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Sheet Metal Bend Allowance and K-Factor Calculator

Bend allowance = bend angle (radians) × (inside radius + K-factor × thickness), and the flat length is the two outside leg lengths minus the bend deduction.

in
in
Typically 0.33 (soft, tight radius) to 0.5. Air-bent steel is about 0.4.
°
in
in
Flat pattern length
4.7749in
Bend allowance0.2749 in
Outside setback0.25 in
Bend deduction0.2251 in

Show the math

BA = θ × (R + K × T) = 90° × π ÷ 180 × (0.125 + 0.4 × 0.125) = 0.2749 in
Setback = tan(90° ÷ 2) × (0.125 + 0.125) = 0.25 in
Bend deduction = 2 × 0.25 − 0.2749 = 0.2251 in
Flat length = 2 + 3 − 0.2251 = 4.7749 in

Rounded the same way as the result above.

How it works

When sheet metal bends, the outside stretches and the inside compresses, but somewhere between them lies a neutral axis that keeps its length. The K-factor locates that axis as a fraction of thickness from the inside face.

The bend allowance is the arc length of that neutral axis. Add the flat legs to the allowance to get the flat length, or subtract the bend deduction from the sum of the outside dimensions. The setback locates the bend line from the mold point.

BA = θ × (R + K × T) (θ in radians) OSSB = tan(θ ÷ 2) × (R + T) BD = 2 × OSSB − BA Flat length = outside leg 1 + outside leg 2 − BD

Worked example

1/8 inch mild steel bent 90° with a 1/8 inch inside radius, K-factor 0.4, legs of 2 and 3 inches outside:

  1. BA = θ × (R + K × T) = 90° × π ÷ 180 × (0.125 + 0.4 × 0.125) = 0.2749 in
  2. Setback = tan(90° ÷ 2) × (0.125 + 0.125) = 0.25 in
  3. Bend deduction = 2 × 0.25 − 0.2749 = 0.2251 in
  4. Flat length = 2 + 3 − 0.2251 = 4.7749 in
InputValue
Material thickness0.125 in
Inside bend radius0.125 in
K-factor0.4
Bend angle (angle through which metal is bent)90 °
Outside leg 12 in
Outside leg 23 in
ResultValue
Bend allowance0.2749 in
Outside setback0.25 in
Bend deduction0.2251 in
Flat pattern length4.7749 in

Assumptions and limits

  • K-factor depends on material, tooling, and method. Bend a test piece to calibrate for your press brake.
  • The bend angle is the angle the metal is turned through (a 90° corner is 90°), not the included angle.
  • Leg lengths are outside dimensions, measured to the virtual sharp.

Common questions

What K-factor should I use?

A common starting point is 0.33 for soft materials with a tight radius, 0.4 for air-bent mild steel, and 0.5 for large radii or hard materials. Always verify on a test bend.

What is the difference between bend allowance and bend deduction?

Allowance is the neutral-axis arc length, added to the inside legs. Deduction is subtracted from the outside leg total. They describe the same geometry.

Does this work for more than one bend?

Compute each bend separately and sum the deductions from the total of the outside dimensions.

Sources

  • Standard sheet metal bend formulas: BA = θ(R + K·T); OSSB = tan(θ/2)(R + T); BD = 2·OSSB − BA (see Machinery's Handbook, sheet metal bending).
  • K-factor is an empirical value and depends on material, tooling and method.

Updated 2026-09-30