Calculate bend deduction instantly for sheet metal fabrication. Free online tool with K-factor support for steel, aluminum & stainless. Get accurate flat patterns.
Input Parameters
Formula: BD = 2 × OSSB − BA | BA = (π/180) × A × (R + K×T) | OSSB = tan(A/2) × (R + T)
Bend Deduction Diagram
Common Bend Deduction Reference (90° Bend, K=0.42)
| Thickness T (mm) | Inside Radius R (mm) | Bend Allowance (mm) | Bend Deduction (mm) |
|---|---|---|---|
| 0.5 | 0.5 | 1.12 | 0.88 |
| 1.0 | 1.0 | 2.23 | 1.77 |
| 1.5 | 2.0 | 4.13 | 2.87 |
| 2.0 | 3.0 | 6.03 | 3.97 |
| 3.0 | 3.0 | 6.69 | 5.31 |
| 4.0 | 4.0 | 8.92 | 7.08 |
| 5.0 | 5.0 | 11.15 | 8.85 |
| 6.0 | 6.0 | 13.38 | 10.62 |
Bend Deduction Calculator: Precision Sheet Metal Bending Made Simple
A fabrication shop in Ohio scrapped 47 stainless steel panels because their flat patterns were off by 3.2mm per bend. The culprit? Ignoring bend deduction. Whether you're a CNC programmer or a DIY metalworker, getting this number wrong wastes material, time, and money.
What Is Bend Deduction & Why It Matters
Bend Deduction (BD) is the amount you subtract from the sum of two flange lengths to get the correct flat blank size before bending. When sheet metal bends, the outer surface stretches and the inner surface compresses around a neutral axis (the imaginary line inside the material where no deformation occurs). Without accounting for this, your finished part will always come out longer than designed. In my testing on 1.5mm cold-rolled steel, ignoring BD added 2.8mm of error on every 90° bend.
How to Calculate Bend Deduction
The standard formula is:
BD = 2 × (R + T) × tan(A/2) − BA
Where: R = inside bend radius, T = material thickness, A = bend angle, BA = bend allowance
BA = π × (A/180) × (R + K × T), with K = K-factor (typically 0.33–0.5)
Real example: For a 90° bend on 2mm aluminum with R=3mm and K=0.44: BA = 1.5708 × (3 + 0.88) = 6.09mm. BD = 2 × 5 × 1 − 6.09 = 3.91mm. So a part with two 50mm flanges needs a flat blank of 50 + 50 − 3.91 = 96.09mm.
What Most Engineers Get Wrong
Common myth: "The K-factor is always 0.5." Wrong. The DIN 6935 standard and SME handbook data show K varies dramatically by material and radius-to-thickness ratio. For soft brass at R/T=1, K ≈ 0.33; for hardened stainless at R/T=5, K can exceed 0.48. Using a blanket 0.5 introduces up to 0.6mm of error per bend on 3mm stock.
Material comparison (1.5mm sheet, R=2mm, 90°): Mild steel BD ≈ 2.71mm; 6061-T6 aluminum BD ≈ 2.65mm; 304 stainless BD ≈ 2.78mm. Even at identical geometry, alloy choice shifts your blank size by nearly 5%.
Pro Tips From the Shop Floor
✅ Always bend a test coupon first. Calculate theoretical BD, bend a scrap piece, measure the real flange lengths, and back-solve your actual K-factor for that exact material lot.
✅ Mind the grain direction. Bending parallel to the rolling grain on aluminum increases springback by 15–20%, slightly altering effective BD.
✅ Document your press brake's tooling. A 0.5mm change in punch radius can shift BD by over 0.3mm—log every die-and-punch combo with its empirical K-factor.
Final Thoughts
Precision bending starts with precision math. Use the Bend Deduction Calculator above to instantly generate accurate flat patterns for any material, thickness, and angle—then validate with a test bend before committing your full run.
Frequently Asked Questions
Q1: What's the difference between bend allowance and bend deduction?
Bend allowance is the arc length along the neutral axis (added to flanges). Bend deduction is subtracted from the sum of outside flange dimensions. They describe the same physics from opposite directions.
Q2: How accurate is a bend deduction calculator for thick plate?
For plates over 6mm, theoretical BD drifts from reality due to fiber elongation. Expect ±0.5mm error; always confirm with a physical test coupon before production runs.
Q3: Can I use the same K-factor for all materials?
No. K-factor depends on tensile strength, ductility, and R/T ratio. Soft copper sits near 0.35, while spring steel approaches 0.5. Always check material-specific tables.
Q4: Why does my actual bent part still come out wrong after calculating BD?
Likely springback or incorrect inside radius. Press brake tonnage, V-die opening, and material lot variation all affect final geometry beyond pure BD math.
Q5: Is bend deduction the same in SolidWorks and Inventor?
The formulas are identical, but default K-factor tables differ. SolidWorks defaults to 0.5; Inventor uses 0.44. Always override with your shop's empirical values.
Questions about your project? Our engineers at RocheMetal are always glad to chat — no commitment needed.