Flat length
-Bend allowance
-Bend deduction
-Outside setback
-All results
| Inside leg A (flat part before the bend) | - |
| Inside leg B | - |
| Neutral axis radius (r + K t) | - |
| Included angle between flanges | - |
Method and formulas
One bend, neutral axis at K t from the inside surface, flanges measured to the virtual sharp (outside mold line).
| Quantity | Formula |
|---|---|
| Bend allowance | BA = θ (r + K t), θ in radians |
| Outside setback | OSSB = tan(θ/2) (r + t) |
| Bend deduction | BD = 2 OSSB − BA |
| Flat length | L = A + B − BD = (A − OSSB) + (B − OSSB) + BA |
Worked example: 2 mm sheet, 90 degree bend
t = 2 mm, r = 2 mm, K = 0.40, θ = 90°, A = 50 mm, B = 30 mm. BA = 1.5708 x (2 + 0.8) = 4.398 mm. OSSB = tan 45° x 4 = 4.000 mm. BD = 2 x 4 − 4.398 = 3.602 mm. Flat length = 80 − 3.602 = 76.398 mm.
Questions
What is bend allowance?
Bend allowance is the length of the neutral axis through the bend. BA = angle(rad) x (inside radius + K x thickness). Add it to the two inside flat leg lengths to get the flat blank length.
What is the difference between bend allowance and bend deduction?
Bend deduction (BD) is how much you subtract from the sum of the two outside flange dimensions (measured to the virtual sharp) to get the flat length: BD = 2 x outside setback - BA. Both give the same flat length when used with the matching dimensions.
What K-factor should I use?
K is the neutral axis position as a fraction of thickness, between 0 and 0.5. Common starting values for air bending are about 0.33 when the inside radius is below the thickness, about 0.40 for radii of one to three thicknesses, and about 0.50 for larger radii. Real values depend on material, tooling and die width, so confirm with a test bend.
Is the bend angle the included angle?
No. Here the bend angle is the angle the sheet is turned through. A flat sheet is 0 degrees and an L-bend is 90 degrees. The included angle between the flanges is 180 minus the bend angle.
How do I use the flat length for several bends?
Compute each bend separately: the flat length is the sum of all the straight segments plus the bend allowance of every bend. This page handles one bend between two flanges.
What does this calculator not cover?
Springback, tooling or die-width effects, grain direction, hemming and offsets. It is an engineering aid for early blank sizing: verify critical parts with a test bend.
Engineering aid for guidance. K-factor depends on material, tooling and process: confirm with a test bend before cutting production blanks.