Sheet Metal Bend Allowance Calculator
Bend allowance, bend deduction and flat length from angle, inside radius, thickness and K-factor
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Calculates bend allowance, bend deduction and setbacks, the flat blank length of a part with several bends, and the K-factor from a test bend.
How to use
- Under "One bend", enter "Thickness T", "Inside bend radius R" and "K-factor", and choose how the angle is entered.
- Read the bend allowance and bend deduction; "Working" shows the formulas with your numbers.
- Under "Flat length of a part", choose "Outside dimensions" or "Inside dimensions", enter the flanges and bends, and use "Add a bend" for more.
- Under "K-factor from a test bend", enter a test strip's flat length and legs, then press "Use this K-factor above".
How it works
- BA = A × π ÷ 180 × (R + K × T), OSSB = tan(A ÷ 2) × (R + T), BD = 2 × OSSB − BA; A is the angle the sheet is bent through.
- Flat length = sum of outside flange dimensions − sum of bend deductions, checked again as straight parts plus bend allowances.
- K is usually 0.3 to 0.5 but depends on material and tooling. Springback is not covered.
Questions and answers
- What is the difference between bend allowance and bend deduction?
- Bend allowance is the neutral-axis length through the bend; bend deduction is subtracted from the outside dimensions. T 2 mm, R 2 mm, K 0.33, 90°: BA 4.178 mm, BD 3.822 mm.
- How do I calculate the flat length of an L bracket?
- Add the two outside legs and subtract one bend deduction: 50 + 30 − 3.822 = 76.178 mm.
- What K-factor should I use?
- Usually 0.3 to 0.5, nearer 0.5 for large radii. A test bend gives the value for your material and press brake.
- Bend angle or included angle?
- The bend angle is how far the sheet is bent; the included angle is what remains between the flanges. They add up to 180°.