Sheet Metal Bend Allowance Calculator
Bend allowance, bend deduction, and minimum bend radius for a press-brake air bend.
Enter material, thickness, bend angle, and inside radius to get the flat-pattern development length correction.
Why bend allowance matters
When sheet metal bends, the material along the bend stretches on the outside and compresses on the inside. Somewhere through the thickness sits a neutral axis that neither stretches nor compresses — bend allowance is the arc length of that neutral axis through the bend.
Flat-pattern development starts from the finished part's outside dimensions and subtracts a correction (bend deduction) at each bend, rather than adding the bend allowance to straight leg lengths. Both numbers describe the same physical neutral-axis shift, expressed for two different layout workflows.
The K-factor sets where that neutral axis actually sits, as a fraction of material thickness from the inside face. In principle it shifts with material ductility and the ratio of inside radius to thickness, along with forming method and tooling. This calculator does not model those geometry effects — when no K-factor is entered, it falls back to a conventional default for the selected material family (a fixed value per material, not a function of the entered thickness or radius).
When to Use This
- Laying out a flat pattern before sending a part to a press brake.
- Checking whether a specified bend radius is achievable for a given material and thickness.
- Converting between bend allowance and bend deduction conventions used by different CAD/CAM tools.
Common Mistakes
- Using a single K-factor for every material — ductile materials (soft aluminum) and harder materials (stainless) shift the neutral axis differently at the same thickness and radius.
- Ignoring minimum bend radius — bending tighter than the material supports risks cracking on the outside face, not just an inaccurate flat pattern.
- Mixing up bend allowance and bend deduction in flat-pattern math — they correct the layout in opposite directions.
FAQ
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the sum of the flange lengths measured to the bend tangent lines. Bend deduction is subtracted from the sum of the flange lengths measured to their outside intersection point. They describe the same neutral-axis correction from two different starting measurements.
Where does the default K-factor come from?
When no K-factor is entered, it defaults to a fixed value per material family (e.g. 0.44 for mild steel, 0.40 for aluminum 5052) — a conventional per-material default, not derived from a specific measurement or from the entered thickness and radius. Enter a K-factor directly if you have a measured or tooling-specific value.
What happens if the inside radius is smaller than the recommended minimum?
The result still calculates, but a warning is returned — bending tighter than the recommended minimum radius for a material and thickness risks surface cracking on the outside of the bend, independent of whether the flat-pattern math is correct.