Sheet metal

Sheet Metal K-Factor Calculator

Estimate the K-factor and bend allowance for an air-bend setup. K-factor is the location of the neutral axis as a fraction of material thickness and must ultimately be confirmed with a coupon test.

mild steel16 ga0.0598 in1.519 mm

Interactive calculator

Sheet Metal K-Factor Calculator

Neutral axis from inside face0.0251 in
Bend allowance0.1334 in
K-factor0.4200.30 to 0.50 is a common practical range.

Static reference

Common thickness, radius, and angle combinations

Swipe table horizontally for more columns.

GaugeThicknessInside radiusAngleBend allowanceBend deduction
16 ga0.0598 in / 1.519 mm0.0598 in / 1.519 mm45 deg0.0667 in0.0324 in
12 ga0.1046 in / 2.657 mm0.1569 in / 3.985 mm90 deg0.3155 in0.2075 in

Formula used

What this calculator computes

BA = (π / 180) × A × (r + K × t)
BA
Bend allowance (length of the neutral axis through the bend) (in or mm)
A
Bend angle (rotation angle of the bend, in degrees) (deg)
r
Inside bend radius (in or mm)
t
Material thickness (in or mm)
K
K-factor (neutral-axis position, fraction of t from the inside surface) (0 to 1)
  • Bend angle A is the angle of rotation through which the sheet is bent (the supplement of the included angle for less-than-180° bends). A flat sheet bent so its flanges meet at 90° corresponds to A = 90°. If your drawing labels the included angle, convert with A = 180° − included angle before entering it.
  • Typical K-factor ranges for air bending: 0.33–0.45 depending on material, radius, and tooling. Bottoming and coining shift K toward the inside surface; large radii shift K toward the centerline.
  • Default K-factors shipped with this calculator are starting estimates. See /reference/data-sources/.

Worked example

One numeric walk-through

Inputs

Material
mild steel
Thickness t
0.0598 in (16 ga)
Inside radius r
0.0598 in (1T)
Bend angle A
90°
K-factor
0.42

Steps

  1. Convert A to radians: 90° × π/180 = 1.5708 rad.
  2. Add radius and K·t: 0.0598 + 0.42 × 0.0598 = 0.0849 in.
  3. Multiply: BA = 1.5708 × 0.0849 ≈ 0.1334 in.

Result

Bend allowance ≈ 0.1334 in. Confirm with a coupon bend before cutting blanks for production.

Common mistakes

Things to check before trusting the number

Related calculators

Move between connected shop calculations

K-factorBend allowanceBend deductionFlat patternMinimum bend radiusReverse K-factorPress brake tonnageData sources

FAQ

Specific checks for this calculator

What K-factor should I use for air bending?

0.38 to 0.43 is a common starting range for air bending mild steel and aluminum. Confirm by measuring a coupon bend and back-solving K with the reverse K-factor calculator.

How does K-factor affect developed flat length?

K-factor sets where the neutral axis sits inside the bend. A higher K shifts the neutral axis toward the centerline of the material, which increases the bend allowance and the developed flat length.