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Metal does not fold at a sharp corner, it wraps around a radius, and that arc is longer than the corner it replaces. So if you cut your blank to the finished outside dimensions it comes out too long after bending. This gives you the bend allowance, the bend deduction, the setback and the flat length to cut, from your thickness, radius, angle and K-factor, in inches or millimeters.
Why the flat blank is not the sum of the flanges
When you bend metal, the outside stretches and the inside compresses. Somewhere between them is a line that neither stretches nor shrinks, the neutral line, and its length through the bend is the bend allowance. Add the straight flange lengths plus the bend allowance and you get the true length of flat stock you need. Cut to the outside dimensions instead and every bend leaves you long.
What the K-factor actually is
The K-factor says where that neutral line sits, as a fraction of the thickness measured from the inside face. A K of 0.5 puts it dead center; 0.33 pulls it a third of the way in, toward the inside, which happens with tight radii and softer metal. It is the single value that most changes your numbers, and it shifts with material, radius and the forming method, so the honest move is to bend a test piece and measure back what K your setup really gives.
Allowance, deduction and setback
Bend allowance is what you add to flange lengths measured to the bend lines. Bend deduction is what you subtract when your flanges are measured to the outside mold lines, the way a drawing usually dimensions them, so flat length is flange A plus flange B minus the deduction. Setback is the distance from the outside mold line back to the start of the bend, which is where you set your brake. Three views of the same geometry, each handy for a different measurement.
Tighten it up on scrap
The formulas here are the standard ones and they get you very close. For production, or for thick plate and tight radii where small errors stack up, fold a sample at your real radius and angle, measure the flat it came from, and back out the K-factor that matches. Feed that K back in and your flat patterns land the first time. A few minutes of scrap saves a pile of short or long parts.
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