Introduction
When you bend sheet metal, the metal stretches. The outside of the bend gets longer, and the inside gets squeezed. Somewhere in the middle is a line that does not change length at all. That line is called the neutral axis. The K factor tells you where that line sits inside the metal.
This calculator works out that number from your bend details. Knowing your K factor lets you cut a flat blank at the right size, so your part comes out the correct length after you bend it. That means less scrap and no guessing at the shear.
The tool works in two ways:
- Empirical Method: Bend a test piece to 90°, measure it, and the calculator finds your real K factor.
- Standard Method: Enter a known K factor and your bend plan to get the numbers before you cut.
You will also get the bend allowance, the Y factor, and the outside setback. Each result comes with the steps shown, plus a chart and a table for common bend angles. You can work in millimeters or inches. Click to switch, and every number changes with you.
How to use our K Factor Calculator
Enter your sheet metal bend details and this calculator gives you the K factor, bend allowance, Y factor, outside setback, and where the neutral axis sits inside the metal. You also get a step-by-step solution, a chart, and a bend allowance table.
Unit System: Pick millimeters (mm) or inches (in). Your numbers and results switch over on their own.
Method Tab: Choose Empirical Method if you bent a test piece and want to find the K factor. Choose Standard Method if you already know the K factor and want the bend allowance.
Empirical Method inputs (90° test bend)
Material Thickness (T): Measure how thick your sheet is and type it in.
Inner Radius (R): Type the radius on the inside of the bend.
Flat Length (FL): Type the full length of the flat blank you measured before you bent it.
Flange A (outside): Type the outside length of the first leg after the bend.
Flange B (outside): Type the outside length of the second leg after the bend.
Standard Method inputs
Material Thickness (T): Type the thickness of the sheet you plan to bend.
Inner Radius (R): Type the inside bend radius your punch and die will make.
Bend Angle: Type the bend angle in degrees, from 0 to 180. A right angle bend is 90.
Material Preset: Pick your metal, like aluminum, mild steel, or stainless steel. This fills in a common K factor for you.
K Factor: Type your own K factor if you have one. It must be between 0 and 1. Most metals fall between 0.3 and 0.5.
Click Calculate to see your results. Click Reset to put the sample values back.
What Is the K Factor in Sheet Metal Bending?
When you bend sheet metal, the metal on the outside of the bend stretches and the metal on the inside gets squeezed. Somewhere in the middle there is a thin layer that does not stretch or squeeze at all. That layer is called the neutral axis. The K factor tells you where that layer sits inside the metal.
The K factor is just a number between 0 and 1. It is the distance from the inside face of the bend to the neutral axis, divided by the material thickness. If K is 0.5, the neutral axis is right in the middle. Most real bends push the neutral axis toward the inside, so K is usually less than 0.5.
Why the K Factor Matters
You need the K factor to cut your flat blank the right size. Cut it wrong and your finished part will be too long or too short after bending. Shops use the K factor every day to work out the flat pattern for parts like brackets, enclosures, panels, and ducts.
Key Terms
- Material Thickness (T): How thick the sheet is.
- Inner Radius (R): The radius on the inside of the bend, set by your punch and die.
- Bend Angle: How far the metal is bent away from flat. A 90° bend makes an L shape.
- Bend Allowance (BA): The length of metal used up inside the bend, measured along the neutral axis. Add it to your flange lengths to get the flat length.
- Y Factor: Another way to write the same idea. Y = K × π ÷ 2. Some CAD programs ask for Y instead of K.
- Outside Setback (OSSB): The distance from where the bend starts to the sharp corner the two outside faces would make if they kept going straight.
The Formulas
Bend Allowance: BA = θ × (R + K × T), with the angle θ in radians.
Outside Setback: OSSB = tan(θ ÷ 2) × (R + T)
For a 90° test bend, you can work backwards to find K: BA = FL − A − B + 2(R + T), then K = (2 × BA ÷ π − R) ÷ T.
Common K Factor Values
| Material | Typical K Factor |
|---|---|
| Soft copper, soft brass | 0.35 |
| Aluminum, semi-hard brass | 0.41 |
| Hard copper, mild steel | 0.45 |
| Stainless steel, hard steel | 0.50 |
Most K factors land between 0.3 and 0.5. The number changes with the material, the bend radius, the thickness, and the tooling you use. A bigger radius compared to the thickness usually pushes K higher.
Finding Your Own K Factor
Charts are a good start, but your own press brake gives the best answer. Cut a flat strip and measure its length. Bend it to 90°. Then measure both outside flanges and the inside radius. Those numbers give you the exact K factor for that material, that thickness, and that tool setup. Save it and reuse it, and your flat patterns will come out right the first time.