K-factor: K-factor is the ratio that locates the neutral axis inside a sheet metal bend, used to calculate the flat length of a part before it is bent.
When sheet bends, the outside stretches and the inside is compressed. Somewhere between them is a layer, the neutral axis, that keeps its length. The K-factor is the distance from the inside face to that axis divided by the thickness, usually somewhere between about 0.3 and 0.5. CAD software uses it to work out the bend allowance and unfold the part into its flat pattern.
What it means for the price
The K-factor never appears on a quote, but a wrong one makes the blank the wrong size, and the flanges come out long or short. Shops tune their K-factors for each combination of material, thickness and tooling from test bends. Using the shop’s values instead of a CAD default avoids scrapped first parts and remakes, which are a real cost even when nobody prices them.
Example: bend allowance equals the bend angle in radians times (inside radius + K × thickness). For a 90° bend with a 2 mm radius in 2 mm sheet and K = 0.44, that is 1.571 × (2 + 0.88), about 4.52 mm. The figures are an example.
Common confusions
K-factor, bend allowance and bend deduction are three ways of expressing the same correction. Check which one a CAD system or a shop expects. The K-factor also depends on the bend radius, so one value does not fit every bend. Read design for sheet metal.
Go deeper: Design for sheet metal: bend radii, reliefs, hole distances and flanges