Bend Allowance Calculator

Bend allowance is the length of the neutral axis that passes through a formed sheet metal bend. Fabricators need this value to convert a finished part drawing into an accurate flat pattern. Material on the inside of a bend compresses while material on the outside stretches. Between those regions is a neutral axis whose length remains approximately unchanged, and its position is represented by the K-factor. The calculator uses the standard bend allowance relationship: bend angle in radians multiplied by the sum of inside radius and K-factor times material thickness. A K-factor of 0.5 places the neutral axis halfway through the sheet, while smaller values place it closer to the inside surface. Common practical values often fall between 0.3 and 0.5, but the correct value depends on alloy, temper, thickness, grain direction, tooling, bend method, and radius-to-thickness ratio. Outside setback is the distance from the mold line to the tangent point on each flange. It equals the inside radius plus thickness, multiplied by the tangent of half the bend angle. Bend deduction equals two outside setbacks minus the bend allowance. When a drawing specifies outside flange dimensions, subtracting the bend deduction from their sum gives the developed flat length for one bend. When tangent dimensions are available instead, bend allowance can be added directly between straight sections. Accurate production work should use a K-factor or bend-deduction table verified on the actual press brake, tooling, and material lot. Air bending, bottoming, and coining move the neutral axis differently, and springback changes the angle that must be formed to achieve the desired finished geometry. Units must remain consistent; this page uses millimeters for thickness, radius, and output. The result is ideal for estimates, CAD checks, and initial layouts. Before cutting a large batch, make a test coupon, measure the formed part, and adjust the shop's bend data so the flat pattern reflects real manufacturing conditions rather than a generic theoretical value.