Bend Allowance & Flat Length Calculator
Work out bend allowance, bend deduction, and outside setback from angle, radius, thickness, and K-factor — then the flat blank length to cut before you bend.
Why the flat isn’t just the sum of the legs
When metal bends, the outside stretches and the inside compresses; somewhere between sits the neutral axis, whose length doesn’t change. The bend allowance is the length of that neutral axis through the bend, and it’s what you add to the straight sections to find the correct flat blank. Get it wrong and every bent part comes out over- or under-size.
Know your material thickness from the Gauge & Thickness Converter, then size the finished part’s weight with the Metal Weight Calculator.
Frequently Asked Questions
What is bend allowance?
Bend allowance is the arc length of the neutral axis as the metal wraps around a bend — the amount of material 'used up' in the bend itself. It's calculated as the bend angle in radians times (inside radius + K-factor × thickness). You add the bend allowance to the flat lengths between bends to get the total flat blank length.
What is the K-factor and what value should I use?
The K-factor locates the neutral axis — the line inside the material that neither stretches nor compresses — as a fraction of the thickness from the inside of the bend. It typically ranges from about 0.33 to 0.50. A value near 0.44 is a common starting point for mild-steel air bends, but the right number depends on your material, tooling, and radius, so confirm it with test bends.
What's the difference between bend allowance and bend deduction?
Bend allowance is added to flat segment lengths; bend deduction is subtracted from outside leg dimensions. Bend deduction equals twice the outside setback minus the bend allowance. This calculator gives both, plus the flat blank length worked out from the outside leg dimensions of a single-bend part.
Why is the flat blank shorter than the sum of the legs?
Because the outside corner of a bend is a mitre point that the material never actually reaches — the metal follows a radius instead. The bend deduction accounts for that geometry, which is why the flat you cut is shorter than adding the outside legs together. Always prove the numbers with a test bend before cutting a batch.
Educational estimate. The correct K-factor depends on material, tooling, and radius — prove it with test bends on your own press brake before cutting a batch.