When sheet metal bends, the inside compresses and the outside stretches. Somewhere between them is a neutral axis that keeps its length; its position as a fraction of thickness is the K-factor, typically 0.3–0.5. CAD programs use the K-factor to unfold flat patterns, and it is best found from a test bend in the actual material and tooling.
How to use the K-Factor Calculator
- Material thickness
- Inside bend radius
- Bend angle
- Bend allowance
- Every result field is editable: type into K-factor and Bend allowance plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the material thickness, inside bend radius, bend angle and the bend allowance from a test piece. Or type a K-factor into the result to calculate the bend allowance.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Material thickness | Units: mm, in, cm |
| Inside bend radius | Units: mm, in, cm |
| Bend angle | Units: °, rad |
| Bend allowance | Units: mm, in, cm |
What each result means
- K-factor = 180×[Bend allowance]÷(pi×[Bend angle]×[Material thickness]) – [Inside bend radius]÷[Material thickness]
- Y-factor = [K-factor]×pi÷2
- Outside setback (OSSB) = ([Inside bend radius] + [Material thickness])×tan(rad([Bend angle])÷2)
- Bend deduction = 2×[Outside setback (OSSB)] – [Bend allowance]
Example values
How the main result changes when Bend allowance changes and everything else stays at the example values above:
| Bend allowance (mm) | K-factor |
|---|---|
| 2.12 | -0.3252 |
| 3.18 | 0.0122 |
| 4.24 | 0.3496 |
| 5.3 | 0.687 |
| 6.36 | 1.0245 |
| 8.48 | 1.6993 |
Formula
K = 180 × BA ÷ (π × θ × T) − Ri ÷ TBA = π/180 × θ × (Ri + K × T)OSSB = (Ri + T) × tan(θ/2); BD = 2 × OSSB − BA; Y = K × π/2Worked example
Inputs — Material thickness: 2 mm; Inside bend radius: 2 mm; Bend angle: 90 °; Bend allowance: 4.24 mm.
Results — K-factor = 0.3496; Y-factor = 0.5492; Outside setback (OSSB) = 4 mm; Bend deduction = 3.76 mm.
Key terms
- Neutral axis
- The layer within the sheet that neither stretches nor compresses during bending.
Frequently asked questions
What is a typical K-factor?
About 0.33 for soft materials or tight bends, 0.4–0.45 for most air-bent steel and 0.5 for large radii.
What is the difference between bend allowance and bend deduction?
BA is the arc length of the neutral axis added to the flange lengths; BD is subtracted from the sum of outside dimensions.
How do I measure the bend allowance?
Bend a test strip of known flat length, measure the flanges and subtract their straight portions.
What formula does the K-Factor Calculator use?
K = 180 × BA ÷ (π × θ × T) − Ri ÷ T.