Calculate bend deduction

The formula

When bending, the material bends around a radius instead of a sharp corner. Inside the bend the material is compressed, outside it's stretched — somewhere in between lies the neutral axis, which doesn't change length. That axis determines the actual flat pattern length.

bend allowance (BA) = angle(rad) × (R + K × thickness)
bend deduction (BD) = 2 × (R + thickness) × tan(angle/2) − BA
flat length = sum of outside dimensions − sum of bend deductions

K-factor is the position of the neutral axis as a fraction of the thickness, measured from the inside surface. Without a value of your own, this tool uses a rule of thumb:

This rule of thumb is good enough for a first estimate — the actual K-factor also depends on the material, its properties, and the bending machine/tooling. For critical tolerances, a test bend remains the only certainty; if in doubt, send the drawing along with your request and we'll verify it.

V-die and inside bend radius

With air bending (the usual bending process), the opening of the bending die — the V-die — determines which inside bend radius is achievable. Rule of thumb:

inside bend radius ≈ V-die width / 6
recommended V-die width ≈ 8 × material thickness

Pick a standard V-die width above the calculator and the radius fills in automatically — or leave "Manual radius" and enter the radius yourself if you already know it. A wider die gives a larger radius and a larger bend deduction (so a shorter flat pattern length) with everything else equal.

L, Z and U profile: ready-made shapes

Pick a shape above the calculator and fill in the legs (A, B and optionally C — see the diagram) instead of adding up the outside dimensions yourself. An L-profile has one bend (two legs), Z- and U-profile have two (three legs). Mathematically, Z and U are identical — the bend deduction only depends on angle and radius, not on which direction the legs point. The difference is purely in the shape of the part.

Other shape or multiple bends

Doesn't your part fit L, Z or U? Choose "Other / multiple bends" and add a bend for every corner with "+ Add bend". The tool assumes the same thickness, radius and K-factor for all bends (the usual case within one part) and adds up the deductions. If you also enter the sum of the outside dimensions — measured to the outer corner point of each flange — the tool subtracts the total deduction from it to get the flat pattern length.

Want this checked, or a quote right away?Send your drawing