Formula
Per-bend bend deduction = (sum of outside leg lengths - measured flat length) / number of bends. Per-bend bend allowance = 2 x outside setback - bend deduction, where outside setback = (inside radius + thickness) x tan(angle / 2). K-factor = ((bend allowance / angle in radians) - inside radius) / thickness.
Standard / source
Reverse calculation from the bend-allowance relation BA = (pi x angle / 180) x (R + K x T), DIN 6935 background, as summarised in the MetalCalz tool-expansion research (2026-10-27). This is distinct from the existing bend-allowance tool: that tool applies a K-factor, this one measures it.
Worked example
Outside legs sum 200 mm, measured flat 192.92 mm, 2 bends, thickness 2 mm, inside radius 2 mm, angle 90 degrees: deduction per bend = (200 - 192.92) / 2 = 3.54 mm. Allowance per bend = 2 x 4.00 - 3.54 = 4.46 mm. K = ((4.46 / 1.5708) - 2) / 2 = 0.42 (0.420 to three decimals from the rounded flat length). These are the calculator defaults.
Safety note
Planning estimate for blank sizing. The back-calculated K-factor applies only to the same material, thickness, grain direction, tooling and bend method as the test piece. Re-test if any of those change, and re-check the measured flat length if the result falls outside the usual 0 to 1 range.