K-factor calculator
Back-calculate the K-factor of your material and tooling from a bend you have measured on a test part.
How it is calculated
K = (BA ÷ θ − R) ÷ T, with θ in radians. Rearranged from BA = θ × (R + K × T).
Worked example
T = 2 mm, R = 4 mm, 90°, measured BA = 7.54 mm: K = (7.54 ÷ 1.5708 − 4) ÷ 2 = 0.40.
Steps
- Bend a test strip of the same material and thickness.
- Measure the developed length between the flats and calculate the bend allowance from the flat length.
- Enter the measured allowance and read the K-factor for your flat patterns.
Frequently asked questions
How do I measure the bend allowance?
Cut a flat strip, measure its flat length, bend it, and measure the outside legs. The allowance is the flat length minus the two outside legs, plus 2 × the outside setback (tan(θ ÷ 2) × (R + T)).
Why does K change with the bend?
The neutral axis moves toward the inside as the bend radius gets smaller, so K falls for tight bends.
Is K-factor the same as the neutral axis position?
Yes. K is the neutral-axis position as a fraction of the thickness, measured from the inside face.
Related calculators
References
K-factor method as in standard sheet metal practice (the Machinery Handbook, sheet metal bending). Confirm with your tooling supplier.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.