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Bend allowance is the arc length of the neutral axis through the bend. It depends on bend angle, inside bend radius, and K-factor (neutral axis position as fraction of thickness).
BA = (π/180) × Bend angle × (Inside radius + K × Thickness)K-factor locates the neutral axis within the sheet thickness during bending. Typical values range 0.33 (tight bend, soft material) to 0.50 (large radius, hard material). Default 0.44–0.45 works for many air bends.
Neutral axis offset = K × Thickness from inner surfaceSum all straight leg lengths plus bend allowances for each bend. Alternatively subtract bend deductions from the total outside dimension of the formed part.
Flat length = Σ(Leg lengths) + Σ(BA) or Σ(Outside dims) − Σ(BD)Updated: July 2026
0.060-inch thick steel, 1× material thickness inside radius (0.060 in), 90° bend, K=0.44. BA ≈ 0.130 inch. Add this to leg lengths for each 90° bend in the flat pattern.
Flat strip 6 inches wide forming a 2-inch channel with 1-inch legs. Two 90° bends at K=0.44, t=0.075 in, R=0.075 in: total BA ≈ 0.33 inch. Flat blank length ≈ 6 + 0.33 = 6.33 inches.
0.125-inch 5052-H32 aluminum, R=0.125 in, 45° bend, K=0.38. BA ≈ 0.092 inch. Aluminum often uses lower K-factors due to greater material flow toward the inner radius.
Bend allowance formulas use inside bend radius (where material contacts the die). Outside radius equals inside radius plus material thickness. Using outside radius overstates the allowance.
Each bend adds its own allowance. A box with four bends needs four BA values added (or deducted). Forgetting even one bend causes cumulative error across the flat pattern.
Sheet metal parts must be cut to a flat pattern length that accounts for material stretching during bending. The bending allowance and K-factor determine how much extra length to add at each bend. This calculator computes flat pattern dimensions for press brake and hand-forming work.