The K-factor is the ratio of the distance from the inside surface to the neutral axis, divided by the material thickness. It captures how far the neutral axis shifts toward the inside of the bend as the material is formed.
K-factor typically ranges from about 0.33 to 0.50. Softer materials and larger radii push K higher; tighter radii and harder materials lower it. A value near 0.44 is a common starting point for air-bending mild steel, but the only reliable K-factor is one calibrated from your own test bends.