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Sheet Metal Bend Allowance Calculator

Work out sheet metal bend allowance instantly with clear inputs, formula shown and shareable results.

About Sheet Metal Bend Allowance Calculator

Metal stretches on the outside of a bend and compresses on the inside, and the neutral axis that changes length least sits at the inside radius plus the K-factor times thickness. Bend allowance is the arc length along that neutral axis, and bend deduction, twice the setback minus the allowance, is what is subtracted from the sum of the outside flange dimensions to get the flat blank.

Formulas

Bend allowance

BA = angle in radians x (R + K x T)

Bend deduction

Setback = tan(angle/2) x (R + T); BD = 2 x setback - BA

Worked example

With the default values below, Sheet Metal Bend Allowance Calculator returns Bend allowance of 6.095. Change any input above and every figure here is recalculated in your browser.

Inputs used

Material thicknessminimum 0.1 mm
2 mm
Inside bend radiusminimum 0.1 mm
3 mm
Bend angleaccepts 1–179 °
90 °
K-factoraccepts 0.2–0.5
0.44

Results

Bend allowance
6.095 mm
Bend deduction
3.905 mm
Setback
5.000 mm
Neutral axis radius
3.880 mm

Step by step

  1. 1Values used
  2. 2Bend allowance
  3. 3Bend deduction
  4. 4Bend allowance= 6.095 mm
  5. 5Bend deduction= 3.905 mm
  6. 6Setback= 5.000 mm
  7. 7Neutral axis radius= 3.880 mm

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Sheet Metal Bend Allowance figure worked out?

BA = angle in radians x (R + K x T). Metal stretches on the outside of a bend and compresses on the inside, and the neutral axis that changes length least sits at the inside radius plus the K-factor times thickness.

What K-factor should I use?

About 0.44 for mild steel at typical radii, dropping toward 0.33 for tight radii and rising toward 0.50 for generous ones. Measuring a test bend on your own press beats any published value.

What values do I need to enter?

This calculator takes 4 inputs: Material thickness, Inside bend radius, Bend angle, K-factor. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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