📄 Sheet Metal Bend Allowance

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Figuring out how long a flat sheet metal blank needs to be to fold up into a finished bent part.

When sheet metal is bent, the material on the outside of the bend stretches and the material on the inside compresses — somewhere in between, at the neutral axis, the material neither stretches nor compresses at all. Bend allowance is the actual arc length of material consumed by the bend, measured along that neutral axis, which is what determines how much of the flat blank the bend "uses up."

The neutral axis doesn't sit at the exact geometric center of the material thickness — where it actually falls depends on the bend radius, the material, and how tightly it's bent, which is what the K-factor (a fraction between roughly 0.3 and 0.5) approximates. A tighter bend relative to material thickness pushes the neutral axis closer to the inside surface.

Bend deduction is a related but different number: it's how much shorter the flat blank needs to be than simply adding up the two outside leg lengths, since those leg dimensions (measured to the outside of the bend) already imply some material that the bend allowance accounts for differently.

Key formula

Bend allowance = bend angle (radians) × (inside radius + K-factor × thickness)
Bend deduction = 2 × (inside radius + thickness) × tan(bend angle ÷ 2) − bend allowance

Variables

K-factor

fraction of material thickness where the neutral axis sits, measured from the inside surface

Notes & limitations

• K-factor isn't a universal constant — it varies by material, bend radius-to-thickness ratio, and bending method (air bending vs. bottoming vs. coining all push material differently), and shops often develop their own K-factor tables from test bends on their specific equipment and material rather than using one generic value.
• This is a single-bend calculation — a part with multiple bends needs each one calculated (and its deduction subtracted) separately to get the total flat pattern length.