K-Factor Calculator

It describes how far the neutral axis sits from the inside surface, as a fraction of material thickness -- 0.33 means it sits about a third of the way in from the inside, while 0.5 means it sits exactly at the material's center. A lower K-factor produces a shorter calculated flat-pattern length for the same bend, because less of the material is being treated as stretched on the outside of the bend.

Because a tight radius compresses the material's inner fibers more severely relative to its thickness, and that concentrated compression pushes the neutral axis outward, away from the inside surface and toward the geometric center. As the Inside Bend Radius to Material Thickness ratio grows past about 1, this calculator lets the K-factor climb toward 0.5, the value used for very generous, thin-stock bends.

Because different press-brake operations apply different amounts of force through the material. Air bending only presses the sheet partway into a V-die, so it deforms the material the least; bottom bending and especially coining force the sheet fully against the die, compressing it further and pushing the neutral axis outward, which is why this calculator adds a small upward adjustment for those two methods.

Treat it as a well-informed starting point, not a guaranteed final answer. K-factor is an empirical approximation that genuinely varies by specific alloy, temper, grain direction, and tooling setup in ways a generic calculator cannot fully capture, so the standard shop practice -- and this calculator's own guidance -- is to make a test bend, measure the actual developed length, and back-calculate the real K-factor for your specific material and setup.

Cracking on the outside surface of the bend. As a sheet bends, the outer surface stretches, and if the radius is too tight relative to the material's thickness and ductility, that outer fiber can exceed its elongation limit and crack. This calculator scales its minimum-radius recommendation to material thickness and tightens it further for less ductile materials like stainless steel and spring steel, which tolerate less stretching before failing.