Sheet Metal Bend Allowance Calculator: Bend Deduction, K-Factor and Flat Pattern Length

Find the flat length of a bent sheet metal part from its outside dimensions.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the thickness, the inside radius of the bend, the bend angle, the K-factor and the two outside flange lengths.

The calculator finds the bend allowance BA = angle x (radius + K x thickness), the outside setback, and the bend deduction BD = 2 x setback - BA.

The flat pattern length is the sum of the flanges less the bend deduction. For more than one bend, subtract the deduction of each bend.

Formulas used

  • BA = \theta\,(r + K t)
  • \mathrm{OSSB} = \tan\frac{\theta}{2}(r + t)
  • BD = 2\,\mathrm{OSSB} - BA
  • L_{flat} = A + B - BD

Worked example

With the default numbers in the calculator above:

  1. Bend allowance: the length of the neutral axis around the bend: 4.40 mm
  2. Outside setback: from the tangent point on the outside to the virtual sharp: 4.00 mm
  3. Bend deduction: how much to take off the sum of the outside flange lengths: 3.60 mm
  4. Flat pattern length for the two flanges you gave (one bend): 76.40 mm
  5. Length of the bend on the inside and the outside surfaces: 3.14 mm inside, 6.28 mm outside

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is the K-factor?

The position of the neutral axis, which keeps its length when the sheet bends, as a fraction of the thickness from the inside. It is usually 0.33 to 0.5.

What is the difference between bend allowance and bend deduction?

The allowance is the length of the neutral axis in the bend, added to the flat lengths of the inside dimensions. The deduction is subtracted from the outside dimensions.

What inside radius should I use?

The radius of the press brake tool or the air-bend radius, which is about the thickness for steel. A radius under the thickness may crack some metals.

How accurate is it?

It is as good as the K-factor. Bend a test piece, measure it, and adjust the K-factor to suit your machine and material.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Machinery's Handbook (sheet metal bending); any sheet metal design guide from a press brake maker.

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