Sheet Metal Bending Allowance Calculator
Calculate bend allowance, bend deduction, and flat pattern length for sheet metal fabrication.
When sheet metal is bent, the material on the outside of the bend stretches while the inside compresses. To produce accurate flat patterns, you must calculate the bend allowance (BA) — the arc length of the neutral axis through the bend.
Bend Allowance Formula: BA = A × (π / 180) × (R + K × T)
Where:
- A = bend angle in degrees
- R = inside bend radius
- T = material thickness
- K = K-factor (position of neutral axis, typically 0.33–0.50)
K-Factor Values:
| Material | Soft/Annealed | Half-Hard | Full Hard |
|---|---|---|---|
| Aluminum | 0.33 | 0.38 | 0.40 |
| Mild Steel | 0.33 | 0.40 | 0.45 |
| Stainless Steel | 0.35 | 0.42 | 0.48 |
| Copper | 0.30 | 0.35 | 0.38 |
Bend Deduction (BD): BD = 2 × (R + T) × tan(A / 2) - BA
Flat Pattern Length: L_flat = Leg1 + Leg2 + BA
Or equivalently: L_flat = Leg1 + Leg2 + (Leg1_to_apex + Leg2_to_apex) - BD
Worked Example — 90° bend in 1.5 mm mild steel, 3 mm inside radius:
- K-factor (soft mild steel): 0.33
- BA = 90 × (π/180) × (3 + 0.33 × 1.5)
- BA = 1.5708 × (3 + 0.495) = 1.5708 × 3.495 = 5.49 mm
- BD = 2 × (3 + 1.5) × tan(45°) - 5.49 = 2 × 4.5 × 1 - 5.49 = 9.0 - 5.49 = 3.51 mm
If Leg1 = 50 mm and Leg2 = 30 mm:
- Flat length = 50 + 30 + 5.49 = 85.49 mm
Minimum Bend Radius: As a rule of thumb, the minimum inside bend radius should be at least equal to the material thickness for soft metals and 2× thickness for hard metals. Going below minimum radius causes cracking on the outer surface.