Bending Angle Calculator (Bend Allowance)
Calculate bend allowance — the length of material consumed by a bend — from your bend angle, inside radius, material thickness, and K-factor. Essential for sizing an accurate flat pattern before cutting.
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This is the bend allowance — add it to your flange lengths (measured to the bend's tangent points) to get the correct flat pattern length before cutting.
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How Sheet Metal Bend Allowance Works
Bend allowance is the length of material consumed by a bend — the arc length along the neutral axis, the layer that neither stretches nor compresses during bending.
How to Calculate Bend Allowance
Bend Allowance = Bend Angle × (π ÷ 180) × (Inside Radius + K-Factor × Thickness).
Bending Angle Calculator Formula
- Neutral Radius = Inside Bend Radius + (K-Factor × Material Thickness)
- Bend Allowance = Bend Angle (radians) × Neutral Radius
What K-Factor Should I Use?
K-factor typically ranges 0.3-0.5 depending on material, thickness, and bending method (air bending, bottoming, or coining). Common starting points are around 0.33 for aluminum and up to 0.5 for mild steel — for precision work, determine your actual K-factor with a test bend.
Bend Allowance Examples
90° bend, 3mm inside radius, 1.5mm thickness, K=0.5: BA = 90 × (π/180) × (3 + 0.5×1.5) ≈ 5.89mm.
Common Bend Allowance Mistakes
Using a generic K-factor without verifying it against your specific material, thickness, and bending method. Forgetting springback — the material's tendency to partially return toward its original shape after bending, requiring overbend or compensation. Confusing bend allowance (material consumed) with bend deduction (a related but different value used in some flat-pattern calculation methods).
How to Use This Calculator
Enter the bend angle in degrees, the inside bend radius, the material thickness, and the K-factor for your material (typically 0.3–0.5; commonly 0.33 for aluminum, up to 0.5 for mild steel — check your material's actual K-factor for precision work).
Formula & Methodology
Bend Allowance = Bend Angle × (π/180) × (Inside Radius + K-factor × Thickness). This calculates the arc length along the neutral axis — the layer of material that neither stretches nor compresses during the bend — which tells you how much material the bend consumes.
Example: 90° bend, 3mm inside radius, 1.5mm thickness, K=0.5
BA = 90 × (π/180) × (3 + 0.5×1.5) = 1.5708 × 3.75 ≈ 5.89mm.
The K-factor varies by material, thickness, bend method (air bending vs. bottoming vs. coining), and tooling — the values used here are common starting points, not universal constants. For precision or production work, determine your actual K-factor empirically with a test bend on your specific material, thickness, and tooling setup.
For estimation and planning purposes. Always verify with a test bend before committing production material, especially for tight tolerances.
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Frequently Asked Questions
- What is bend allowance?
- The length of material along the neutral axis consumed by a bend — used to calculate the correct flat pattern length before cutting sheet metal.
- What K-factor should I use?
- K-factor typically ranges 0.3–0.5 depending on material, thickness, and bending method. Common starting points are around 0.33 for aluminum and up to 0.5 for mild steel, but for precision work, determine your actual K-factor with a test bend.
- What is the neutral axis?
- The layer inside a bent material that neither stretches (like the outer surface) nor compresses (like the inner surface) — it's where the K-factor tells you the actual material length stayed constant during the bend.
- Does this account for springback?
- No — springback (the material's tendency to partially return toward its original shape after bending) is a separate factor. Overbend slightly or apply your process's known springback compensation.
Last updated 7/12/2026