Sheet Metal Bend Allowance Calculator – Developed Length
Calculate bend allowance, bend deduction, setback and developed flat length before bending sheet metal. Enter thickness, inside radius, bend angle, K-factor and flange dimensions.
What the calculation shows
- Bend allowance (BA) — neutral-axis arc length consumed by the bend.
- Outside setback — distance from the theoretical bend apex to the tangent point based on inside radius and bend angle.
- Bend deduction (BD) — amount subtracted from the sum of two outside flange dimensions to obtain a common developed length.
- Developed length — estimated flat length before forming, based on the selected dimension convention.
- Neutral-axis length — straight developed segments plus the bend allowance.
For a single bend, a commonly used relationship is:
BA = θ × (R + K × T)
where θ is the bend angle in radians, R is inside radius, K is the K-factor and T is material thickness.
Why the dimension convention matters
A drawing dimensioned to outside flange ends is not handled the same way as dimensions measured along the neutral/developed path. The calculator therefore separates the two approaches rather than silently mixing conventions.
This is an engineering estimate, not a guaranteed press-brake setup value. Actual results depend on material grade, thickness variation, tooling, die opening, punch radius, forming method, grain direction, springback and shop-specific bend tables. Validate production dimensions with your established tooling and bend test results.
Understanding sheet metal bend calculations
- Inside radius (R) is measured from the inside surface of the sheet to the bend center region.
- Thickness (T) affects both the neutral-axis position and the relationship between outside dimensions and the bend.
- K-factor is dimensionless and describes the neutral-axis location as a fraction of material thickness.
- Bend angle in this calculator is the included amount of material rotation through the bend, from 0° to less than 180°.
- Setback grows as the bend angle changes because the tangent geometry changes.
- Bend allowance is proportional to the angle in the standard K-factor equation.
Important terminology: Shops and CAD systems can use different definitions for bend angle, bend deduction, outside setback and flange dimensions. Always match the calculator's convention to the drawing and software you are using.
Common bend-angle reference
The formula uses radians internally. For example, a 90° bend is π/2 radians.
Frequently Asked Questions
What is bend allowance?
Bend allowance is the neutral-axis arc length through the bend. A common estimate is BA = angle in radians × (inside radius + K × thickness).
What is a K-factor?
The K-factor describes where the neutral axis lies through the sheet thickness. It is used with thickness and inside radius to calculate bend allowance.
What is bend deduction?
For the outside-dimension convention used here, bend deduction is the amount subtracted from the two outside flange dimensions to obtain a developed length.
How is developed length calculated?
For two outside flange dimensions, a common relationship is flange 1 + flange 2 − bend deduction. For neutral-axis segments, the bend allowance is added instead.
Is K-factor universal?
No. Material, tooling and forming conditions affect the practical neutral-axis location. Shop-specific bend tables or test results should take priority for production.
Does the bend angle affect bend allowance?
Yes. With the same radius, thickness and K-factor, a larger bend angle creates a longer neutral-axis arc and therefore a larger bend allowance.