Sheet metal stretches slightly as it bends, so a formed part is always longer, unfolded, than the simple sum of its finished flange lengths. Bend allowance quantifies that stretch so you can size the flat blank correctly the first time, instead of trimming or re-cutting after a test bend comes up short or long.
How the bend allowance formula works
Bend allowance treats the bend as an arc traced by the material's neutral axis — the layer inside the thickness that neither stretches nor compresses. The arc length is angle (in radians) times the radius from the bend's center to that neutral axis, which is R + K x t: the inside radius plus the K-factor's fraction of the thickness. Multiplying by the angle in radians (angle in degrees x pi/180) converts that radius into the actual arc length added by the bend.
The K-factor is the part of the formula that's genuinely variable in practice. It depends on the bend method (air bending vs. bottoming vs. coining), the material's hardness and grain direction, and the ratio of inside radius to thickness. K = 0.44 is a reasonable default for a general air bend in mild steel, but shops that run consistent tooling and material often dial in their own K-factor from test bends — see the K-Factor Reference tab for a few common starting points.
Inputs and what they mean
Bend angle is the included angle of the bend in degrees — 90° for a right-angle flange, smaller for a shallower bend. Inside radius is measured on the concave face of the bend and is usually set by the punch or die radius on the press brake. Thickness is the sheet's thickness before forming. K-factor has no fixed 'correct' value — it's an empirical ratio, and the default of 0.44 is a starting point, not a universal constant.
On the Flat Length tab, each flange is a straight segment measured to the bend line (not to the part's centerline), and each bend between two flanges has its own angle entry, while sharing the radius, thickness, and K-factor from the Bend Allowance tab — the common case where every bend on a part uses the same tooling and material.
Limits and edge cases
This calculator assumes every bend on a part shares the same inside radius, thickness, and K-factor — realistic for a single sheet formed on one set of tooling, but not for parts mixing bend radii or bend methods (air bend vs. bottoming) in the same layout. It also doesn't model springback, minimum bend radius or flange-length limits for a given material and thickness, or bend relief/notching needed near part edges — those are press-brake and material-specific concerns that a K-factor formula alone can't capture. For high-precision or high-volume work, validate the K-factor and resulting blank size against a physical test bend before committing a production run.