When sheet metal bends, the material along the inside face compresses and the outside stretches; somewhere between lies the neutral axis, the fiber that keeps its length. The K-factor locates it as a fraction of the thickness measured from the inside face — K = 0.44 puts the neutral axis at 44% of the thickness. It exists as a tunable number because bending shifts the axis inward from the geometric middle: it can never exceed 0.50 (the axis cannot sit past mid-thickness), and how far below that it lands depends on material, radius-to-thickness ratio, and bending method.
The working values are narrower than the theory suggests. Air-bent low-carbon steel over a V-die opening near 8× the thickness lands at the industry baseline K = 0.44 — the default most CAD sheet-metal engines ship — with air bending generally producing 0.40–0.45. Harder forming (tight radii, bottoming) pulls K down toward 0.40 or below, gentle large-radius bends drift toward 0.50, and published charts rarely run below about 0.30 even for severe coining. For production accuracy the advice is empirical: bend a test coupon, measure the flat that produced the right part, and back-solve K — a measured value always beats a chart.
K earns its keep in the flat-pattern arithmetic: the neutral radius is R + K·T, so the bend allowance — the arc of flat material the bend consumes — is BA = θ·(π/180)·(R + K·T). For 2 mm mild steel bent 90° over a 3 mm inside radius at K = 0.44, the neutral radius is 3.88 mm and the arc works out to 6.095 mm, the number the whole blank length calculation hinges on.