Bend Allowance Calculator

Sheet Metal Bend Allowance

When a flat blank is bent, the neutral axis shifts inside the material. Bend allowance is the arc length along that neutral axis through the bend. Bend deduction is how much shorter the flat pattern must be compared to the sum of outside leg lengths.

Formulas

Bend allowance for steel (K = 0.33):

$$BA = \frac{\pi}{180} \times \text{angle} \times (R + K \times T)$$

Outside setback and bend deduction:

$$OSS = R + T$$ $$BD = 2 \times OSS - BA$$

Where angle is in degrees, $R$ is inside bend radius, and $T$ is material thickness.

Frequently Asked Questions

What is the K factor for steel?

This calculator uses K = 0.33, a common default for mild steel. Actual K varies with tooling, grain direction, and material type.

What is bend deduction used for?

Bend deduction tells you how much to subtract from the total outside flange lengths when laying out a flat pattern so the bent part matches the desired dimensions.

What is outside setback (OSS)?

OSS is the distance from the bend tangent line to the outside mold line: $R + T$. It appears in both bend deduction and setback calculations.

Does this work for acute bends?

The formula applies to any bend angle up to 180°. Very tight radii or thick plate may need shop-tested K values instead of the steel default.

Report

Sheet Metal Bend Allowance

Formulas

Frequently Asked Questions

Related Tools

Recent Visits