What is bend allowance?
Bend allowance is the length of the neutral axis through the bend, which is the arc that keeps its original length when the sheet is formed. It is added to the straight legs to give the flat blank length. Bend deduction is the amount subtracted from the outside dimensions instead.
How are bend allowance and bend deduction calculated?
Bend allowance is the bend angle in radians multiplied by the neutral axis radius, R + K·t. Bend deduction is twice the outside setback minus the bend allowance. Use one method or the other, never both, when you calculate the flat pattern.
| Method | Formula | Used when |
|---|---|---|
| Bend allowance | BA = θrad · (R + K·t) | You add the allowance to the straight legs |
| Bend deduction | BD = 2(R + t)·tan(θ/2) − BA | You subtract the deduction from the outside dimensions |
| Flat length (legs method) | L = A + B − BD | A and B are outside dimensions |
What K factor should I use?
For general air bending a K factor between 0.33 and 0.50 is typical, with about 0.44 a common starting value. Tight radii in thin material sit near the lower end, and large R/t ratios sit near the upper end.
| Condition | R/t ratio | Typical K factor |
|---|---|---|
| Tight radius in thin material | R < t | 0.33 – 0.40 |
| General air bending | R ≈ t to 3t | 0.40 – 0.46 |
| Large radius | R > 3t | 0.46 – 0.50 |
What is the difference between bend allowance and bend deduction?
They are two ways of expressing the same correction. Bend allowance adds the arc length of the neutral axis to the leg lengths. Bend deduction subtracts the overlap you get when you measure the legs to the outside corner instead.
Mixing the two in one calculation is the most common cause of a blank that comes out too short or too long. Decide which one your drawing uses and stay with it.
Related technical pages
K factor calculator
Work back to the K factor from a measured flat length.
Bend allowance guide
The theory behind the neutral axis, setback and flat length.
Minimum bend radius
How tight the radius can be before the material cracks.
Press brake dies
Which die geometry produces which radius.
Next step
Send the drawing and material grade and we will check the flat pattern and recommend the die and radius that match your bending method.
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