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Manufacturing & Materials

Sheet Metal Bend Allowance Calculator

Calculate bend allowance, bend deduction, flat pattern length, and neutral axis location for a sheet metal bend.

Cited standard/category: Manufacturing & Materials.

Inputs

All flange lengths use the same unit and the same bend setup.

mm
mm
deg
Typical starting range: 0.33 to 0.44.
Enter at least two finished flange lengths, separated by commas, spaces, or new lines. Bend count is inferred as flange count minus one.
BA = angle(rad) x (radius + K x thickness)
BD = 2 x (radius + thickness) x tan(angle / 2) - BA
Flat length = sum(flange lengths) +/- total BD

Results

Results update as inputs change.

Calculated report

Bend allowance (BA) -
Bend deduction (BD) -
Flat pattern length -
Neutral axis from inside surface -
Neutral axis radius -
Bend angle in radians -

Input summary

Flange sum -
Inferred bend count -
Total bend deduction adjustment -
Treatment -

Assumes every inferred bend uses the same thickness, inside radius, bend angle, and K-factor.

Self-tests

Golden checks verify the core BA and BD formulas.

Self-tests: not run

About the Sheet Metal Bend Allowance Calculator

Sheet metal fabricators, CAD designers, estimators, and press brake operators use a sheet metal bend allowance calculator to estimate flat-pattern length from thickness, inside radius, bend angle, and K-factor. Enter material and bend data to compare allowance, setback, and bend deduction. It helps reduce trial cuts when moving from 3D part geometry to a blank size.

How it works

  1. Enter material thickness, inside bend radius, and bend angle.
  2. Choose or enter a K-factor based on material and tooling.
  3. Calculate bend allowance, outside setback, and bend deduction.
  4. Apply the result to the flat pattern or press brake setup sheet.
  5. Validate critical parts with shop data and test bends.

Frequently asked questions

What is bend allowance?

Bend allowance is the arc length of the neutral axis through a bend. It is added to flange lengths to estimate the flat blank length needed before forming.

What does K-factor mean in sheet metal bending?

K-factor is the neutral-axis location as a fraction of material thickness, measured from the inside surface. It changes with material, tooling, bend radius, bend method, and forming conditions.

Are bend allowance and bend deduction the same?

No. Bend allowance is the developed length through the bend, while bend deduction is subtracted from outside flange dimensions to obtain the flat pattern.

Why do shop results differ from the calculator?

Actual bends depend on material batch, grain direction, punch and die, inside radius, springback, coating, press brake condition, and operator setup. Production parts should use proven shop tables when available.

Should bend angle be entered as included angle or bend angle?

Use the angle definition required by the calculator and your CAD system. Confusing included angle with bend angle is a common reason flat patterns come out wrong.

References