K-Factor Calculator

K-factor is a ratio that describes where the neutral bend line sits within the material's thickness during a bend, measured from the inside surface. A K-factor of 0.5 means the neutral axis sits at the exact center of the material thickness; lower values mean it sits closer to the inside surface.

Bend allowance equals the bend angle in radians multiplied by the inside radius plus K-factor times material thickness: BA = angle(rad) x (R + K x T). For a 90 degree bend with a 2 mm inside radius, 2 mm thickness, and K-factor 0.44, BA = 1.5708 x (2 + 0.44 x 2) = 4.524 mm.

0.44 is a commonly used default estimate, sometimes called the 'rule of 0.44', suitable for general air bending in mild steel. Tighter bend radii relative to thickness tend to have lower K-factors (around 0.33), while more generous radii tend toward 0.5. Precise production work often determines K-factor experimentally for the specific material and tooling.

Bend allowance is added to the sum of the flat leg lengths (measured to the bend apex) to get the total flat pattern length. Bend deduction is subtracted from the sum of the flat leg lengths (measured to the outside bend corners) instead. Both give the same flat pattern length, they simply use different reference measurement points.

Outside setback (OSSB) is the distance from the bend tangent point to the theoretical sharp corner of the bend, calculated as tan(angle/2) times (radius + thickness). It is used to derive bend deduction from bend allowance, and to lay out tooling and part geometry.

Yes. Different materials deform differently during bending, softer, more ductile materials tend to have the neutral axis shift differently than harder, less ductile materials, which affects the appropriate K-factor for accurate flat pattern calculations.

Yes, the ratio of bend radius to material thickness (R/T ratio) significantly affects K-factor. Tighter radii relative to thickness compress the inside material more, shifting the neutral axis closer to the inside surface and lowering the effective K-factor.

Bend allowance lets designers and fabricators calculate the correct flat pattern size needed to produce a part with specific finished dimensions after bending. Getting bend allowance wrong results in a formed part that is too long or too short compared to the intended design.

It is uncommon but possible in certain bending conditions, though most practical air-bending and bottom-bending operations produce K-factors between roughly 0.3 and 0.5. Values well outside this range usually indicate unusual tooling, material, or bending process conditions worth verifying experimentally.

Total flat pattern length equals the sum of all flat leg lengths (measured from the part's outside edge to the bend tangent line) plus the bend allowance for each bend, giving the total length of flat material needed before forming.