What is bend allowance in sheet metal?

Bend allowance is the arc length of material along the neutral axis through the bend zone. When metal bends, the inner surface compresses and the outer stretches. The neutral axis sits between them. Bend allowance accounts for the material consumed in the bend so your flat pattern is cut to the right length before bending.

What K-factor should I use for mild steel?

For mild steel up to 3mm thick, use K = 0.33. For thicker mild steel, use K = 0.40. These values assume a standard air bend on a press brake. If you have empirical data from your specific machine and tooling, enter that K-factor directly for more accurate results.

What is the difference between bend allowance and bend deduction?

Bend allowance is the arc length through the bend zone along the neutral axis. Bend deduction is the amount subtracted from the sum of the two outside leg lengths to get the flat pattern length. Bend deduction is the simpler number to apply when marking out from outside dimensions — most fabricators use this.

How does inside bend radius affect the flat pattern length?

A tighter radius means more compression on the inner face and more stretching on the outer face. The neutral axis shifts inward, effectively increasing the K-factor. As a rule: for radius less than the material thickness use K = 0.33; for radius equal to thickness use K = 0.40.

Can I use this calculator for stainless steel bends?

Yes. Select Stainless Steel 304 — the tool uses K = 0.40. Stainless has higher springback than mild steel, so plan for 2–5° overbend to compensate when setting up the press brake, especially on material above 2mm.

Bend Allowance Calculator

Calculate accurate flat pattern lengths, bend deductions, and K-factors for your press brake operations.

Live SVG Diagram K-Factor Presets Flat Pattern Length

1 Parameters

Material

Thickness (t)

Inside Radius (r)

Bend Angle (θ)

deg

K-Factor (K)

Leg 1 (A)

Leg 2 (B)

Flat Pattern Length

-- mm

Outside Setback (SB)

-- mm

Mould-line apex to bend tangent — mark this to locate your bend line on the sheet.

Bend Allowance (BA)

-- mm

Bend Deduction (BD)

-- mm

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How to Use This Tool

Select Material — Choose a preset to automatically load the correct K-Factor, or select "Custom" to enter your own empirical K-Factor data.
Enter Thickness and Radius — Enter material thickness (t) and inside bend radius (r) in millimeters.
Enter Bend Angle — Enter the desired bend angle (θ) in degrees. (Usually 90° for standard right-angle bends).
Enter Leg Dimensions — Enter the outside dimensions for Leg 1 (A) and Leg 2 (B).
Calculate — Hit calculate to instantly get the flat pattern length, bend allowance, bend deduction, and outside setback to use on your press brake.

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