Sheet Metal Bend Allowance Calculator
Calculate bend allowance, bend deduction, outside setback and the flat blank length for a sheet metal bend. Uses the K-factor method: BA = (pi/180) * A * (R + K * T), which is the standard model used inside SolidWorks, Fusion and most press brake CAM packages.
How to use this calculator
- Enter material thickness T, inside bend radius R and bend angle A.
- Enter the K-factor for your material and process, or use the table hint as a starting point.
- Enter the two flange lengths to get the flat blank length and the bend deduction.
Frequently asked questions
What K-factor should I use?
As a starting point use 0.33 for a cold-rolled steel air bend with R near T, 0.40 to 0.45 for R between 1 and 3 times T, and 0.5 for a large radius. Always calibrate against a test bend on your own press brake and tooling.
What is the difference between bend allowance and bend deduction?
Bend allowance is the arc length of the neutral axis through the bend. Bend deduction is how much you subtract from the sum of the flange lengths to get the flat length: BD = 2 * OSSB - BA.
Why is my flat pattern short in reality?
Real bends stretch more than the K-factor model predicts, especially with tight radii, and material thickness and grain direction vary within tolerance. Measure a test piece and back-solve the K-factor.