Sheet Metal Bend Allowance Calculator

When sheet metal is bent, material on the inside of the bend is compressed and material on the outside is stretched, so between them lies a neutral surface whose length does not change. Bend allowance is the arc length of that surface through the bend, and it is what replaces the missing corner material when a folded part is unfolded into a flat blank. The neutral surface does not sit at mid-thickness; it shifts toward the inside of the bend, and the K-factor states its position as a fraction of material thickness t, so its radius is R + K x t and its arc length is the included bend angle in radians times that radius. Outside setback, (R + t) x tan(angle/2), locates the intersection of the extended outside faces, and bend deduction follows as twice the setback minus the bend allowance. K-factor is empirical and rises with the ratio of inside radius to thickness: roughly 0.33 for R/t below 1, about 0.38 to 0.42 for R/t between 1 and 3, and approaching 0.5 for generous radii or coined bends. Air bending in a press brake produces an inside radius near one sixth of the V-die opening, and a common shop default is a die opening eight times material thickness. The formulas assume a constant radius, uniform thickness, and no thinning, and they ignore springback, which for cold rolled steel typically requires one to three degrees of overbend. Grain direction and coil-to-coil variation shift the result too, so a test bend on production material remains the only reliable way to fix K.

Flat pattern development is as old as tinsmithing, but numerical bend allowance tables came out of the aircraft industry. All-metal airframes of the 1930s and 1940s required thousands of formed aluminum parts whose blanks had to be cut before bending, and manufacturers issued shop standards giving setback and bend allowance per degree for each combination of thickness and radius. Those tables passed into military and civil training material and survive today in the US Federal Aviation Administration advisory circular AC 43.13-1B on acceptable methods, techniques, and practices for aircraft inspection and repair, which still presents bend allowance, setback, and sight line calculations in essentially their 1940s form. The same tables were reproduced in general references such as Machinery Handbook, published by Industrial Press since 1914, where they served press brake and sheet metal ductwork shops. The K-factor formulation, which replaced lookup tables with a single dimensionless position for the neutral surface, became the standard description once computer aided design systems needed a parametric unfold rather than a chart. From the mid 1990s onward the sheet metal modules in packages such as SolidWorks, Autodesk Inventor, and Solid Edge asked the user directly for a K-factor, bend allowance, or bend deduction. In parallel, press brake controls began storing empirical bend tables measured on the specific machine, tooling, and material, which is how most fabrication shops calibrate the value in practice today.