Sheet Metal Bend Allowance Calculator

Compute bend allowance, bend deduction, and flat-pattern length for a sheet-metal bend from thickness, inside radius, bend angle, and K-factor — in inches or millimeters. Get the blank size right before you cut.

Bend allowance & K-factor

Material thickness (in)Inside bend radius (in)Bend angle (°)K-factorFlange A — optional (in)Flange B — optional (in)

Enter thickness, inside radius, angle, and K-factor. K-factor varies with material, tooling, and radius — confirm against your shop's test bends for production.

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The formulas

Bend allowance: BA = (π / 180) × angle × (radius + K × thickness)
Bend deduction: BD = 2 × (radius + thickness) × tan(angle / 2) − BA
Flat length (BD method): outside flange A + outside flange B − BD

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Frequently asked questions

What is bend allowance?: Bend allowance is the arc length of the neutral axis through a bend. You add it to the flat portions of the flanges to get the correct flat-pattern (blank) length, so the part comes out to size after forming. BA = (π/180) × angle × (inside radius + K × thickness).
What is the K-factor?: The K-factor is the ratio of the distance from the inside surface to the neutral axis, divided by material thickness. It typically ranges ~0.3–0.5 and depends on material, thickness, inside radius, and tooling. Soft aluminum trends lower (~0.33); stainless trends higher (~0.45). Confirm with test bends for production.
Bend allowance vs. bend deduction?: Bend allowance is added to flat lengths; bend deduction is subtracted from the sum of the outside flange dimensions. Both get you to the same flat length — use whichever matches how your flanges are dimensioned.

Sheet Metal Bend Allowance Calculator

Bend allowance & K-factor

Material thickness (in)Inside bend radius (in)Bend angle (°)K-factorFlange A — optional (in)Flange B — optional (in)

Enter thickness, inside radius, angle, and K-factor. K-factor varies with material, tooling, and radius — confirm against your shop's test bends for production.

Get sheet-metal parts quoted by U.S. shops Find finishing partners

The formulas

Bend allowance: BA = (π / 180) × angle × (radius + K × thickness)

Bend deduction: BD = 2 × (radius + thickness) × tan(angle / 2) − BA

Flat length (BD method): outside flange A + outside flange B − BD

Frequently asked questions

What is bend allowance?

Bend allowance is the arc length of the neutral axis through a bend. You add it to the flat portions of the flanges to get the correct flat-pattern (blank) length, so the part comes out to size after forming. BA = (π/180) × angle × (inside radius + K × thickness).

What is the K-factor?

The K-factor is the ratio of the distance from the inside surface to the neutral axis, divided by material thickness. It typically ranges ~0.3–0.5 and depends on material, thickness, inside radius, and tooling. Soft aluminum trends lower (~0.33); stainless trends higher (~0.45). Confirm with test bends for production.

Bend allowance vs. bend deduction?

Bend allowance is added to flat lengths; bend deduction is subtracted from the sum of the outside flange dimensions. Both get you to the same flat length — use whichever matches how your flanges are dimensioned.