Bend Allowance
Bend allowance, bend deduction, setback, and flat length from thickness, radius, angle, and K-factor — or the K-factor from a test bend.
The metal stretches,
the blank shrinks.
Four formulas, and where the K-factor comes from.
The formulas
Bend allowance is the arc length of the neutral axis: BA = θ × (R + K × T), with θ in radians, R the inside radius, T the thickness. Outside setback runs from the bend tangent to the mould line: OSSB = tan(θ/2) × (R + T). Bend deduction is what to take off the sum of the outside flange lengths: BD = 2 × OSSB − BA, so flat length = L1 + L2 − BD. θ is the bend angle — how far the flange is deflected from flat — and the included angle (180° − θ) is shown next to it, because drawings use either.
What the K-factor is, and the presets
K is where the neutral axis sits through the thickness (0 = inside face, 0.5 = middle). It moves inward as the bend gets tighter. The presets are the usual rules of thumb for air bending mild steel — 0.33 for sharp bends (R < T), 0.42 for R between T and 2T, 0.50 for large radii — and are estimates. Bottoming and coining, hard alloys, and thick plate move K; your own value from a test bend is always better.
Measuring your own K-factor
Cut a blank of known length, bend it with the production tooling, and measure both outside flange lengths. The second tab solves K by running the same formulas backwards: BD = L1 + L2 − flat, BA = 2 × OSSB − BD, K = (BA/θ − R)/T. A result outside 0–1 is flagged: it means a measurement (usually the inside radius) is off. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.