About this tool
The Sheet Metal Bend Allowance Calculator gives the numbers a fabricator needs to lay out a flat blank that folds to the right size: the bend allowance, the bend deduction, the outside setback and the overall flat-pattern length. Enter the material thickness, inside bend radius, bend angle and K-factor, add the two flange (leg) lengths, and it computes everything in millimetres or inches, locally in your browser.
The bend allowance — the length of the neutral axis through the bend — is BA = (π / 180) × A × (R + K·T), where A is the bend angle in degrees, R the inside radius, T the thickness and K the K-factor locating the neutral axis (typically 0.33 to 0.45; about 0.44 for soft and 0.33 for hard material). The outside setback is OSSB = tan(A/2) × (R + T), and the bend deduction is BD = 2·OSSB − BA. The flat length for a single bend is then the sum of the outside leg lengths minus the bend deduction, so blank = leg A + leg B − BD. These relations come straight from unrolling the bend around its neutral axis.
As a worked example, a 90° bend in 2 mm sheet with a 3 mm inside radius and K = 0.44 gives a bend allowance of (π/180) × 90 × (3 + 0.44 × 2) ≈ 6.10 mm and a bend deduction of about 3.90 mm; two 50 mm outside legs then need a flat blank of 50 + 50 − 3.90 ≈ 96.1 mm. Getting the K-factor right for your material and tooling is the key to accurate parts — dial it in from a test bend and reuse it for the same setup.