Sheet Metal Bend Allowance Calculator
Estimate bend allowance and flat blank length for a single-bend sheet-metal part using angle, radius, thickness, and K-factor.
- Enter bend geometry Set bend angle (degrees), inside bend radius, material thickness, and K-factor for your tooling/material combination.
- Enter straight flange lengths Provide flange lengths on each side of the bend so the tool can estimate developed blank length.
- Calculate the flat pattern Press Calculate to get bend allowance and resulting flat blank length in inches.
What is bend allowance in sheet-metal fabrication?
Bend allowance is the arc length of the neutral axis through the bend region. It is commonly computed as BA = θ(rad) × (R + K × T), where θ is bend angle, R is inside radius, T is thickness, and K locates the neutral axis.
What K-factor should I start with?
A common starting range is 0.30 to 0.50 depending on material and tooling, with many mild-steel air-bend setups near 0.42 to 0.45. Use this tool for first-pass values, then calibrate against your brake and bend-test coupons.
Why can finished parts still be off after using bend allowance?
Real parts vary with die opening, punch radius, grain direction, springback, and material lot. Production work usually applies shop-specific bend deduction tables from measured test bends rather than pure theoretical values.