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Sheet Metal Bend Allowance Calculator

Flat pattern length and bend deduction for sheet metal.

Your measurementsEvery assumption editable01

The angle turned through — 90 for a right-angle bend.

Where the neutral axis sits, as a fraction of thickness. 0.42 is a common default for mild steel.

Outside dimension of the first flange.

Flat patternCalculated live02

Result

4.8673in flat

Cut the blank 4.8673 in — bend deduction 0.1327 in

Bend allowancearc length along the neutral axis
0.16729 in
Setbacktan(angle ÷ 2) × (radius + thickness)
0.15 in
Bend deduction2 × setback − bend allowance
0.13271 in
Legsoutside dimensions
2 + 3 in
Flat pattern lengthlegs less the bend deduction
4.86729 in

Metal stretches on the outside of a bend and compresses on the inside, so the flat blank is shorter than the sum of the finished legs. The bend deduction is that difference, and it is why a part folded from a blank cut to the sum of its legs always comes out oversized.

K-factor describes where the neutral axis sits through the thickness — the layer that neither stretches nor compresses. It varies with material, thickness and tooling, and 0.42 is a starting point for mild steel rather than a constant. If your shop has measured values, use theirs.

Verify with a test bend before cutting a batch. Measure a folded sample, work backwards to the K-factor your press and tooling actually produce, and use that figure from then on.

Metal stretches on the outside of a bend and compresses on the inside, so the flat blank is shorter than the sum of the finished legs. This works out by how much, which is the difference between a part that fits and one that is oversized.

Why use this tool?What it does differently03

Why use this tool?

The whole set

Bend allowance, setback, bend deduction and flat length — the four figures a brake operator works from.

K-factor explained

Where the neutral axis sits through the thickness. It is a material and tooling property, not a constant.

Verify by test bend

Measure a folded sample and work backwards to the K-factor your press actually produces.

Any bend angle

Not just 90°, since acute and obtuse bends are common in fabrication.

How this worksThe method04

How this sheet metal bend allowance calculator works

Bend allowance is the arc length along the neutral axis: the bend angle in radians times the inside radius plus the K-factor times the thickness. That is the material actually consumed by the bend.

Setback is the tangent of half the bend angle times the radius plus the thickness. Bend deduction is twice the setback less the bend allowance, and the flat pattern is the sum of the outside legs less that deduction.

K-factor describes where through the thickness the material neither stretches nor compresses. It varies with material, thickness and tooling, and 0.42 is a starting point for mild steel rather than a universal value.

How to use itStep by step05

How to use it

  1. Step 1: Measure thickness and radius

    Material thickness and the inside radius your tooling produces, which is not always what is asked for.

  2. Step 2: Enter the bend angle

    The angle turned through — 90 for a right-angle bend.

  3. Step 3: Set the K-factor

    0.42 for mild steel as a start. If your shop has measured values, use theirs.

  4. Step 4: Enter the outside legs

    The finished outside dimensions of the two flanges.

Example usageWorked figures06

Example usage

A 90 degree bend in light steel
0.075 in material, 0.075 in inside radius, K 0.42, legs of 2 and 3 in: bend allowance 0.16729 in, setback 0.15, deduction 0.13271 — so the blank is 4.86729 in, not 5.
Why the blank is shorter
Cutting that blank at 5 in — the sum of the legs — produces a part 0.133 in oversized. Across a folded box that error appears at every bend.
Checking your K-factor
Fold a test piece, measure the finished legs, and work backwards. A press that produces 0.45 rather than 0.42 shifts every part in a batch.
Frequently asked questionsCommon questions07

Frequently asked questions

What is bend allowance?

The arc length of material consumed by the bend, measured along the neutral axis. It is added to the flat legs to give the blank length.

What is the difference between bend allowance and bend deduction?

Allowance is added to flat leg lengths; deduction is subtracted from outside dimensions. Most shop drawings give outside dimensions, so deduction is usually the one you need.

What K-factor should I use?

0.42 is a reasonable start for mild steel. It varies with material, thickness and tooling, so a measured value from your own press beats any published figure.

Why is my folded part too big?

Almost certainly the blank was cut to the sum of the legs without deducting for the bend. Metal consumed by the bend has to come out of the flat length.

Does this work for aluminium?

The geometry is identical; only the K-factor changes. Aluminium typically runs a slightly different neutral axis position, so measure a test bend in your actual material.

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