What bend allowance actually tells you
When sheet metal bends, the inside face compresses and the outside stretches. Between them sits a neutral line whose length does not change when the part is flattened — and that arc length is what you must account for through the bend. Get it wrong and every flange comes out long or short. Bend allowance (BA) is that neutral-axis arc length through the bend region.
The bend allowance formula
BA = (A x pi / 180) x (R + K x T), where A is the bend angle in degrees, R is the inside bend radius, T is the material thickness, and K is the K-factor that locates the neutral axis as a fraction of T measured from the inside surface.
Where the K-factor comes from
K usually runs from about 0.3 to 0.5. Tight radii and softer materials draw the neutral axis inward (lower K); generous radii and harder tooling push it toward the middle (higher K). When in doubt, 0.33 is a reasonable starting point for air-bending mild steel and aluminum.
| Inside radius vs thickness (R/T) | Typical K-factor |
|---|---|
| R less than T | 0.33 |
| R = 1 to 3 x T | 0.38 – 0.42 |
| R greater than 3 x T | 0.45 – 0.50 |
Bend allowance vs bend deduction
Both get you to a flat pattern, just from opposite directions. Bend allowance is added to flange lengths measured to the bend lines; bend deduction is subtracted from the sum of the outside dimensions. This tool reports both, so you can use whichever your CAD or press-brake workflow expects.
Worked example
A 90-degree bend in 2 mm steel with a 3 mm inside radius and K = 0.33 gives BA = (90 x pi/180) x (3 + 0.33 x 2) = 5.75 mm. The flat blank needs 5.75 mm of material through the bend, not the 6 mm you might guess from the outside corner.
FAQ
Is the bend angle the included angle or the angle of bend?
It is the angle of bend — how far the metal is rotated away from flat. A part formed to a 90-degree corner has been rotated 90 degrees, so enter 90.
What units does it use?
Whatever you enter. Choose mm or inches and the result returns in the same unit; the formula itself is unit-agnostic.