Bend Allowance Calculator
Calculate bend allowance, bend deduction and outside setback for sheet metal, plus the flat blank length of a bent part.
Inputs
Result
Flat length
96.18mm
- Bend allowance
- 4.18mm
- Bend deduction
- 3.82mm
- Outside setback
- 4mm
Formula
BA = π ÷ 180 × A × (R + K × T) · BD = 2 × tan(A ÷ 2) × (R + T) − BA
Results are estimates and should be verified for critical engineering or manufacturing applications.
How it works
When sheet metal is bent, the outside stretches and the inside compresses. Somewhere in between is the neutral axis, which keeps its original length. The K-factor says where that axis sits, as a fraction of the thickness from the inside surface.
- Bend allowance (BA) — the length of the neutral axis through the bend. Add it to the flat (straight) lengths.
- Outside setback (OSSB) — the distance from the tangent of the bend to the mold line (the virtual sharp corner).
- Bend deduction (BD) — what to subtract from the sum of the outside flange dimensions to get the flat blank length.
Enter both flange lengths, measured to the outside of the part, to get the flat length of a single-bend part. For parts with several bends, subtract one bend deduction per bend.
Formula
- BA = π ÷ 180 × A × (R + K × T)
- OSSB = tan(A ÷ 2) × (R + T)
- BD = 2 × OSSB − BA
- Flat length = Flange A + Flange B − BD
- A
- Bend angle in degrees
- R
- Inside bend radius
- T
- Material thickness
- K
- K-factor (0–0.5)
The K-factor depends on material, thickness, radius and tooling. Values around 0.33 are common for air bending with a radius close to the thickness; larger radii move it toward 0.5. Test bends give the most accurate value for your press brake.
Example
A 90° bend in 2 mm sheet, 2 mm inside radius, K = 0.33, flanges 50 × 50 mm
- 1BA = 1.5708 × (2 + 0.33 × 2) = 4.18 mm
- 2OSSB = tan(45°) × (2 + 2) = 4 mm
- 3BD = 2 × 4 − 4.18 = 3.82 mm
- 4Flat length = 50 + 50 − 3.82 = 96.18 mm
Cut the blank at 96.18 mm.
Frequently asked questions
What K-factor should I use?
Start with 0.33 for air bending mild steel with an inside radius close to the material thickness, and around 0.4–0.45 for larger radii. The most reliable value comes from bending a test piece and measuring it.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the flat lengths measured to the bend tangents. Bend deduction is subtracted from the outside dimensions. Both give the same flat length; use the one that matches how your drawing is dimensioned.
Does the bend angle mean the included angle?
No. Enter how far the flange is bent from flat. A part bent into an L shape has a 90° bend; a flange bent to a 135° included angle has a 45° bend.