Sheet Metal K-Factor Calculator
Calculate bend allowance (BA), outside setback and bend deduction (BD) from a K-factor — or work backwards from a test bend to find your shop’s real K-factor. Use any consistent length unit.
Formulas
The K-factor is the location of the neutral axis — the layer that neither stretches nor compresses during bending — as a fraction of the thickness measured from the inside surface. K = 0.5 would put it in the middle; in practice the neutral axis shifts toward the inside, so real values range from about 0.3 to 0.5. The bend allowance is the arc length of the neutral axis through the bend, and the bend deduction is how much shorter the flat blank is than the sum of the outside flange lengths. The angle here is the bend angle (how far the sheet is bent, 90° for an L-bracket), not the included angle.
Worked example
A 2 mm mild steel sheet is bent 90° with a 2 mm inside radius and K = 0.44. θ = π/2 = 1.5708. BA = 1.5708 × (2 + 0.44 × 2) = 4.524 mm. OSSB = tan 45° × (2 + 2) = 4.000 mm. BD = 2 × 4.000 − 4.524 = 3.476 mm. So for an L-bracket with 50 mm and 30 mm outside flanges, the flat blank is 50 + 30 − 3.476 = 76.52 mm.
Finding your K-factor: bend a test strip of known flat length, measure both outside flange lengths, and compute BD = (flange₁ + flange₂) − flat length. Enter BD with the thickness, radius and angle to get the K-factor your press brake and tooling actually produce.
Typical K-factor values
| Material / condition | Typical K-factor |
|---|---|
| Soft materials (copper, soft aluminium), R ≥ T | 0.35 – 0.45 |
| Mild steel, air bending, R ≈ T | 0.40 – 0.44 |
| Stainless steel, air bending, R ≈ T | 0.42 – 0.46 |
| Any material, tight bend R < T | 0.30 – 0.38 |
| Bottoming | 0.38 – 0.42 |
| Coining | 0.33 – 0.38 |
| Large radius R > 3T | 0.45 – 0.50 |
| Common CAD default | 0.44 (SolidWorks/Inventor often use 0.5 or 0.44) |
Bend allowance table (90° bends)
| T | R (inside) | K | BA (90°) | BD (90°) |
|---|---|---|---|---|
| 1 mm | 1 mm | 0.33 | 2.089 mm | 1.911 mm |
| 1 mm | 1 mm | 0.44 | 2.262 mm | 1.738 mm |
| 1.5 mm | 1.5 mm | 0.33 | 3.134 mm | 2.866 mm |
| 1.5 mm | 1.5 mm | 0.44 | 3.393 mm | 2.607 mm |
| 2 mm | 2 mm | 0.33 | 4.178 mm | 3.822 mm |
| 2 mm | 2 mm | 0.44 | 4.524 mm | 3.476 mm |
| 3 mm | 3 mm | 0.33 | 6.267 mm | 5.733 mm |
| 3 mm | 3 mm | 0.44 | 6.786 mm | 5.214 mm |
Tips for accurate flat patterns
- For air bending, the inside radius is set by the die opening (roughly 15–16% of V-die width for mild steel), not the punch radius.
- Use one K-factor per material, thickness and tooling combination; a bend table is better still.
- Measure springback separately; K-factor only affects length, not the final angle.
Frequently asked questions
What is a good default K-factor?
0.44 is a common default for air-bent mild steel with R ≈ T. Many CAD systems default to 0.5, which slightly overestimates the blank length.
What is the difference between bend allowance and bend deduction?
Bend allowance is the neutral-axis arc length added to the flat (inside-tangent) lengths; bend deduction is subtracted from the sum of outside flange lengths. Both give the same flat blank.
Can the K-factor be greater than 0.5?
Physically the neutral axis does not move past the mid-plane in normal bending, so K above 0.5 usually indicates a measurement or input error.
Does the K-factor depend on bend angle?
Only weakly. It mainly depends on material, the R/T ratio and the bending method (air bending, bottoming, coining).
What units should I use?
Any consistent length unit. Thickness, radius and the measured BA or BD must all be in the same unit (mm or inches).