Bend allowance and flat length: K-factor formula and calculator

Define the inputs first

This example handles one bend in a uniform-thickness sheet. All lengths are in mm. T is thickness, R is the formed inside radius, and K is the ratio of the neutral-axis offset from the inside surface to thickness. θ is the angle turned from flat: enter 90° for a right-angle bend. It is not always the drawing’s included angle.

A and B are outside flange dimensions extended to the virtual sharp corner, not straight lengths ending at the arc tangent points. The flat-length formula below requires this outside-dimension convention.

Calculate allowance, setback, and deduction

BA is the neutral-axis arc length. Each outside dimension includes a setback, so subtract two setbacks and add the arc length. These equations do not automatically compensate for springback or replace trial-bend calibration.

A hand-checkable example

Use A = 60 mm, B = 40 mm, T = 2 mm, R = 3 mm, θ = 90°, and an assumed K = 0.4. BA = 1.9π ≈ 5.969 mm; SB = 5 mm; BD ≈ 4.031 mm; L ≈ 95.969 mm.

K = 0.4 is an example assumption, not a recommendation for every steel or machine. Actual values should come from trial measurements or validated bend tables for the process.

Calibrate with trial measurements

Record the actual thickness, formed radius, angle, outside flange dimensions, and blank length. With consistent conventions, calculate K = (BA / θrad − R) / T from the measured allowance; θrad is the bend angle in radians. Do not mix results from different materials, tooling, or processes without verification.

Method references

For the neutral-axis definition, K-factor, bend tables, and unfolding methods, see Autodesk Fusion sheet metal rules and Inventor unfold equations. This page implements an example of the single-bend geometry above.