Bending Guide
Bend Allowance vs Bend Deduction
Two terms for the same bend, measured from two different starting points — and why the distinction actually matters when you're cutting a blank.
Bend allowance and bend deduction get used almost interchangeably in casual shop talk, but they answer two different questions. Bend allowance asks: "how much material does the bend itself consume, measured along the neutral axis?" Bend deduction asks: "how much shorter is the flat blank than the sum of the part's outside flange dimensions?" Mixing them up is one of the most common ways a flat pattern ends up the wrong length.
Let the calculator handle both
Enter your flanges, radius, thickness and K-factor — bend allowance, bend deduction and the final flat length come out together.
Bend allowance, in one line
Bend allowance (BA) is the arc length of the neutral axis as it passes through the bend — the length of material "used up" by the fold itself:
See how to calculate bend allowance for the full breakdown of this formula.
Bend deduction, in one line
Bend deduction (BD) starts from a different reference point: the outside dimensions of the flanges, the way most drawings are actually dimensioned. It's the amount you subtract from the sum of those outside flange lengths to land on the correct flat length:
OSSB (outside setback) is the distance from the bend's apex back to where the flange's outside edge would meet if it weren't bent at all. Two setbacks minus the bend allowance gives you the deduction.
Side by side
| Bend allowance | Bend deduction |
|---|---|
| Measured along the neutral axis | Measured from the outside flange dimensions |
| Represents material consumed by the bend | Represents the correction to outside-dimensioned flanges |
| Used when flanges are dimensioned to the bend line | Used when flanges are dimensioned to the outside (most common) |
| Flat length = Σ (flange-to-bend-line) − Σ BA — rarely used this way directly | Flat length = Σ (outside flange) − Σ BD — the standard method |
Which one should you actually use?
In practice, almost every real-world drawing dimensions flanges to their outside edges, which makes bend deduction the number you want for laying out a flat pattern. Bend allowance still matters — it's the building block bend deduction is calculated from, and understanding it makes the deduction formula make sense rather than feeling like a memorised rule.
Frequently asked questions
What is the difference between bend allowance and bend deduction?
Bend allowance is the developed length of material within the bend, measured along the neutral axis. Bend deduction is the amount subtracted from the sum of the outside flange dimensions to reach the correct flat length. Same bend, two reference points.
Which one do I use to calculate a flat pattern?
Bend deduction. If your flange lengths are measured to the outside of the part — the more common way to dimension a drawing — subtract the bend deduction from the sum of those flanges.
How do you convert bend allowance to bend deduction?
BD = 2 × OSSB − BA, where OSSB = tan(Angle/2) × (R + T). The calculator works both out for you automatically from the same inputs.