K-Factor (Bend Allowance) Calculator

Find the flat-pattern length needed for a sheet metal bend.

How the Math Works

The K-Factor (Bend Allowance) Calculator uses the formula BA = (π/180) × angle × (IR + K × thickness) to determine how much material length is consumed when bending metal. This equation converts the bend angle from degrees to radians (via π/180), then multiplies by the sum of the inner radius (IR) and the K-factor times the material thickness. The K-factor represents the ratio of the neutral axis depth to the material thickness, accounting for how the material deforms during bending. The result, called Bend Allowance (BA), gives the arc length of the bend's neutral axis that must be added to flat pattern dimensions for accurate part fabrication.

Practical Applications

To apply this calculation, start by determining your material's K-factor through testing or manufacturer specifications. Next, measure the inner bend radius (IR) and material thickness, then identify the bend angle needed for your part. Input these values into the calculator to find the bend allowance, which you'll add to your flat pattern layout when creating sheet metal parts. For example, if fabricating a bracket requiring a 90-degree bend with a 0.5mm inner radius, 2mm thickness, and K-factor of 0.4, the calculator determines exactly how much material length to allocate for the bend itself, ensuring your final part achieves the correct dimensions after forming.

Day-to-Day Use

This calculator helps you avoid costly mistakes in everyday metalworking projects, whether you're building a DIY workbench, repairing home appliances, or creating custom automotive parts. By accurately calculating bend allowances, you'll minimize material waste, reduce the need for test bends, and achieve professional-quality results without expensive rework. It's especially valuable for hobbyists and small shop fabricators who don't have access to expensive manufacturing equipment but still need precise, repeatable bends for projects ranging from furnace repairs to custom furniture fabrication.

Worked example

90° bend, IR 2 mm, 1 mm thick, K 0.33 → BA ~3.66 mm.

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