Sheet Metal Bend Allowance Calculator

What Is Bend Allowance?

When sheet metal is bent, the material on the outside of the bend stretches and the material on the inside compresses. The bend allowance (BA) is the length of the arc through the neutral axis of the bend — it tells you how much material is "used up" in the bend.

Without accounting for bend allowance, your flat pattern will be the wrong size. Parts won't fit. Holes won't align.

The Formula

BA = angle × (π / 180) × (radius + K × thickness)

Where:

• angle = bend angle in degrees
• radius = inside bend radius
• K = K-factor (0 to 1, typically 0.33 to 0.5)
• thickness = material thickness

K-Factor

The K-factor represents where the neutral axis sits within the material thickness:

• K = 0.33 — typical for air bending with a sharp radius
• K = 0.40 — typical for air bending with a moderate radius
• K = 0.50 — the neutral axis is exactly at the center (theoretical, for very large radii)

Common K-Factor Values

Bend Deduction

Bend deduction (BD) is the difference between the sum of the two flange lengths and the flat pattern length:

BD = 2 × (radius + thickness) × tan(angle / 2) - BA

This is what SolidWorks uses internally when you unfold a sheet metal part.

Practical Example

Given: 16-gauge mild steel (1.52mm thick), 90° bend, inside radius = 2mm, K = 0.33

BA = 90 × (π / 180) × (2 + 0.33 × 1.52)
BA = 1.5708 × 2.5016
BA = 3.93mm

So when you unfold this bend, you need 3.93mm of material length to form the bend.

In Software

When building sheet metal automation tools:

• Read the K-factor and bend radius from SolidWorks sheet metal feature data
• Use these values to calculate flat pattern dimensions for nesting
• Store material-specific K-factor tables in your application for quick lookup
• Always validate against SolidWorks' own unfolded flat pattern to catch edge cases