Bend allowance & blank length calculator
Developed length is the number that decides whether your blanks fit. It comes from the bend allowance — and the bend allowance comes from the K-factor, which is an estimate until you have bent a test piece.
Uses the standard K-factor method, BA = (π/180) × bend angle complement × (R + K × t). K is estimated from the radius-to-thickness ratio when left blank — treat it as a starting point and confirm with a test bend on your material.
What the K-factor actually is
When sheet bends, the outside surface stretches and the inside surface compresses. Somewhere between them is a layer that keeps its original length — the neutral axis. The K-factor is where that axis sits, expressed as a fraction of the thickness measured from the inside surface.
K = 0.33 means the neutral axis sits a third of the way through the material. Because the neutral axis does not stretch, the developed length is the length of that axis rather than either face. Get K wrong and every blank in the run comes out the same wrong length — which is exactly the kind of error that survives inspection until the parts do not fit.
Why the automatic estimate is only a starting point
The estimate here follows the usual published table against radius-to-thickness ratio: sharper bends push the neutral axis further in, gentler bends let it drift toward the middle. That gets you close on mild steel.
What it cannot know is your material. Alloy and temper change spring-back. Tooling condition and lubrication change friction. Ram speed changes how the material flows. A harder stainless bends differently from the same gauge of soft steel, and two coils of the same specification can still behave differently.
The practical method is to bend one test piece on your actual setup, measure the resulting legs, and solve backwards for the K that reproduces them. Then use that K for the run. It takes one piece of scrap and about two minutes, and it removes the largest source of error in the calculation.
Inside versus outside dimensions
This calculator expects the two leg lengths measured to the inside of the bend, which is how most drawings dimension a part. If your drawing gives outside dimensions, use the bend deduction instead: flat length = outside leg 1 + outside leg 2 − BD. The BD row above is there for that case.
Need a machine that holds the angle consistently?
Send us the part, the material and the tolerance you have to hit. Repeatable angles come from the machine and the tooling together, and that is what we build for.