How it works
For a single bend, the bend deduction is BD = 2 · OSSB − BA, where the outside setback is OSSB = (R + t) · tan(angle/2) and the bend allowance is BA = (π/180) · angle · (R + K · t). Here R is the inside radius, t the thickness, and K the K-factor (the neutral-axis position as a fraction of the thickness). The flat (blank) length is the sum of the outside (mould-line) flange dimensions minus one bend deduction per bend: L = A + B − BD.
Worked example
Two 50 mm outside flanges, a 90° bend, 3 mm inside radius, 2 mm thick, K = 0.33:
the outside setback is (3 + 2) × tan 45° = 5.0 mm and the bend
allowance is (π/180) × 90 × (3 + 0.33 × 2) ≈ 5.749 mm, so the
bend deduction is 2 × 5.0 − 5.749 ≈ 4.25 mm. The flat blank is
50 + 50 − 4.25 ≈ 95.75 mm. The calculator returns exactly this.
Frequently asked questions
- What is bend deduction?
- Bend deduction is the length you subtract from the sum of a part’s outside flange dimensions to get the flat (blank) length. For one bend it is BD = 2·OSSB − BA, where OSSB = (R + t)·tan(angle/2) is the outside setback and BA = (π/180)·angle·(R + K·t) is the bend allowance.
- How do I calculate the flat (blank) length?
- Add up the outside (mould-line) flange dimensions and subtract one bend deduction for each bend: flat length = Σ outside flanges − Σ bend deductions. For a single 90° bend joining two 50 mm outside flanges with R = 3 mm, t = 2 mm, K = 0.33, the bend deduction is ≈ 4.25 mm, so the blank is 100 − 4.25 ≈ 95.75 mm.
- What is the difference between bend deduction and bend allowance?
- Bend allowance (BA) is the developed length of the arc along the neutral axis through the bend itself. Bend deduction (BD) is what you remove from the summed outside flanges: BD = 2·OSSB − BA. Use bend allowance when you sum the flat flange lengths plus the arc; use bend deduction when you work from the outside (mould-line) dimensions — the more common shop method.
- Why do my bent parts come out the wrong length?
- Almost always a wrong K-factor or inside radius. The K-factor is empirical and varies with material, thickness and tooling, and the actual inside radius from air bending differs from the punch tip. Cut one test piece, measure the flanges after bending, back-calculate the bend deduction, and use that K and radius for the batch.
- How do I handle a part with multiple bends?
- Subtract a bend deduction for every bend. Flat length = sum of all outside flange segments − (BD₁ + BD₂ + …). If every bend has the same angle, radius, thickness and K-factor, each bend removes the same BD; bends with different angles or radii each get their own.
- Does this work in metric and imperial?
- Yes — enter the flanges, radius and thickness in mm or inches; the bend deduction, outside setback and flat length convert to your unit system. The bend angle and K-factor are unitless.
Method & assumptions
- Flange A and B are outside (mould-line), not inside, dimensions.
- Subtract one bend deduction for each bend; multiply for repeated identical bends.
- The K-factor is empirical (~0.33–0.45) — confirm it with a sample bend before cutting a batch, as it varies with material, thickness and tooling.
- The inside radius is the formed radius; air bending can differ from the punch-tip radius.
Related calculators
- Bend Allowance Calculator — Bend allowance, deduction and setback from angle, radius, thickness and K-factor.