K Factor Calculator

What is K factor?

Many designers use the SolidWorks sheet metal module.

There are a lot of sheet metal parameters in the module, so how do you set them?

What is the K-factor in sheet metal parameters? How to set it up?

The K-factor is a key basic concept in SolidWorks sheet metal design and must be understood in order to learn sheet metal design and fabrication well.

What is the K-factor?

The K-factor is the ratio between the distance from the neutral layer to the bent inner surface and the thickness of the sheet metal.

As shown in figure 1 below, K = t/T.

K Factor

The K-factor is defined as a constant that is greater than 0 and less than 1.

Neutral layer

Since the K-factor is related to the location of the neutral layer, what is the neutral layer?

In the bending deformation zone, the material near the inner surface is compressed, and the closer the inner surface is, the more it is compressed.

Likewise, the material near the outer surface is stretched, and the closer it is to the outer surface, the more it is stretched.

The transition from the inner surface to the outer surface is from compression to stretching.

Assuming that a material is made up of thin layers stacked one on top of the other (most metals are actually layered), there must be a layer in the middle of the material that is neither compressed nor stretched. .

This layer we call the neutral layer.

In general, the neutral layer is invisible and inaccessible, because it is inside the metal.

Its position is related to the intrinsic properties of the material, i.e. the K-factor is related to the material.

From the definition of the neutral layer, the unfolded dimension of the sheet metal is equal to the width of the neutral layer, as shown in the figure above.

Unfolding dimension of the sheet metal = straight line A + straight line B + arc C (length of the neutral layer in the deformation zone).

K factor calculator

Online Calculator for K-factor, Y-factor, Bend Allowance, Neutral Axis, Arc Length

K factor chart

The following are K-factors for common materials.

  • Soft copper or soft brass: K=0.35
  • Semi-hard copper or brass, mild steel, aluminium etc.: K=0.41
  • Bronze, hard bronze, cold rolled steel, spring steel, etc.: K=0.45

Definition of the K factor: the K factor is the ratio of the neutral layer position thickness (t) of the sheet metal to the overall thickness of the sheet metal material (T), i.e..

K = t / T

Sheet metal k factor chart


GI Sheet & MS Sheet

K Factor

All angles, including R angle


Why the K-factor cannot exceed 0.5?

To figure this out the reason why the K factor can’t exceed 0.5, you first need to know what the K-factor is.

To figure out the K-factor, you need to know what the neutral layer is.

You understand that bending a sheet metal part is to make it produce a very small arc, like roll bending, except that the radius of the roll bending is larger than that of sheet metal bending.

No matter how you bend, it’s impossible to get a right angle, it will always have a little arc.

If the lower die radius is small, the workpiece radius is small; if the lower die radius is large, the workpiece radius is large.

Then we come to the neutral layer.

As you know, sheet metal parts have a thickness.

When bending it into an arc, you will notice that the length dimensions of its inner surface are reduced, while the length dimensions of its outer surface are enlarged.

This is where the bending allowance comes from.

For example, if you bend an angle-like part with an outside diameter of 20 x 20, it will always unfold to less than 40, no matter how thick the plate is.

This is because the dimensions of its outer surface become larger after bending.

So if you design the unfolded size to be 40, the bent size is 20 on one side and over 20 on the other.

But most of the time, we need to already know the dimensions of the resulting arc (bend).

But most of the time, we need to know the dimensions of the resulting arc (bend) to extrapolate its unfolded dimensions.

It has long been believed that no matter how thick the sheet is, no matter how much the inner dimensions get smaller and how much the outer dimensions get larger, the size of middle layer of the sheet will not change.

People just call the middle layer that doesn’t change the neutral layer.

With the increasing demand for dimensional accuracy of products, it has also been found that the amount that gets smaller on the inside does not necessarily equal the amount that gets larger on the outside.

Especially when the resulting arc is small (such as a bend), it tends to get 0.3 smaller on the inside, but 1.7 larger on the outside.

At this point one realizes that the layer (neutral layer) that does not change size is not necessarily in the middle of the thickness of the sheet, but is more near the inside.

Divide the distance from the inside to the neutral layer by the entire thickness of the sheet, which is known as the K-factor.

Yes, you’re right, the furthest the neutral layer is from the inside is in the middle of the plate thickness.

So the distance from the inside to the middle divided by the entire plate thickness is 0.5, which means the K factor is equal to 0.5, it can’t be any greater.

These are the reasons why the sheet metal K-factor should not exceed 0.5, and I hope you understand after reading this article.

Further reading:


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8 thoughts on “K Factor Calculator”

  1. Bruce Lewandowski

    On this k-factor-calculator page it says the k-factor cannot/should not exceed 0.5, but in your Thickness / K-Factor example chart it does exceed 0.5, it is 0.615 for the 0.8 thickness. And it exceeds 0.5 when I use your K-Factor Calculator for 0.0751 thick material with an 11″ inside bend radius! The Calculator indicates a k-factor of 0.8359. Is the calculator not working correctly? So is there a chart or calculator for specific materials, say SA240T304L 14Ga (0.0751″) at an 11″ I.R.?

    1. Generally,
      If the bending angle is equal to the plate thickness, the k factor = 0.5
      If the bending angle is less than the plate thickness, the k factor < 0.5 If the bending angle is greater than the plate thickness and the k factor > 0.5, this situation is rare.
      You should at least input the correct inner radius to get the K factor.

  2. How do you calculate the k-factor for different kinds of steel (or materials)?
    Say I have a stainless steel AISI 316L and a hot rolled steel S235 JR, will they have the same k-factor for similar plate thickness and bend angle? Or will it be different for the different types of steel, and how do you calculate that k-factor?

    Thanks for a great article!

  3. Why does this page sometimes have a calculator for k-factor and other times its gone? please advise as this is getting annoying with this page.

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