Bend Allowance Calculator
Calculate flat pattern length for a sheet metal bend using the K-factor.
The K-factor varies by material and process. A tighter radius (R < 2×T) commonly uses around 0.33; a larger radius (R ≥ 2×T) is often closer to 0.40–0.50.
Enter both flange lengths to also calculate the total flat pattern length (optional).
Bend allowance
Enter values to see the bend allowance.
Usage examples
Working out flat pattern length from a drawing
Use the flange lengths and bend radius from a finished part drawing to calculate the flat dimensions before laser cutting or punching.
Comparing results across different K-factors
See how the flat pattern length changes as you adjust the K-factor, for the same radius and thickness.
Cross-checking a bend deduction value against CAD software
Compare the calculated bend deduction against the sheet metal settings in your CAM/CAD software to confirm they match.
Frequently asked questions
What K-factor should I use?
The K-factor genuinely varies with material, bending method (air bending vs. bottoming), and the radius-to-thickness ratio. For precise work, measure it from a test bend or use the value from your CAM software or sheet metal supplier. The 0.33 default here is just a common starting point.
What's the difference between bend allowance and bend deduction?
Bend allowance (BA) is the arc length of the bent region when flattened out. Bend deduction (BD) is the amount subtracted from the sum of the mold-line flange dimensions to get the flat pattern length, related by BD = 2×Setback − BA.
Does setback depend on the K-factor?
No. Setback is determined purely by the outer (mold-line) geometry of the bend, independent of where the neutral axis sits inside the material.
Does this work for angles other than 90 degrees?
Yes. The same formulas apply to any bend angle between 0 and 180 degrees.