Sheet Metal & Fabrication
K-Factor Calculator
Calculate the flat blank length for press-brake and HVAC duct work. Enter your material thickness, inside bend radius, bend angle, and K-Factor — the calculator returns Bend Allowance (BA), Bend Deduction (BD), Outside Setback (OSSB), and the exact flat pattern dimension. Material presets for steel, aluminium, stainless steel, and copper. Supports metric (mm) and imperial (inches).
K-Factor Calculator
Sheet metal bend allowance, bend deduction & flat pattern length
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Enter dimensions above to calculate
Bend Geometry Diagram
The neutral axis (dashed) sits at K × T from the inside face. All arc lengths are measured along this axis to calculate the Bend Allowance.
Step-by-Step Formulas
1. K-Factor Definition
When sheet metal is bent, the outer surface stretches and the inner surface compresses. Somewhere in between lies the neutral axis — the layer that neither stretches nor compresses. K-Factor locates that axis as a fraction of the total thickness. A K of 0.5 means the neutral axis is at the material mid-plane. Soft, ductile materials like aluminium have a neutral axis closer to the inside (lower K ≈ 0.33–0.38). Hard materials like stainless steel shift it toward the centre (K ≈ 0.45–0.50).
2. Outside Setback (OSSB)
The Outside Setback is the distance from the bend tangent line to the outside mold line (OML). It is the amount that must be subtracted from each leg before summing them with the Bend Allowance to get the flat pattern. When fabricators dimension drawings "to the outside" of the part — the most common convention — OSSB is always in play.
3. Bend Allowance (BA)
Bend Allowance is the arc length of the neutral axis as it travels through the bend zone. It is the actual material used in the curved portion of the part. A 90° bend with a tight radius consumes less material than a gentle, large-radius bend at the same angle.
4. Bend Deduction (BD)
Bend Deduction is the shorthand number most press-brake operators use. When you know Leg A and Leg B measured to the outside mold lines, simply subtract BD once from the total (Flat = A + B − BD) to get the flat blank length. BD is always positive for angles less than ~150° and simplifies the calculation to one subtraction.
5. Flat Pattern Length
The total flat blank length from which the part will be bent. For parts with multiple bends, add each Bend Allowance and subtract each Bend Deduction sequentially. Many CAD/CAM systems calculate this automatically once the correct K-Factor is entered — verifying with a physical test bend ensures the software K matches your actual tooling and material condition.
Worked Example
Cold-rolled steel bracket: T = 1.5 mm, R = 3.0 mm, θ = 90°, K = 0.38, Leg A = 50 mm, Leg B = 50 mm.
💡 Pro tip
Cut a 100 mm wide test coupon at exactly the calculated flat length (96.61 mm), bend it at the same setup, then measure Leg A and Leg B. If they're both exactly 50 mm — your K is correct. If they're short, increase K; if they're long, decrease K. Each 0.01 change in K shifts the flat by roughly 0.024 mm per bend for this setup.
K-Factor Reference Values
| Material | Air Bend | Bottoming | Notes |
|---|---|---|---|
| Soft Aluminium / Copper | 0.33–0.35 | 0.35–0.38 | Most ductile; neutral axis nearest inside face |
| Cold-Rolled Steel (CRS) | 0.38–0.41 | 0.41–0.44 | Most common sheet metal; grain direction matters |
| Hot-Rolled Mild Steel | 0.40–0.43 | 0.42–0.46 | Slightly softer than CRS; more scale variation |
| Stainless Steel 304/316 | 0.44–0.47 | 0.46–0.50 | Work-hardens; use larger radius and higher K |
| Hard Brass / Bronze | 0.45–0.50 | 0.48–0.50 | Brittle; strict minimum bend radius required |
Frequently Asked Questions
- What is K-Factor in sheet metal bending?
- K-Factor is the ratio of the neutral axis location to the material thickness (K = t/T). It describes where inside the material the neutral axis (the layer that doesn't stretch or compress) is located during bending. Typical values range from 0.33 for very soft materials to 0.50 for hard materials.
- What K-Factor should I use for mild steel?
- For mild steel (hot-rolled or cold-rolled) bent on a standard V-die press brake, a K-Factor of 0.38–0.42 is typical. Use 0.38 for air bending with tight radii, 0.41 for standard air bending, and up to 0.44 for coining or bottoming. Always verify with a test bend before cutting a production run.
- What is the difference between Bend Allowance and Bend Deduction?
- Bend Allowance (BA) is the arc length of the neutral axis through the bend — the actual material consumed in the curved zone. Bend Deduction (BD = 2×OSSB − BA) is a shortcut: when your legs are measured to the outside mold line, subtract BD once to get the flat blank length. BD is more intuitive for press-brake work; BA is used for CNC programming and formed-length calculations.
- What is an acceptable minimum inside bend radius?
- A common rule of thumb is that the inside bend radius should be at least equal to the material thickness (R/T ≥ 1). For aluminium and stainless steel, R/T ≥ 1.5–2 is safer to prevent cracking, especially across the grain direction. Tighter radii require annealing, coining, or special tooling.
- Does K-Factor change with bend angle?
- Yes, slightly. The neutral axis shifts inward (lower K) for tight angles and sharper radii due to greater compressive strain. For most practical press-brake work, a single K-Factor per material gives results accurate to within 0.1–0.3 mm. For precision aerospace or HVAC duct work, calibrate K empirically by bending a test coupon and back-calculating from the measured flat length.