Sheet Metal & Fabrication
K-Factor Calculator
Last Updated On
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
Material Preset
Results
Flat Pattern Length
96.6077 mm
Bend Allowance (BA)
5.6077 mm
Leg A (actual)
45.5000 mm
Outside mold line − OSSB
Leg B (actual)
45.5000 mm
Outside mold line − OSSB
R/T Ratio
2.00
✓ Safe bend radius
Y-Factor
0.5969
Y = K × π/2
Expert Note
Flat blank = 96.608 mm. Always make a test bend in scrap material to verify K-Factor before cutting your production batch — K varies with die width, punch radius, material hardness, and grain direction.
Legs measured to outside mold lines (intersection of flange extensions). K-Factor presets are typical values — always validate with a test bend.
Step-by-Step Calculation
Step 1 — Bend angle in radians
90.00° × π ÷ 180 = 1.57080 rad
Step 2 — Bend allowance (arc length on the neutral axis)
BA = 1.57080 × (3.000 + 0.380 × 1.500) = 5.6077
Step 3 — Outside setback
OSSB = tan(45.00°) × (3.000 + 1.500) = 4.5000
Step 4 — Bend deduction
BD = 2 × 4.5000 − 5.6077 = 3.3923
Step 5 — Flat pattern
50.000 + 50.000 − 3.3923 = 96.6077
Result — cut the blank 96.6077 long
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.
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 |
Flat Patterns and Bend Allowance
Sheet metal stretches on the outside of a bend and compresses on the inside, and the K-factor describes where the boundary sits:
- The Neutral Axis Shifts Inward: Under bending the neutral axis moves toward the inside face, typically to 0.33–0.5 of material thickness. K-factor is exactly that ratio, and it is what makes flat patterns accurate.
- Flat pattern size: Get the bend allowance right and parts come off the brake at the correct finished dimension. Get it wrong and every bend compounds the error along the part.
- K-Factor Is Not Universal: It varies with material, thickness, bend radius, and tooling. A value taken from a chart is a starting point; production accuracy comes from measuring test bends on your own brake.
Example Calculations
Three scenarios worked through step by step:
Example 1 — 90° Bend in Mild Steel
0.0625 in thick, 0.0625 in inside radius, K = 0.42, 90°.
Angle in radians = 90 × π ÷ 180 = 1.5708
BA = 1.5708 × (0.0625 + 0.42 × 0.0625)
BA = 1.5708 × 0.08875 = 0.1394 in
OSSB = tan(45°) × (0.0625 + 0.0625) = 0.125 in
BD = (2 × 0.125) − 0.1394 = 0.1106 in
Example 2 — Flat Pattern
Two flanges of 2 in each, one 90° bend, BD = 0.1106 in.
Sum of flanges = 2 + 2 = 4 in
Flat length = 4 − 0.1106 = 3.889 in
Cut the blank at 3.889 in
After bending, the outside dimensions read 2 × 2 in
Result: the deduction is what makes it land right
Example 3 — 135° Bend
0.125 in thick aluminium, 0.125 in radius, K = 0.40, 135°.
Angle in radians = 135 × π ÷ 180 = 2.3562
BA = 2.3562 × (0.125 + 0.40 × 0.125)
BA = 2.3562 × 0.175 = 0.4123 in
OSSB = tan(67.5°) × 0.25 = 0.6036 in
BD = (2 × 0.6036) − 0.4123 = 0.7949 in
Press Brake Tip
Press Brake Tip
Bend a test coupon in your actual material with your actual tooling and measure the result, then back-calculate the real K-factor from it. Published values assume ideal conditions; grain direction, die opening, and material batch all shift the answer enough to matter on a tight tolerance part.
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.
- How do I calculate flat pattern length?
- Add the outside dimensions of all flanges, then subtract the bend deduction for each bend. A two-flange part with 2 in legs and a 0.11 in deduction gives 4 − 0.11 = 3.89 in of blank.
- What is outside setback?
- The distance from the bend tangent point to the apex where the two outside faces would intersect if extended. It equals tan(angle ÷ 2) × (inside radius + thickness), and it links bend allowance to bend deduction.