The Engineer's Ephemeris

Bend Allowance & Flat Length Calculator

Determine the bend allowance (BA), outside setback (OSSB), bend deduction (BD), and optional flat pattern length from bend angle, inside radius, thickness, and K‑factor. Choose metric or imperial units. Results update live.

Neutral‑axis location as a fraction of thickness (0–0.5). 0.33 is a common starting point.
Using bend angle A (0–180°). Switch to included angle β if needed.
Included β = 180° − A.
Numbers here are in mm.
Numbers here are in mm.
Numbers here are in mm.
Numbers here are in mm.

Formulas used

Bend allowance (BA): BA = (π/180) · A · (R + K·T) (A in degrees). In radians: BA = A · (R + K·T).

Outside setback (OSSB): OSSB = (R + T) · tan(A/2).

Bend deduction (BD): BD = 2·OSSB − BA.

Flat pattern (single bend): Flat = L1 + L2 − BD (equivalently Flat = L1 + L2 + BA − 2·OSSB).

K‑factor positions the neutral axis at a distance K·T from the inside surface (typ. 0.30–0.50).

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