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Sheet metal bend allowance calculator. Bend deduction and flat length from the K-factor.

Bend allowance is the neutral axis’s arc through the bend, BA = (π/180) × A × (R + K × T), and this calculator turns it into the bend deduction and the flat length to cut.

Updated

Units

How far the flange is bent from flat: 90 for an L.

Starting value 0.33: replace it with your shop’s tested value.

Typical starting K-factors (air bending)

Generic reference values, not measurements of your material: replace them with a K-factor tested on your own brake and tooling. Tap a value to use it. Your R:T is 1.00.

K-factor by material and inside radius to thickness ratio
MaterialSuggestedR ≤ TT – 3TR > 3T
Softaluminium, copper
Mediummild steel
Hardstainless steel
Back-solve your K-factor from a test bend

Bend a test piece with your tooling, measure both outside flanges into the fields above, then enter the flat blank length you cut it from.

01— Method

How to calculate bend allowance

From thickness, radius, angle and K-factor to the flat blank you cut. Five steps, and what this calculator does with each.

  1. 01Collect the bend’s numbers

    Material thickness T, the inside bend radius R your tooling forms, and the bend angle A, measured as how far the flange turns from flat (90° for an L).

    This tool: takes all three, in millimetres or inches, and converts them when you switch units.

  2. 02Choose a K-factor

    The K-factor locates the neutral axis, the layer that neither stretches nor compresses, as a fraction of T from the inside face. It depends on material, tooling and R:T, so generic tables only give a starting point.

    This tool: starts at 0.33 and shows the generic air-bending table by material and R:T. It can back-solve your own K from a test bend.

  3. 03Work out the bend allowance

    BA = (π/180) × A × (R + K × T): the length of the neutral axis through the bend, which is the material the bend consumes.

    This tool: shows the formula with your numbers filled in.

  4. 04Work out the setback and the bend deduction

    The outside setback OSSB = tan(A/2) × (R + T) runs from the bend’s tangent to the outside mold line. The bend deduction BD = 2 × OSSB − BA is what to subtract from the outside dimensions.

    This tool: computes both and shows each formula.

  5. 05Get the flat length

    Flat length = L1 + L2 − BD, where L1 and L2 are the flange lengths measured to the outside mold line (the outside dimensions on a drawing). A part with several bends subtracts one BD per bend.

    This tool: handles one bend between two flanges; for more bends, add each flange and subtract each bend’s deduction.

02— FAQ

Questions about the sheet metal bend calculator.

What it does, what it leaves to you, and what it costs.

Bend allowance is the length of material through the bend itself, measured along the neutral axis. Bend deduction is what you subtract from the two outside flange dimensions to get the flat blank: twice the outside setback minus the bend allowance.

Start from a typical value (this page starts at 0.33) and replace it with one tested on your own press brake, tooling and material: bend a test piece, measure it, and use the back-solve. The table on the page is the generic air-bending table widely reproduced in sheet-metal references; it gives starting values, not your shop’s numbers.

They are the generic air-bending values commonly published in sheet-metal trade references: about 0.33 to 0.40 for an inside radius up to the thickness, 0.40 to 0.45 up to three times the thickness, and 0.5 beyond that, rising from soft to hard material. Bottoming and coining give different values.

The bend angle: how far the flange turns from flat. An L bracket is 90°. If your drawing gives the included angle between the flanges (for example 135°), enter 180° minus it (45°).

Outside: measured to the outside mold line, where the outside faces of the two flanges would meet. For bends other than 90° that point is the virtual sharp, beyond the material.

It calculates one bend between two flanges. For a part with several bends, add every flange and subtract each bend’s deduction. It does not model springback, hems or the radius your tooling actually produces; your test bend captures those.

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Working out flat patterns for every order?

SBFS builds product configurators that do this math for every order automatically: the customer picks the size, the configurator works out the blanks, the price and the quote. See configurator demos by industry.

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