Sheet metal bend allowance calculator
Calculate the bend allowance and bend deduction you need to build a flat pattern for a sheet metal part.
How it is calculated
Bend allowance BA = θ × (R + K × T), with θ in radians. Bend deduction BD = 2 × (R + T) × tan(θ ÷ 2) − BA.
Worked example
T = 2 mm, R = 4 mm, 90°, K = 0.4: BA = 1.5708 × (4 + 0.8) = 7.54 mm, BD = 2 × 6 × 1 − 7.54 = 4.46 mm.
Steps
- Enter the material thickness and the inside bend radius from the drawing.
- Enter the bend angle and the K-factor for your material and tooling.
- Use the bend allowance for the flat length, or the deduction for the outside-dimension method.
Frequently asked questions
What K-factor should I use?
About 0.33 for small radii, and about 0.45 to 0.5 when the radius is large. Measure your own bends with the K-factor calculator to confirm.
Allowance or deduction?
Use the allowance when you add the bend length to the flanges. Use the deduction when you measure to outside dimensions.
Does it apply to all materials?
It applies to ductile sheet metal. Harder or thicker materials need a larger radius and a tested K-factor.
Related calculators
References
Bend allowance and K-factor method as in standard sheet metal practice (the Machinery Handbook, sheet metal bending).
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.