Bend Allowance Calculator

Flat-pattern sheet metal calculator. Enter thickness, inside radius, bend angle, and K-factor. Returns bend allowance, neutral axis offset, bend deduction, and developed length.

Mild steel air-bend K

0.42

Aluminum 5052 K

0.40

Stainless 304 K

0.43

Practical K range

0.33–0.50

Units Switches linear inputs and outputs

Material thickness T (mm) Nominal sheet thickness

Inside bend radius R (mm) Punch tip radius, measured or spec'd

Bend angle (degrees) Complement of included angle

K-factor See K-factor reference

Bend allowance

6.03mm

90° bend, K = 0.42

Neutral axis offset

0.84mm

Bend deduction

3.97mm

Developed length (single bend)

6.03mm

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Tip Bend allowance uses the neutral axis arc length. Bend deduction subtracts from summed OML flange lengths. Pick one method for the whole flat — do not mix BA and BD on the same drawing.

Worked example

T = 2 mm, R = 3 mm, 90° bend, K = 0.42 (air bend, mild steel, ~1.5 R/T).

1. Neutral axis offset offset = K × T = 0.42 × 2 = 0.84 mm 2. Bend allowance BA = (π/180) × angle × (R + K × T) BA = 1.5708 × (3 + 0.84) BA = 6.03 mm 3. Bend deduction BD = 2 × (R + T) × tan(angle/2) − BA BD = 2 × 5 × tan(45°) − 6.03 BD = 10 − 6.03 BD = 3.97 mm

K-factor reference by material

Material	Soft (tight R/T)	Medium	Hard (large R/T)
Aluminum 1100-H14	0.33	0.38	0.44
Aluminum 5052-H32	0.34	0.40	0.45
Mild Steel 1018	0.35	0.41	0.46
Stainless 304	0.37	0.43	0.48
Copper	0.33	0.39	0.44
Brass	0.34	0.40	0.45

Full tooling-method breakdown in the K-factor & bend allowance reference.

Common mistakes

Using outside radius. Bend allowance uses the inside radius R. Outside radius = R + T.
Entering included angle for bend angle. A 90° bend means the material turns 90°, not the included angle of 90°. A U-channel has two 90° bends, not 180°.
Trusting one K value across all thicknesses. K for 0.030" mild steel ≠ K for 0.250" mild steel. Calibrate per thickness-tooling combo.
Forgetting springback. BA gets the length right but not the final angle. Overbend 1–2° on mild steel, more on aluminum and stainless.

Warn Production tolerances tighter than ±0.010" per leg require a test coupon on the exact tooling and material lot. Theoretical K gets you close, but the final bend table is empirical.

FAQ

What is bend allowance in sheet metal?

The neutral-axis arc length assigned to the bend zone of a formed part. It is the amount added to the flat pattern for the curved region so the cut blank matches the final bent geometry.

What is K-factor and how does it relate to bend allowance?

K-factor is the fractional position of the neutral axis through the material thickness, measured from the inside face. It appears in the bend allowance equation as R + K × T.

How accurate is a theoretical bend allowance calculation?

Good enough for quoting and first-pass CAM flat patterns. For tight tolerances, validate with a test coupon and adjust K until the measured flat matches the calculation.

What's the difference between bend allowance and bend deduction?

BA is the arc length added to the neutral-axis method. BD is the value subtracted from summed OML flange lengths. Same geometry, two workflows.

How do I find the right K-factor for my material?

Start from a reference table, cut a coupon, bend on the production setup, measure, and back-solve K from the measured flat length.

Sources

Ind. Press Machinery's Handbook — Sheet Metal Protocase Bend Allowance & K-Factor guide Xometry Sheet Metal Bending reference Fabricator Bending Basics (Wilson Tool / Benson)

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