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Sheet-Metal K-Factor & Neutral Axis

Relate bend allowance, angle, inside radius, thickness and neutral-axis position for an ideal circular sheet-metal bend.

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
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For thickness 2 mm and K = 0.5, the neutral axis lies 1 mm from the inner surface. A 180° bend with inside radius 2 mm has allowance 3π mm.

BA = θ(π/180)(Rᵢ + KT); neutral-axis offset = KT

Enter these known values and leave the other values blank.

K is the neutral-axis offset divided by thickness. The retained model allows positive K up to 1; practical values depend on material and forming process.
Use the swept bend angle, not the angle remaining between the flanges. This tool models a single bend up to 360°.
Thickness and bend allowance must be positive. Zero inside radius is an idealized geometric limit, not a claim that a particular material can be formed that tightly.
A measured bend test or qualified tooling data is needed to choose K for production. The formula alone does not predict springback or a minimum safe bend radius.

No. K is a dimensionless ratio; bend allowance is the length of the neutral arc through the bend.

Bookify permits a zero-radius geometric limit and bounds the single swept bend to 360 degrees.