# K-Factor Calculator

URL: https://speedycalculator.com/calculators/construction/k-factor/
Last updated: 2026-10-25

> The K-factor calculator finds the sheet-metal K-factor (neutral axis location) from a measured bend allowance, thickness, inside radius and bend angle, plus the bend deduction, outside setback and Y-factor.

## Inputs

- Material thickness (mm, in, cm)
- Inside bend radius (mm, in, cm)
- Bend angle (°, rad)
- Bend allowance (mm, in, cm)

## Results

- K-factor
- Y-factor
- Outside setback (OSSB)
- Bend deduction

## Formula

```
K = 180 × BA ÷ (π × θ × T) − Ri ÷ T
BA = π/180 × θ × (Ri + K × T)
OSSB = (Ri + T) × tan(θ/2); BD = 2 × OSSB − BA; Y = K × π/2
```

## Explanation

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

FieldWhat to enterMaterial thicknessUnits: mm, in, cmInside bend radiusUnits: mm, in, cmBend angleUnits: °, radBend allowanceUnits: 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-factor2.12-0.32523.180.01224.240.34965.30.6876.361.02458.481.6993

## Worked 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.

## FAQ

**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.