Bend Allowance Calculator

Bend allowance is the length of the neutral axis through a bend — the number that turns a folded part back into a flat pattern. Enter thickness, inside radius and bend angle, pick a K-factor, and this returns the allowance to add to your flat length.

Thickness (t)mmInside radius (R)mmBend angle°K-factorK

4.46mm

Bend allowance

3.54mm

Bend deduction

BA = angle × π/180 × (R + K × t) — the arc length along the neutral axis. Bend deduction is what you subtract from the sum of the two outside flange lengths (BD = 2 × tan(θ/2) × (R + t) − BA). Use these to sanity-check a flat pattern, not to design one — for production we unfold from the 3D model with the tooling we will actually run.

K-factor values we programme

The K-factor is where the neutral axis sits, as a fraction of thickness. It is not a material constant — it moves with the inside radius, the bend method and the tooling. These are the working values for air bending on our press brakes.

Air-bending K-factor guidance by inside radius
Condition	K-factor	Why
Inside radius < 1 × thickness	0.30 – 0.35	Sharp bends stretch the outside surface and pull the neutral axis inward.
Inside radius ≈ 1 × thickness	0.40 – 0.44	The default for most air-bent parts in this shop.
Inside radius > 3 × thickness	0.50	Large radii bend with almost no stretching; the axis sits near mid-thickness.

Questions about bend allowance

BA = angle × π/180 × (R + K × t). K-factor guidance follows air-bending practice.

What K-factor should I use?

For air bending, 0.42 is a safe default for an inside radius near one thickness. Below one thickness the metal stretches and K drops toward 0.33; above three thicknesses it approaches 0.50. For production we unfold with the K-factor programmed for the tooling that will actually run the part.

Is bend allowance the same as bend deduction?

No. Bend allowance is the arc length along the neutral axis — what you add when building a flat pattern from the outside dimensions. Bend deduction is what you subtract from the sum of the two flange lengths. Both describe the same bend from opposite directions.

Why does my CAD flat pattern disagree with yours?

Different CAD templates carry different K-factors. When we quote, we unfold with the tooling-specific value and flag any difference larger than 0.2 mm on a critical dimension, so the flat you approve is the flat we bend.

Keep going

CNC press brake bending: ±0.5° angle accuracy DFM design rules for sheet metal Send a flat pattern for a bend review

Calculators — enter dimensions, get a number

Sheet Metal Weight CalculatorWeight per piece and per batch from L × W × t, with real alloy densities.Sheet Metal Cost CalculatorWhere a part’s cost actually sits — material, cutting, forming, finishing and setup.Press Brake Tonnage CalculatorBending force in kN and tons from thickness, material and bend length, with a 20% margin.K-Factor Calculator (Reverse)Reverse a K-factor from a real bend: outside dimensions and flat length in, K out.Hole-to-Bend Distance CalculatorMinimum hole-to-bend and hole-to-edge clearances from thickness and inside radius.

Charts and checklists — read, print, work through

Sheet Metal Gauge to mm ChartSteel (MSG) and aluminium (B&S) gauge dimensions, and why we quote in mm.Sheet Metal DFM ChecklistThe 24 checks we run on a drawing before we quote it, grouped and explained.Sheet Metal RFQ ChecklistThe 12 items that make an RFQ quotable in one round, with the reason each matters.Sheet Metal Supplier Audit Checklist13 remote-verifiable checks for a sheet metal supplier, in four passes.

Send the flat pattern — we check the bends

Upload STEP, IGES, DXF or DWG and our engineers unfold it with the K-factor programmed for the tooling that will run your part, then flag anything that will not bend cleanly.

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