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Sheet Metal Bend Allowance Calculator

K-factor, bend allowance, bend deduction and flat blank length for press brake parts.

Work out the flat blank length of a bent sheet metal part from thickness, inside radius, bend angle and flange lengths. Enter flanges as outside (mould line) dimensions from the drawing or as flat tangent lengths.

  • Live results
  • Full step-by-step working
  • PASS / FAIL checks
  • Free PDF report

Inputs

Results

Flat blank length
76.16 mm
Bend allowance
4.163 mm
Bend deduction
3.837 mm
K-factor
0.325
Step-by-step calculation (7 steps)
  1. 1K-factor (neutral axis position)
    FormulaK = k/2, k = 0.65 + 0.5·log₁₀(R/t) (DIN 6935, R/t ≤ 5)
    SolutionR/t = 1, k = 0.65
    ResultK = 0.325
  2. 2Bend allowance (arc length of neutral axis)
    FormulaBA = θ·(π/180)·(R + K·t)
    SolutionBA = 90 × (π/180) × (2 + 0.325 × 2)
    ResultBA = 4.163 mm
  3. 3Outside setback
    FormulaOSSB = tan(θ/2)·(R + t)
    SolutionOSSB = tan(45°) × (2 + 2)
    ResultOSSB = 4 mm
  4. 4Bend deduction
    FormulaBD = 2·OSSB − BA
    SolutionBD = 2 × 4 − 4.163
    ResultBD = 3.837 mm
  5. 5Flat blank length
    FormulaLflat = L1 + L2 − BD
    SolutionLflat = 50 + 30 − 3.837
    ResultLflat = 76.16 mm
  6. 6Flange length check
    Formulaflat leg ≥ 2.5·t + R (practical press brake minimum)
    Solutionflat legs = 46 and 26 mm, minimum = 7 mm
    ResultLegs long enough to formPASS
  7. 7Inside radius check
    FormulaR ≥ ~0.8·t for mild steel; check material data for hard alloys
    SolutionR / t = 1
    ResultRadius acceptable for most mild steelsPASS

Engineering takeaways

  • K-factor depends on material, tooling and bending method. Bend a test strip and back-calculate K for production parts.
  • Air bending inside radius ≈ 15 to 17% of V-die opening for mild steel. Use the radius your tooling actually produces, not the drawing nominal.
  • Keep holes at least 2t + R away from the bend line to avoid distortion.

The PDF report contains every input, formula, substitution and check shown above, in LTM calculation-sheet format.

Worked Example

Use a fixed K-factor or let the calculator derive it from the radius to thickness ratio using the DIN 6935 correction formula. Results include bend allowance, outside setback and bend deduction, with checks on minimum flange length and inside radius.

The calculator above opens with this example. Change any input to run your own case.

Inputs

Sheet thickness t2 mm
Inside bend radius R2 mm
Bend angle θ (90 for an L-bend)90 °
K-factorAuto from R/t (DIN 6935)
Leg lengths are measured toOutside (mould line, as on drawings)
Leg length L150 mm
Leg length L230 mm

Results

Flat blank length76.16 mm
Bend allowance4.163 mm
Bend deduction3.837 mm
K-factor0.325

Bend Allowance Formulas Used

Bend allowance

BA=θ⋅π180⋅(R+K t)BA = \theta\cdot\frac{\pi}{180}\cdot(R + K\,t)
  • θbend angle (degrees)
  • Rinside radius
  • tthickness
  • KK-factor, neutral axis position

Outside setback

OSSB=tan⁡(θ2)(R+t)OSSB = \tan\left(\frac{\theta}{2}\right)(R + t)

Bend deduction

BD=2⋅OSSB−BABD = 2\cdot OSSB - BA

Flat blank length

Lflat=L1+L2−BD  (outside dims),Lflat=L1+L2+BA  (tangent dims)L_{flat} = L_1 + L_2 - BD\ \ (\text{outside dims}),\qquad L_{flat} = L_1 + L_2 + BA\ \ (\text{tangent dims})

K-factor from DIN 6935

K=k2,k=0.65+0.5 log⁡10 ⁣(Rt)  (R/t≤5)K = \frac{k}{2},\qquad k = 0.65 + 0.5\,\log_{10}\!\left(\frac{R}{t}\right)\ \ (R/t \le 5)

Basis: DIN 6935 correction factor; neutral axis (K-factor) method. Results are for preliminary design and should be confirmed against supplier data and the governing standard.

Bend Allowance Calculator FAQ

What is the K-factor in sheet metal?

K is the position of the neutral axis as a fraction of thickness, measured from the inside of the bend. It is typically 0.3 to 0.5: lower for tight radii and hard materials, approaching 0.5 for large radii.

What is the difference between bend allowance and bend deduction?

Bend allowance is the length of the neutral axis through the bend, added to flat tangent lengths. Bend deduction is subtracted from the sum of outside flange dimensions. Both give the same flat blank when used correctly.

Which bend angle do I enter?

Enter the angle the sheet is bent through. A 90° L-bracket is 90. If your drawing shows an included angle of 120° between flanges, the bend angle is 60.

Why does my flat pattern not match the CAD unfold?

CAD uses its own K-factor or bend table. Set the same K-factor in both, and use the radius your tooling actually produces. For air bending, inside radius is about 15 to 17% of the V-die opening in mild steel.

How short can a flange be?

The flat flange must rest on both shoulders of the V-die. A practical minimum is about 2.5 × thickness plus the inside radius for standard tooling; shorter flanges need special tools.

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