Sheet metal / interactive calculator

Sheet Metal Bend Allowance & Bend Deduction Calculator

Calculate bend allowance, outside setback, and bend deduction from bend angle, inside radius, material thickness, and conventional K-factor.

01 / Engineering inputs

Live calculation

Material thickness

Use measured thickness when production accuracy matters.

Inside bend radius

Finished inside radius, not punch-tip radius.

Bend angle

deg

Angle through which the flat sheet is bent; 90° is a right-angle bend.

K-factor (ANSI convention)

Neutral-axis location divided by material thickness.

Formula, scope & sources

How this calculator works

Bend allowance

BA = (A × π / 180) × (R + K × T)

A is the angle through which the sheet is bent. R is inside radius, T is thickness, and K is the conventional ANSI K-factor.

Setback and deduction

OSSB = tan(A / 2) × (R + T); BD = 2 × OSSB − BA

Bend deduction applies when the two flange dimensions are measured to the theoretical outside sharp.

Production application disclaimerConfirm the K-factor with a controlled bend coupon or the fabricator’s bend table. Grain direction, tooling, bend method, radius, material condition, springback, and angle convention can change the production flat pattern. Do not substitute a DIN K-factor without converting its definition.

Reference basis: FreeCAD SheetMetal reference implementation

Implementation: Original MfgWorkbench calculation code, independently verified against cited formulas and worked examples. Last reviewed September 4, 2026.

Check the angle convention before calculating

The input is the angle bent from flat. A finished included angle of 120° therefore uses a 60° bend angle. Both examples below use T = 2 mm, R = 3 mm and an illustrative K = 0.40; they are not material-specific bend data.

Example links switch to metric and recalculate. Choose BA or BD correctly · Check the model limits. Without JavaScript, use the formulas and worked table below.

A complete 90° outside-dimension example

Use thickness T = 2 mm, finished inside radius R = 3 mm, bend angle A = 90° and an illustrative ANSI K-factor of 0.40. The neutral-axis radius is R + KT = 3.8 mm. The K-factor is an assumed example, not a verified value for a material/tooling combination.

Original schematic, not to scale. Outside flange dimensions extend to the theoretical sharp intersection.

Step	Result
BA = (π/2) × 3.8	5.969026 mm
OSSB = tan(45°) × (3 + 2)	5.000000 mm
BD = 2 × 5 − 5.969026	4.030974 mm
Flat length for 50 mm and 30 mm outside flanges	50 + 30 − BD = 75.969026 mm

A second check uses tangent lengths: (50 − 5) + (30 − 5) + BA = 75.969026 mm. If the two methods disagree, check whether the drawing dimensions go to tangent points, inside corners or the theoretical outside sharp.

For another bend angle, enter the angle through which the flat sheet rotates, not automatically the finished included angle. Use a measured bend coupon or approved bend table to establish K for the process. See BA, BD versus BA and K-factor. Definition reference: Ansys bend-allowance documentation.

Use the result with the right drawing dimensions

Bend allowance (BA) is the neutral-axis arc length through the bend. Add it to straight lengths measured up to tangent points. Bend deduction (BD) is subtracted from the sum of flange dimensions measured to the theoretical outside sharp. Do not subtract BD from tangent lengths or add BA to outside-sharp lengths: those mix two different dimensioning methods.

Second example: 120° included angle

Load the second example: bend angle A = 60°, T = 2 mm, R = 3 mm and K = 0.40. The neutral-axis radius is 3.8 mm. BA = (π/3) × 3.8 = 3.979351 mm; outside setback = tan(30°) × 5 = 2.886751 mm; BD = 2 × 2.886751 − 3.979351 = 1.794152 mm. For outside-sharp flanges of 50 mm and 30 mm, flat length is 80 − 1.794152 = 78.205848 mm. The calculator’s included-angle output should read 120°.

When this estimate needs a different model

The supported K-factor range here is 0.01–0.50 under the conventional neutral-axis-offset/thickness definition. An accepted value is not evidence that it matches your material or tooling.

Bend angle must be above 0° and below 180°. Hems and very tight included angles are not reliable applications of the outside-setback model; near 180°, the tangent term becomes extremely sensitive.

Zero inside radius is accepted mathematically, not recommended as a physically achievable radius. Use the finished measured radius for a real job.

For multiple bends, corner interactions, reliefs and process sequencing can affect the blank. Do not infer a complete manufacturable flat pattern from one bend result.

Why doesn’t the calculated blank match my coupon?

Check thickness, finished inside radius, angle convention and measurement points first. Then use the actual process bend table or a controlled coupon to calibrate K. Springback, grain direction and tooling make a universal material K-factor unreliable.

Calculator and examples reviewed October 19, 2026. Values are mathematical examples, not measured bend trials or production approval. Editorial method · Report a correction.