Bend Deduction Calculator
Enter the bend angle, inside radius, material thickness, and K-factor to find the bend allowance and bend deduction.
How bend deduction is calculated
Bend allowance (BA) is the length of the neutral axis arc through the bend, calculated as BA = angle(radians) × (radius + K-factor × thickness). The K-factor represents where the neutral axis sits within the material's thickness, typically between 0.3 and 0.5 depending on the material and bend radius.
Bend deduction (BD) is the amount to subtract from the sum of the flat flange lengths to get the correct flat pattern length, calculated as BD = 2 × outside setback − bend allowance, where outside setback is (radius + thickness) × tan(angle/2).
Frequently asked questions
What's the difference between bend allowance and bend deduction?
Bend allowance is added to the sum of the flange lengths measured to the bend's inside edges. Bend deduction is subtracted from the sum of the flange lengths measured to the bend's outside edges (the more commonly measured dimension). Most shops use bend deduction since outside dimensions are easier to measure.
What K-factor should I use?
0.33 to 0.4469 is a common range depending on the bending method and material. Many shops use 0.33 for a general default, but check your specific material and process — some reference tables provide more precise K-factors for different thickness-to-radius ratios.
Why does this matter for sheet metal fabrication?
Getting the bend deduction right ensures the flat pattern, once bent, produces a part with the correct final dimensions. An incorrect value leads to parts that are too long or too short after bending, wasting material and requiring rework.