What is bend allowance?

Bend allowance is the arc length of material along the neutral axis as it wraps around the bend radius. Because the neutral axis neither stretches nor compresses, this length is what you add between the flat flanges to lay out the part before bending.

How does bend allowance differ from bend deduction?

Bend allowance is added to the inside flange dimensions, while bend deduction is subtracted from the sum of the outside flange dimensions. Both describe the same bend, just measured from different reference points, and you pick whichever matches how your drawing is dimensioned.

What is the K-factor?

The K-factor is the ratio of the distance from the inside surface to the neutral axis, divided by the material thickness. It typically falls between about 0.3 and 0.5 and depends on the material, the bend method, and the radius-to-thickness ratio. A test bend gives the most reliable value.

Which formula does this tool use?

Bend allowance equals the bend angle in radians times the quantity inside-radius plus K-factor times thickness. Outside setback is the tangent of half the angle times inside-radius plus thickness, and bend deduction is twice the setback minus the bend allowance.

Why is my finished part slightly off?

K-factor and springback vary with tooling, material batch, and grain direction. The math here is exact for the inputs you give, but the real K-factor for your setup may differ, so always confirm with a sample bend before running a production batch.

Sheet Metal Bend Allowance Calculator

Sheet metal does not bend at a sharp corner. It curves around the punch over the bend radius, and the metal on the outside stretches while the inside compresses. Getting a flat blank to fold up to the right finished size means accounting for that behaviour, which is exactly what bend allowance describes.

How it works

The neutral axis is the layer inside the bend that neither stretches nor compresses. Its position is set by the K-factor, and its arc length through the bend is the bend allowance.

angle_rad = bend_angle x pi / 180
BA   = angle_rad x (IR + K x T)
OSSB = tan(angle_rad / 2) x (IR + T)
BD   = 2 x OSSB - BA
flat = leg1 + leg2 - BD

Here IR is the inside bend radius, T is material thickness, and K is the K-factor. The bend angle is the angle the metal turns through, so a part folded into a right angle uses 90 degrees. The flat-pattern length is the sum of the outside flange legs minus the bend deduction, which is what you cut before forming.

Tips and notes

The K-factor is the single most important and most variable input. As a starting point, soft aluminum air bends near 0.33, mild steel near 0.42, and harder or coined bends climb toward 0.5. Bottoming and coining push the neutral axis outward compared with air bending.

For multi-bend parts, calculate the deduction for each bend separately and subtract them all from the summed outside dimensions. Springback is not modelled here, so for tight tolerances bend a test coupon, measure the result, and back-calculate the true K-factor for your tooling and material before committing to a run.