When you bend sheet metal, the material stretches around the bend, so the flat blank you cut is smaller than the two finished legs added together. This calculator finds that difference for you.
Bend deduction calculator
Estimates for a single air bend. Actual values vary with tooling. Darioo confirms every bend before cutting.
What the numbers mean
- Bend allowance: the length of material consumed by the bend itself, measured along the neutral axis.
- Bend deduction: the amount you subtract from the sum of the outside legs to get the flat length. This is the number you use most.
- Outside setback: the distance from the bend line to the outside apex of the bend, useful for laying out your flat pattern.
- Flat pattern length: the size of the flat blank before bending. This is what actually gets cut.
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Get an instant quoteK-factor explained
The K-factor tells you where the neutral axis sits inside the material during a bend. The metal on the outside of the bend stretches, the metal on the inside compresses, and somewhere in between is a line that does neither. The K-factor is how far that line sits from the inside surface, as a fraction of the thickness.
K ranges from about 0.3 to 0.5. Softer materials and larger bend radii push it lower; harder materials and tighter radii push it higher. If you do not know the exact value for your setup, the material defaults in the calculator are good starting points: about 0.33 for aluminum and 0.44 for mild steel.
How to use the flat length
The flat pattern length is the number you feed into your CAD flat pattern or DXF. The formula behind it is simple:
Flat length = Leg A + Leg B − Bend deduction
Where Leg A and Leg B are the outside dimensions of your finished part. For parts with several bends, subtract the bend deduction once per bend.
Tips for accurate bends
- Use an inside bend radius at least equal to the material thickness to avoid cracking.
- Keep the same radius across all bends where possible, so one tool and one K-factor apply.
- Give each flange enough length to grip the press brake, at least about 4 times the thickness.
- When exactness matters, let us confirm the bend deduction against our actual tooling. It is part of every quote review.