Sheet Metal Bend Allowance Calculator

Formula

• Bend allowance BA = (π/180) × θ × (R + K × T)
• Outside setback = tan(θ/2) × (R + T)
• Bend deduction BD = 2(R + T)tan(θ/2) − BA
• Flat length = Leg A + Leg B + BA

About this calculator

Part of our Sheet Metal Calculators collection. Bend allowance, K-factor, flat pattern length, and minimum bend radius.

How it works

Bend allowance equals bend angle times the sum of inside radius plus K-factor times thickness. Flat pattern length adds both leg dimensions plus bend allowance. Use your shop K-factor when you have bend test data.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

90° bend, 2 mm thick, inside radius 2 mm, K = 0.33, legs 40 mm each.

1. Neutral axis radius = 2 + 0.33×2 = 2.66 mm
2. BA = (π/2) × 2.66 = 4.18 mm
3. Setback = tan(45°) × 4 = 4.00 mm
4. Flat length = 40 + 40 + 4.18 = 84.18 mm

Result: Bend allowance 4.18 mm, flat pattern 84.18 mm.

When to use

• Flat pattern layout before laser or punch programming
• Checking leg lengths after bend allowance
• Comparing K-factor from bend tests

Limitations

• Single bend only; no multi-bend unfolding
• K-factor should come from your bend table when available
• Air-bend springback not included in flat length

FAQ

What is K-factor?

K-factor locates the neutral axis during bending as a fraction of sheet thickness. Typical values are 0.25 to 0.45 depending on material and inside radius.

Bend allowance vs bend deduction?

Bend allowance is arc length along the neutral axis added to flat length. Bend deduction is the amount removed from leg-to-leg distance when unfolding.

Inside radius vs bend radius?

This calculator uses inside radius Ri at the inner surface of the bend. Outside radius equals Ri + thickness.