Bending Guide
Bend Deduction: Formula and Table
Bend deduction is the amount subtracted from the sum of outside flange dimensions to calculate a sheet metal flat 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 along a straight outside face from the theoretical sharp corner to the bend tangency point. Two setbacks minus the bend allowance gives you the deduction.
Bend deduction table for 90° bends
Calculated estimates in millimetres using the site’s mild-steel preset K = 0.33. At 90°, OSSB = R + T, so BD = 2 × (R + T) − (π/2) × (R + 0.33 × T). Radius is the finished inside radius; verify against a test bend.
| Thickness T (mm) | Inside radius R (mm) | K-factor | Bend deduction (mm) |
|---|---|---|---|
| 1 | 1 | 0.33 | 1.911 |
| 1.5 | 1.5 | 0.33 | 2.866 |
| 2 | 2 | 0.33 | 3.822 |
| 3 | 3 | 0.33 | 5.733 |
| 4 | 6 | 0.33 | 8.502 |
For outside legs of 50 mm and 40 mm, T = R = 2 mm and K = 0.33, BA = 4.178 mm and BD = 3.822 mm. Flat length = 50 + 40 − 3.822 = 86.178 mm. Keep full precision until the final result. Compare the bend allowance and deduction chart or check one bend in the sheet metal layout calculator.
Angle means material rotation in degrees, not the remaining opening. Evaluate the tangent in degrees or convert to radians. For straight tangent lengths, add BA; for outside mould-line dimensions, subtract BD.
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 with straight lengths measured to bend tangency points | Used when flanges are dimensioned to the outside (most common) |
| Flat length = Σ straight tangent lengths + Σ BA | 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.