What is Barlow's formula?

Barlow's formula estimates the internal pressure at which hoop stress in a thin-walled pipe reaches the allowable stress. It is pressure equals two times allowable stress times wall thickness divided by outside diameter. It is the standard quick check for pipe pressure capacity.

What design factor should I use?

The design factor multiplies the yield strength to give an allowable stress with a safety margin. Pipelines under ASME B31.4 and B31.8 commonly use around 0.72 in standard locations, while plant piping uses lower values. Always use the factor required by your governing code.

Does this account for corrosion?

No. The result uses the nominal wall thickness with no corrosion allowance, joint efficiency reduction, or temperature derating. Subtract a corrosion allowance from the wall and apply code derating factors for any real design.

Where do the dimensions come from?

Outside diameter and wall thickness come from the ASME B36.10 dimension tables for welded and seamless wrought steel pipe. Inside diameter is the outside diameter minus twice the wall, and weight uses the standard steel pipe weight formula.

Can I use this for copper or stainless?

The pressure changes with material because yield strength changes. The tool lets you pick stainless or copper to set a lower yield, but the dimension table is the steel schedule table; verify dimensions against the correct standard for non-steel piping.

What is the Pipe Schedule & Pressure Rating Calculator?

Looks up nominal pipe size from 1/8 in to 36 in and schedule (10, 40, 80, 120, 160, XXS) in an embedded ASME B36.10 table to return outside diameter, wall thickness, inside diameter, and weight. Calculates working pressure with Barlow's formula for steel, stainless, and copper. It runs free in your browser on Gera Tools, with nothing uploaded.

Pipe Schedule & Pressure Rating Calculator

Name: Pipe Schedule & Pressure Rating Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Picking a pipe means balancing size, schedule, and the pressure it must hold. This calculator reads the real ASME B36.10 dimensions for your size and schedule, then applies Barlow’s formula to estimate the working pressure for steel, stainless, or copper.

How it works

Wall thickness and outside diameter come from the standard pipe table; the pressure follows Barlow’s thin-wall relation:

ID       = OD - 2 × wall
S        = yield_strength × design_factor
pressure = 2 × S × wall / OD
weight   = 10.69 × (OD - wall) × wall   (lb/ft, steel)

A higher schedule means a thicker wall, which raises the pressure rating but narrows the bore and adds weight. The design factor sets how much of the yield strength you are willing to use.

Pipe schedule primer

The schedule number is a legacy designation — it does not correspond to a fixed wall thickness but rather to a thickness that varies with the nominal size. The original intent was that Schedule 40 represented a medium-weight pipe suitable for most general service at common pressures. Higher schedules have thicker walls and higher pressure ratings:

Schedule	Character
10	Thin wall, light service, low pressure
40	Standard weight, general purpose
80	Extra-strong, higher pressures, reduced bore
120	Heavy wall, high-pressure industrial
160	Very heavy wall
XXS	Double extra-strong, extreme service

For a given nominal size, the outside diameter is fixed across all schedules — only the wall thickness changes. This means a larger schedule always reduces the inside diameter and the flow area.

Barlow’s formula explained

Barlow’s formula estimates the hoop stress in a pressurised cylindrical wall:

hoop stress = pressure × OD / (2 × wall)

Rearranged for working pressure:

working pressure = 2 × S × wall / OD

where S is the allowable stress (yield strength × design factor). Hoop stress is the circumferential tension trying to split the cylinder along its length — it is always greater than the axial stress from end caps for thin-walled pipes.

The formula is accurate for thin-walled pipe (wall/OD less than about 10%). For thicker walls (some Schedule 160 and XXS cases at smaller sizes), a Lamé or thick- wall calculation would be more precise, but Barlow remains the standard quick check in most piping codes.

Example and tips

A 4 in Schedule 40 carbon-steel pipe has a 4.5 in OD and 0.237 in wall, giving roughly 2,650 psi of working pressure at a 0.72 design factor. Stepping up to Schedule 80 nearly doubles the wall and the pressure rating but cuts the flow area noticeably, which affects velocity and friction loss.

Always subtract a corrosion allowance from the wall before calculating, and confirm the result against the ASME B31 code that governs your system. This tool does not apply temperature derating, joint efficiency factors, or code-specific stress multipliers — those are required steps before any real design decision.