Sheet metal bend allowance calculator
Metal stretches on the outside of a bend and compresses on the inside. The neutral axis sits somewhere between, and the K-factor says where.
degrees
Flat blank length
96.461mm
- Bend allowance
- 4.4611
- Bend deduction
- 3.5389
- Outside setback
- 4.0000
- Neutral axis radius
- 2.8400
- Y-factor
- 0.6597
- Minimum practical flange
- 10.00
Show the working
- Material thickness = 2
- Inside bend radius = 2
- Bend angle = 90
- K-factor = 0.42
- First leg = 50
- Second leg = 50
- Bend allowance = rad(90) * (2 + 0.42 * 2) = 4.461 06
- Outside setback = (2 + 2) * tan(rad(90)/2) = 4
- Bend deduction = 2 * 4 - 4.461 06 = 3.538 94
- Flat blank length = 50 + 50 - 3.538 94 = 96.4611
- Neutral axis radius = 2 + 0.42 * 2 = 2.84
- Y-factor = 0.42 * 1.5707963267948966 = 0.659 734
- Minimum practical flange = 4 * 2 + 2 = 10
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- bend allowance = angle in radians x (inside radius + K x thickness)
- outside setback = (radius + thickness) tan(angle/2)
- bend deduction = 2 x setback - bend allowance
- flat length = sum of the legs - bend deduction
K-factor depends on material, radius-to-thickness ratio and tooling. Measure a test bend rather than trusting a default when tolerances are tight.
For a 90-degree bend in mild steel with a radius near one thickness, K is usually between 0.38 and 0.45.