Sheet Metal Bending Allowance Calculator
Calculate bend allowance, bend deduction, and flat blank length for sheet metal.
Enter material thickness, bend angle, inside radius, and K-factor.
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.