Plan precise bends with dependable deduction and allowance outputs, every job today. Save time on fabrication by validating radii, angles, and thickness before cutting.
| Unit | Thickness | K | Angle | Radius | Flange 1 | Flange 2 | BD (result) | Flat (result) |
|---|---|---|---|---|---|---|---|---|
| mm | 2.0 | 0.33 | 90 | 3.0 | 60 | 40 | ≈ 7.394 | ≈ 92.606 |
Bend deduction (BD) converts finished flange dimensions into a flat pattern you can cut with confidence. It prevents common site issues like misaligned holes, short returns, and gaps in brackets, trays, flashings, and sheet-metal closures—especially when parts repeat across floors or bays. That means fewer recuts, fewer trips, and cleaner inspections.
BD depends on thickness (T), inside radius (IR), bend angle (A), and K-factor (K). In day-to-day work, IR and K create most of the variation. If either changes with tooling or material batch, flat length changes even when flange lengths stay the same. Even a small radius shift can move hole-to-edge distances.
K locates the neutral axis as a fraction of thickness. For many air-bent steel and aluminum parts, K commonly lands around 0.30–0.50, with 0.33 a practical starting value. Bottoming/coining often shifts behavior, so record K per tooling setup.
Use the radius you actually form, not a wish value. Air bending can yield different IR for the same die if thickness or grade changes. A quick coupon bend and a radius gauge can lock in IR for the job, improving repeatability across crews.
Outside setback connects the bend to layout: OSSB = (IR + T) × tan(A/2). At 90°, tan(45°)=1, so OSSB becomes (IR + T). This makes a handy field check when verifying templates.
Bend allowance is the arc length through the bend at the neutral axis: BA = (π/180) × A × (IR + K×T). Bend deduction then ties it to finished flanges: BD = 2×OSSB − BA. Higher K increases BA and typically reduces BD.
For multi-bend parts, apply BD per bend and estimate flat length as Σ(flange lengths) − Σ(BD). When bends are close, sequence can affect results. Document the bend order and tooling so your BD stays valid for rework or future phases.
Cut one test blank, bend with planned tooling, then measure finished flanges. If parts come out long, BA is too large (often K too high); if short, BA is too small. Adjust K in steps of 0.02–0.03, then export CSV/PDF to keep a job record.
Bend allowance is the arc length added through the bend at the neutral axis. Bend deduction is what you subtract from the sum of flange lengths to get the flat length for cutting.
Start with 0.33 for typical air bending. Run a single test bend, measure the finished flanges, then adjust K up or down until the flat pattern predicts the real outcome.
Yes. Both OSSB and BA depend on the bend angle. Acute bends generally reduce OSSB and BA compared to 90°, changing BD and therefore the calculated flat length.
Use the radius you actually get with your tooling and material. If you only have a target, bend a coupon first, measure the real inside radius, and then calculate production parts.
Different presses, dies, and operators shift the neutral axis location and effective inside radius. That changes K and BA. Calibrate each machine/tooling combination and save those settings per job.
Yes. Add one row per bend. The calculator totals bend deductions and estimates flat length by subtracting total BD from the sum of your flange lengths.
Use the same unit system throughout the job and measure thickness and radius in that unit. Consistency matters more than the unit choice; the math scales correctly either way.
Accurate bend deductions reduce waste and rework onsite significantly.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.