Sheet Metal Bend Allowance Calculator
Free Sheet metal bend allowance Calculator for materials specifications projects. Enter dimensions to get material lists and cost estimates.
Reviewed by Abdullah, Technical Content Specialist
Formula
BA = (Angle in radians) x (Inside Radius + K-factor x Thickness)
Bend allowance equals the bend angle converted to radians multiplied by the neutral axis radius. The neutral axis radius is the inside bend radius plus the K-factor times the material thickness. Bend deduction is calculated as: BD = 2 x OSSB - BA, where OSSB (outside setback) = (R + T) x tan(Angle/2).
Worked Examples
Example 1: 90-Degree Bend in 2mm Steel
Problem:Calculate bend allowance for 2mm mild steel with 3mm inside radius at 90 degrees (K=0.40).
Solution:Angle in radians = 90 x pi/180 = 1.5708\nBA = 1.5708 x (3 + 0.40 x 2)\nBA = 1.5708 x 3.80 = 5.969 mm\nOSSB = (3 + 2) x tan(45) = 5.000 mm\nBD = 2 x 5.000 - 5.969 = 4.031 mm
Result:Bend Allowance = 5.969 mm, Bend Deduction = 4.031 mm
Example 2: 120-Degree Bend in 1.5mm Aluminum
Problem:Find bend allowance for 1.5mm aluminum, 2mm radius, 120 degrees, K=0.35.
Solution:Angle in radians = 120 x pi/180 = 2.0944\nBA = 2.0944 x (2 + 0.35 x 1.5)\nBA = 2.0944 x 2.525 = 5.288 mm
Result:Bend Allowance = 5.288 mm
Frequently Asked Questions
What is bend allowance in sheet metal?
Bend allowance is the arc length of the neutral axis through the bend zone. It represents the amount of material consumed by the bend and must be added to the flat flange dimensions to calculate the total flat pattern (blank) length. Without accurate bend allowance, the finished part will be either too long or too short. The formula is BA = (Bend Angle in radians) x (Inside Radius + K-factor x Thickness).
What is the difference between bend allowance and bend deduction?
Bend allowance (BA) is the arc length of the neutral axis and gets added to flat dimensions to calculate blank size. Bend deduction (BD) is subtracted from the sum of the outside flange dimensions. They are related by: BD = 2 x Outside Setback - BA. Both methods produce the same flat pattern length when applied correctly. CAD systems typically use bend allowance internally, while shop floor personnel often prefer bend deduction for manual calculations.
How does the K-factor affect bend allowance?
The K-factor determines where the neutral axis sits within the material thickness during bending. A higher K-factor (closer to 0.50) places the neutral axis near the center, producing a larger bend allowance. A lower K-factor (closer to 0.33) places it toward the inside of the bend, giving a smaller bend allowance. For standard air bending of mild steel, 0.40-0.45 is typical. The K-factor depends on material type, bend radius to thickness ratio, and bending method.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy