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Sheet Metal Bend Allowance Calculator

Bend allowance is the length of the neutral axis through a bend, and it is what turns a formed-part drawing into a correct flat blank. This calculator gives bend allowance, outside setback, bend deduction and the total flat length for a single bend.

Your numbers

Material thickness

Actual measured thickness, not the nominal gauge.

Inside bend radius

Inside radius produced by the punch nose and V die, not the drawing radius if they differ.

K-factor

Position of the neutral axis as a fraction of thickness. Typical range 0.33 to 0.50. Calibrate with a test bend.

Bend angle (deviation from flat)

A 90 deg value means a flange turned 90 deg from the flat sheet.

Outside leg 1

Outside dimension from the bend apex to the end of flange 1.

Outside leg 2

Outside dimension from the bend apex to the end of flange 2.

Your results

Bend allowance

4.3982

Arc length of the neutral axis through the bend, in mm.

Flat blank length

126.3982

Cut length of the blank for a single bend using outside dimensions.

tan(angle/2)

Helper value used for the outside setback.

Outside setback

Distance from the bend apex to the tangent line where the bend starts.

Bend deduction

3.6018

Amount subtracted from the sum of outside legs.

Results use the standard neutral-axis formula. Real bends vary with material lot, grain direction, tooling and bend method, so confirm on a test piece before cutting production blanks.

Save results:

Bend allowance vs thickness (radius and K held)

Sweeps Material thickness from 0.5 to 6, holding your other inputs fixed.

Connect bend data to your ERP routings

Optional: a short review of how verified K-factors and bend tables could feed your ERP routing and quoting data.

Optional. The tool and every export work without this. No spam.

How to use the calculator

Enter the measured thickness, the inside radius your tooling actually produces, a K-factor and the bend angle. Then enter the two outside leg dimensions taken from the formed-part drawing. The calculator returns the bend allowance, the bend deduction and the flat blank length. For parts with several bends, repeat the calculation per bend and subtract each bend deduction from the sum of all outside dimensions. Use the chart to see how allowance grows with thickness when radius and K-factor stay fixed.

Measure thickness with a micrometer, not from the gauge name.
Use the radius the die and punch really form, which is often about 15 to 20 percent of the V opening for air bending in mild steel.
Enter outside dimensions, since that is how most flange callouts are dimensioned.

The formulas behind the result

Bend allowance is BA = angle (in radians) x (R + K x T), where R is the inside radius, T is thickness and K is the K-factor. Outside setback is OSSB = (R + T) x tan(angle / 2). Bend deduction is BD = 2 x OSSB - BA. Flat length for one bend is the sum of the two outside legs minus BD. The same neutral-axis idea sits behind the flat-pattern functions in most CAD packages, which let you set a K-factor, a bend allowance table or a bend deduction table.

BA is an arc length along the neutral axis.
BD is negative for very large radii at shallow angles, which is normal.
If you know BD from a bend table, you can back-solve K with the formula.

Worked example and how to read it

With 2 mm material, a 2 mm inside radius, K of 0.40 and a 90 degree bend, BA is about 4.40 mm, the outside setback is 4.00 mm and BD is about 3.60 mm. Two outside legs of 50 mm and 80 mm give a flat length of about 126.4 mm. If the first part comes off the brake 0.3 mm short or long, change K rather than the dimensions, then save the corrected K for that material, thickness and tooling combination so the next job starts with a known value.

A higher K-factor lengthens the blank.
Check grain direction on tight bends, since cracking can force a larger radius and a different K.

Common mistakes

The most common error is mixing inside and outside dimensions. A second is using the drawing radius when the tooling forms a different one, which changes both the allowance and the setback. A third is applying a K-factor from one material and thickness to another. Bottom bending and coining also shift the neutral axis compared with air bending. In ERP and routing data, store the verified K-factor or bend deduction with the material, thickness, tooling and press so that CAD, quoting and the shop floor use the same number.

Do not reuse a K-factor across different materials.
Do not round the blank length before the last step.
Record the verified value after the first-article bend.

Frequently Asked Questions

What is the difference between bend allowance and bend deduction?

Bend allowance is the neutral-axis arc length through the bend and is added to the flat portions. Bend deduction is subtracted from the sum of the outside dimensions. They describe the same bend from two sides and are linked by BD = 2 x outside setback - BA.

What K-factor should I use if I do not know it?

A starting value between 0.33 and 0.50 is common, with smaller values for tight air bends and larger values as the radius grows relative to thickness. Treat it as an estimate, form a test piece, measure the flat length, and adjust.

Does this work for bends over 90 degrees?

Yes, if you enter the bend angle as the deviation from flat. A 120 degree entry means the flange is turned 120 degrees from the original flat. Be careful with CAD drawings that quote the included angle instead, which is 180 degrees minus that value.

Can I use inch inputs?

The formulas are unit-consistent, so you can enter all lengths in inches and read the answers in inches. Just do not mix units. The labels say mm because most shop drawings are metric, but the math does not change.

If bend tables and K-factors live in spreadsheets, we can help connect verified bend data to your routings and quoting inside your ERP. Netray builds this kind of integration on private infrastructure.

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