National Engineering Handbook. 1995. United States Department of Agriculture.