What is the difference between bend allowance and bend deduction?

Bend allowance is the arc length of the neutral axis through the bend, which is the material you actually gain by forming. Bend deduction is the amount you subtract from the sum of the outside flange dimensions to get the flat pattern. Bend deduction equals 2 x (R + T) x tan(angle/2) minus the bend allowance.

What K-factor should I use?

K-factor is the position of the neutral axis as a fraction of thickness. It is about 0.33 for air bending at a radius near the material thickness, drops towards 0.25 with a tight radius and rises towards 0.50 with a very large radius. Your brake and tooling supplier can give you a measured value, and using theirs is always better than a default.

How do I estimate press brake tonnage?

For air bending, required force is proportional to bend length, thickness squared and the material's tensile strength, and inversely proportional to the die opening. The calculator uses the standard 1.42 factor formula. Doubling thickness roughly quadruples the tonnage, which is why thick plate needs a very different machine.

What die opening should I choose?

The usual starting rule is a die opening about eight times the material thickness for air bending. A narrower die needs more tonnage and gives a tighter radius but risks marking and tool damage; a wider die reduces tonnage and increases the achievable inside radius.

Sheet Metal Bend Allowance & Tonnage Calculator

Work out the flat length before you bend, and the tonnage the press brake will need. Enter thickness, inside radius, angle and die opening and the flat pattern follows.

Bend and flange

Material

Thickness T (mm)

Inside radius R (mm)

Bend angle (°)

K-factor

Flange A (mm)

Flange B (mm)

Die opening V (mm)

Rule of thumb: 8 × T.

Bend length (mm)

Calculated locally in your browser. Flat pattern figures are for a single bend on a simple part — verify against a trial bend before cutting a full sheet.

Result

Neutral axis radius— Bend allowance— Bend deduction— Flat pattern length— Outside setback— Required tonnage—

BA = angle(rad) × (R + K × T). BD = 2 × (R + T) × tan(angle/2) − BA. Flat = A + B − BD. Tonnage = 1.42 × L × T² × UTS ÷ (1000 × V), in kN.

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Reference

K-factor by bend condition

Bend condition	R / T	Typical K-factor
Tight radius, coining	0.5 or less	0.25 – 0.30
Air bend, radius near thickness	1.0	0.33
Air bend, generous radius	2 – 4	0.38 – 0.42
Large radius, soft material	5 and above	0.45 – 0.50

Tonnage scales with thickness squared. Going from 2 mm to 4 mm roughly quadruples the required force for the same die opening, so a part that runs comfortably on a small brake can be out of reach on a thicker gauge. Verify your machine's rated tonnage per metre against the die opening you intend to use, because the rating falls as the die narrows.

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Basis & references: ISO 2768 — General tolerances · NIST — Manufacturing process data

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