Sheet Metal Bend Allowance Calculator

Calculator Inputs

Units

Material

Tooling Method

Thickness

Inside Radius

Bend Angle (degrees)

K-Factor

Typical range is 0.30 to 0.50.

Leg A

Leg B

Number of Bends

Known Flat Length (optional)

Show bend allowance chart

Plotly Bend Allowance Graph

The graph shows allowance changes across common bend angles using your current thickness, radius, and K-factor.

Example Data Table

Material	Thickness	Inside Radius	Angle	K-Factor	Leg A	Leg B	Bend Allowance	Flat Length
Mild Steel	2.00 mm	3.00 mm	90°	0.33	50.00 mm	40.00 mm	5.7496 mm	85.4219 mm
Aluminum	1.50 mm	2.00 mm	120°	0.40	35.00 mm	35.00 mm	5.4454 mm	67.6269 mm
Stainless Steel	1.20 mm	1.50 mm	45°	0.42	25.00 mm	30.00 mm	1.5627 mm	53.7594 mm

Formula Used

Bend Allowance:
BA = (π / 180 × Bend Angle) × (Inside Radius + K × Thickness)

Outside Setback:
OSSB = tan(Bend Angle / 2) × (Inside Radius + Thickness)

Bend Deduction:
BD = 2 × Outside Setback − Bend Allowance

Flat Length for One Bend:
Flat Length = Leg A + Leg B − Bend Deduction

Bend allowance estimates the neutral axis arc length through the bend. The K-factor places that neutral axis inside the material thickness.

Use measured shop data whenever possible. Real results vary with punch radius, die width, grain direction, material hardness, and forming method.

This calculator also estimates total developed length for repeated bends by multiplying single-bend deduction and allowance values by the bend count.

How to Use This Calculator

Choose your working unit, material, and tooling style.
Enter thickness, inside radius, and the bend angle.
Set the K-factor manually or start from a preset.
Enter both flange lengths and the number of bends.
Submit the form to view bend allowance and flat length.
Review the graph to compare allowance across bend angles.
Export the results to CSV or save them as PDF.
Compare calculated flat length with shop-tested values for calibration.

Frequently Asked Questions

1. What is bend allowance?

Bend allowance is the arc length of the neutral axis through the bend. It helps convert formed dimensions into flat blank length before fabrication.

2. Why does K-factor matter?

K-factor locates the neutral axis within the material thickness. A small change can noticeably affect bend allowance and final flat pattern accuracy.

3. What bend angle should I enter?

Enter the included bend angle used in your bend formula workflow. Shops often use the actual formed angle, so stay consistent with your established method.

4. Is bend deduction different from bend allowance?

Yes. Bend allowance is added along the neutral axis arc. Bend deduction is subtracted from summed flange lengths to estimate flat length.

5. Why might my shop result differ?

Actual results vary with tooling, springback, grain direction, die opening, press setup, and material lot. Calibrate with test bends for production accuracy.

6. Can I use inches instead of millimeters?

Yes. The calculator supports inches, millimeters, and centimeters. Keep every value in the same unit for correct outputs.

7. Does this work for multiple bends?

Yes. Enter the total bend count to estimate total bend allowance, total deduction, and developed length across repeated similar bends.

8. Should I trust preset K-factors completely?

Use presets as starting points only. The best K-factor comes from your own test coupons, measured samples, and press brake setup history.