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Sheet Metal Bend Allowance & Tonnage Calculator

Bend allowance, deduction, developed flat length and the press tonnage to make the bend.

Metric · IS practiceFormula shown with your numbersNothing stored

How this calculator works

A bend stretches the outside of the material and compresses the inside, and the neutral axis sits somewhere between — its position is the K-factor. Bend allowance is the arc length along that neutral axis. Outside setback and bend deduction convert between the outside dimensions on the drawing and the flat blank you have to cut. Air bending force is estimated from the tensile strength, the material thickness and the die opening.

Formulas used

Bend allowanceBA = (π ÷ 180) × A × (R + K × T)
Outside setbackOSSB = tan(A ÷ 2) × (R + T)
Bend deductionBD = 2 × OSSB − BA
Flat lengthL_flat = Σ flanges − Σ BD
Air bend forceF = 1.42 × Rm × T² ÷ V (tonne/m)

Checked against

3 mm mild steel in a 24 mm V die needs 24.4 tonnes per metre of bend length.

Worth knowing

K-factor is typically 0.33–0.42 for mild steel and depends on the punch radius relative to thickness. Die opening V is normally 6–8× material thickness for air bending.

Sheet Metal Bend Allowance & Tonnage Calculator — common questions

How do I calculate flat length for a sheet metal part?
Add the outside flange dimensions and subtract the bend deduction for each bend. The bend deduction is twice the outside setback minus the bend allowance.
What tonnage press brake do I need?
Air bending force is roughly 1.42 × tensile strength × thickness² ÷ die opening, in tonnes per metre. Multiply by the bend length in metres. A 2 m bend in 3 mm mild steel needs about 49 tonnes.

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