Inputs
The angle bent through — 90° is a square corner. Works for acute and obtuse bends.
Neutral-axis ratio. Mild steel ≈ 0.38–0.42. See the K-factor calculator.
Results
Let Bendline do the bend maths
Upload a STEP part and Bendline unfolds it to a production DXF — bend allowance, deduction and every flat dimension computed from the geometry, in seconds.
Bend allowance vs. bend deduction
Both describe how much a bend affects the flat blank, but you use them differently. Bend allowance (BA) is the length of material consumed inside the bend region — the arc of the neutral axis. You add it to your flat leg lengths when you measure legs to the start of the bend.
Bend deduction (BD) is what you subtract from the sum of the
outside leg dimensions (the flat legs measured all the way to the theoretical
sharp corner, or apex). Most shop drawings dimension to outside edges, so the
bend-deduction method is the one used on the fabrication floor:
flat length = Σ outside legs − Σ BD.
The formulas
- Outside setback (OSSB) =
tan(θ/2) × (r + t)— the distance from the bend tangent line to the apex, where θ is the bend angle. - Bend allowance =
θ(rad) × (r + K·t)— the neutral-axis arc through the bend. - Bend deduction =
2 × OSSB − BA.
For a 90° bend, tan(45°) = 1, so OSSB simplifies to r + t.
For acute bends (θ > 90° swept) the setback grows quickly, and for obtuse bends
(θ < 90° swept) it shrinks — the tan(θ/2) term handles the general
case. Note that "bend angle" here is the angle the flange is bent through;
if your drawing gives the included angle A between the flanges, the bend angle is
180° − A.
Worked example
t = 2 mm, r = 2 mm, 90° bend, K = 0.38: OSSB = 1 × (2 + 2) = 4.000 mm; BA = 1.5708 × (2 + 0.38×2) = 4.335 mm; BD = 2×4 − 4.335 = 3.665 mm. A part with two 50 mm outside legs then flattens to 50 + 50 − 3.665 = 96.335 mm — try it in the flat-length calculator.