Engineering Tools

Sheet Metal Bend Allowance

Compute bend allowance, bend deduction, outside setback and the flat-pattern (blank) length for a single bend, using the K-factor method. Pick a material preset for a starting K-factor, or enter your own.

Material (sets K-factor)

K-factor

Thickness in

Inside radius in

Bend angle °

Leg A — outside in

Leg B — outside in

Flat-pattern length (A + B − BD)

0.000in

Bend allowance

0.000 in

Bend deduction

0.000 in

Outside setback

0.000 in

The formulas used

For a bend angle A (the number of degrees the material is bent through), inside radius R, thickness T and K-factor K, the neutral-axis arc length is the bend allowance:

BA = (π / 180) · A · (R + K · T)

The outside setback is OSSB = (R + T) · tan(A / 2), and the bend deduction is BD = 2 · OSSB − BA. When the two flange lengths are measured to the outside mold lines, the flat blank length is simply Leg A + Leg B − BD. The K-factor sets where the neutral axis sits through the thickness; it typically runs from about 0.33 for soft alloys to 0.45–0.5 for harder materials, and is best confirmed by a test bend on your press and tooling.

Typical starting K-factors

Material	K (approx.)
Soft aluminum, soft brass/copper	0.33
Mild / cold-rolled steel	0.38–0.42
Stainless, hard / spring materials	0.45–0.50

Results are theoretical and depend on the actual K-factor for your material, tooling and forming method (air bend vs. bottoming vs. coining). For production parts, confirm with a sample bend and adjust K to match measured results.

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