Bend Allowance Calculator — Sheet Metal Flat-Pattern
Calculate how much flat sheet metal is consumed in a bend (bend allowance), how much to subtract from your flat blank (bend deduction), and the outside set-back — the three numbers needed to design an accurate flat-pattern in any CAD or CNC environment.
°
mm
mm
K-factor
Arc length of material consumed in the bend zone
Neutral axis radius
Bend allowance
Outside set-back
Bend deduction
- 1
Neutral axis radius
2 + 0.33 × 1 = 2.33 mmK-factor locates the neutral axis as a fraction of material thickness. - 2
Bend angle in radians
90° × π ÷ 180 = 1.5708 rad - 3
Bend allowance BA
1.5708 × 2.33 = 3.660 mm
How does this calculator work?
Bend allowance (BA) = (π/180) × angle × (inside radius + K × thickness) — the arc length of neutral-axis material consumed in the bend. Bend deduction = 2 × OSSB − BA, where OSSB = tan(angle/2) × (radius + thickness). K = 0.33 for air bending, 0.50 for coining.
Formula
How this is calculated
When sheet metal is bent, the outer surface stretches and the inner surface compresses. Somewhere between them lies the **neutral axis** — the layer that neither stretches nor compresses. The K-factor (0 < K < 1) describes where the neutral axis sits as a fraction of the material thickness: K = 0.33 means it is one-third of the way from the inside surface, typical for air bending of soft materials like aluminium. K = 0.50 (the geometric mid-plane) applies in coining or bottoming dies.
The **bend allowance (BA)** is the arc length of the neutral axis through the bend zone — i.e., how much flat material is "used up" by the bend. To make a flat blank that folds into the right finished part, you add all straight (flange) lengths and then add one BA for each bend. The **bend deduction (BD)** is an equivalent shortcut: subtract one BD from the sum of the two outer flange lengths measured to the mold-line intersection.
The **outside set-back (OSSB)** is the distance from the tangent line (where the straight flange meets the curved bend zone) to the mold-line corner on the outside surface. It depends on the geometry of a right triangle formed by the bend angle: OSSB = tan(A/2) × (R + T). These three values — BA, BD, OSSB — are the core outputs for generating flat patterns in sheet-metal CAD (SolidWorks, Creo, Inventor, Fusion 360) or CNC press-brake programming.
Frequently asked questions
K-factor depends on material and bending method. A widely accepted starting point is 0.33 for air bending of soft materials (aluminium, soft copper), 0.38 for mild steel in typical fabrication, 0.42 for semi-hard materials, and 0.50 for coining/bottoming. Your press-brake or material supplier may provide empirically measured values specific to your setup — always use measured data if available.
The inside bend radius is the radius of curvature at the inner (compressed) face of the bend. It is often roughly equal to the material thickness for air bending. A larger inside radius produces a gentler bend and a larger bend allowance; a smaller radius (approaching zero for sharp bends) produces a shorter arc but risks cracking, especially in hard materials.
Measure your two flange lengths from the outside mold-line corners to the part edges. Add them together, then subtract one bend deduction (BD) per bend. The result is the flat blank dimension in that direction. Bend allowance does the same job but works from the tangent-line flange lengths (inside the bend zone), so choose one method and use it consistently.
Also known as
TG we-Calculate Editorial Team. (2026). Bend Allowance Calculator — Sheet Metal Flat-Pattern [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/bend-allowance-calculator
TG we-Calculate Editorial Team. "Bend Allowance Calculator — Sheet Metal Flat-Pattern." TG we-Calculate. 2026. https://we-calculate.com/calculator/bend-allowance-calculator.
TG we-Calculate Editorial Team, "Bend Allowance Calculator — Sheet Metal Flat-Pattern," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/bend-allowance-calculator
@misc{wecalculate_bend_allowance_calculator, title = {Bend Allowance Calculator — Sheet Metal Flat-Pattern}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/bend-allowance-calculator}}, year = {2026}, note = {TG we-Calculate} }
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