BenchBend Allowance & Flat Pattern Calculator
Bend allowance, bend deduction, outside setback, and the flat length to cut before forming — from thickness, inside radius, bend angle, and K-factor.
Geometry · your K-factor, our arithmetic
How the bend is measured
Formula
BA = θ(rad) × (R + K·T) arc length of the neutral axis
OSSB = tan(θ/2) × (R + T) outside setback, one leg
BD = 2·OSSB − BA bend deduction
flat = Σ(outside legs) − Σ(BD) developed length
Assumptions
- The K-factor is your input, not our recommendation. It is where the neutral axis actually sits as a fraction of thickness, and it moves with material, temper, tooling, and forming method. Published ranges run roughly 0.33 for a tight radius in soft material to 0.5 for a generous one. Shops calibrate it by bending a coupon and measuring the result. A calculator that hands you a K-factor is guessing on your behalf.
- Legs are outside dimensions, measured to the apex where the outside surfaces would meet. Measuring to the tangent points instead means adding the bend allowance rather than subtracting the deduction.
- One deduction is taken per bend, and n legs means n−1 bends of the geometry entered. Parts whose bends differ need one pass per distinct bend.
- Air bending, springback, bend relief, minimum flange length, and grain direction are not modeled. Nothing here is a forming-limit check.
Worked example
0.125″ material, 0.125″ inside radius, K = 0.446, bent 90°:
BA = (π/2)(0.125 + 0.446 × 0.125) = 0.2839″.
OSSB = tan45° × (0.125 + 0.125) = 0.25″.
BD = 2(0.25) − 0.2839 = 0.2161″. Two 2″ outside legs cut
flat at 4 − 0.2161 = 3.7839″.
Related
- Sheet metal gauge chart — thickness by gauge and material
- All calculators