Bend Allowance Calculator
Bend allowance, bend deduction, and flat pattern length from thickness, radius, angle, and K-factor
When sheet metal bends, the material stretches on the outside and compresses on the inside of the bend. The neutral axis — the layer that keeps its length — sits somewhere in between, and the K-factor describes where: K = t/T, the neutral axis depth as a fraction of thickness. That position is what decides how long your flat blank must be.
Bend allowance (BA) is the arc length of the neutral axis through the bend: BA = (π/180) × angle × (R + K×T). Bend deduction (BD) is what you subtract from the sum of the outside flange dimensions: BD = 2×OSSB − BA, where OSSB = (R + T) × tan(angle/2). Enter your flanges and this calculator returns the exact flat length.
K-factor in practice: ~0.33 for sharp bends (R < T), ~0.40 for typical air bending (R ≈ T), approaching 0.50 for large radii (R > 2T). For precision work, measure it: bend a test strip, measure the flat and formed lengths, and back-solve — your press, die, and material combination gives a repeatable K.
Frequently Asked Questions
What is the K-factor in sheet metal bending?
The K-factor is the position of the neutral axis (the non-stretching layer) as a fraction of material thickness, measured from the inside of the bend. It ranges 0.30–0.50 in practice; 0.40 is the common default for air-bent steel with R ≈ T.
What is the difference between bend allowance and bend deduction?
Bend allowance is the length of material consumed by the bend arc (added when working from tangent-to-tangent leg lengths). Bend deduction is subtracted from the sum of outside flange dimensions. They describe the same geometry from two measuring conventions: BD = 2×OSSB − BA.
How do I calculate flat pattern length?
Measure both flanges to the outside apex of the bend, then: flat = flange A + flange B − bend deduction. For a 90° bend in 2 mm steel with a 2 mm inner radius and K = 0.40, BD ≈ 3.6 mm.
Why does my CAD software give a different value?
CAD systems default to their own K-factor (SolidWorks 0.5, others 0.44 or bend tables). If the K differs from your shop’s real value, flats will be off by a few tenths per bend — set your measured K in the CAD sheet metal settings for parts that must fit.