Mastering Sheet Metal Bend Allowance in Solidworks

Today we will learn the method to calculate bend allowance.

Further reading

You can directly use our bend allowance calculator to calculate the bending allowance. Besides, the fabrication calculator can also help you calculate K-factor, Y-factor, bend allowance, bend deduction, etc.

You may be wondering what exactly bend allowance is if you’ve never worked with sheet metal before.

When a sheet is bent in a press brake, the part of the sheet close to and in contact with the punch elongates to compensate for the given bend.

If you compare the length of this part before and after the bending, you’ll find that they’re different.

As an engineer, if you don’t compensate for this variation, the final product won’t have accurate dimensions.

This is more critical for parts where you have to maintain a tighter allowance or precision.

In this post, I cover some of the basic problems and principles you have to deal with regularly when working with sheet metal.

Before we get started, I want to comment on something – there is not really a scientific method or formula for determining the exact calculation of the bend allowance, because there are so many factors at play during the production of your sheet metal part.

For example, actual material thickness, an infinite variety of tooling conditions, forming methods, and so on.

There are many variables here, and in reality, many methods are used to calculate the bend allowance.

Trial and error is probably the most popular method, while bend tables are another commonly used technique.

Bend tables are typically available from metal suppliers, manufacturers, and engineering textbooks. Some companies develop their own bending tables based on their standard formulas.

Now, let’s return to Solidworks. How does Solidworks calculate bend allowance exactly? Solidworks uses two methods: bend allowance and bend deduction.

Bending Allowance

I’m going to explain what these methods are and show you how they are used in Solidworks.

Bend Allowance Drawing

The bend allowance method is based on the formula that appears in my diagram.

  • Lf = L1 + L2 + BA
  • BA = Bend Allowance

The total length of the flattened sheet is equal to the sum of L1 (the first length), L2, and the bend allowance.

The bend allowance region is shown in green on my diagram. This is the region where all deformation occurs during the bending process.

Generally, the bend allowance will be different for each combination of material type, material thickness, bend radius, bend angle, and different machining processes, types, speeds, and so on. The list of potential variables is extensive.

The value of the bend allowance from sheet metal suppliers, manufacturers, and engineering textbooks is provided in bend tables. A bend table looks like the following Excel spreadsheet.

Bend Allowance Table

The bend table approach is probably the most accurate method for calculating bend allowance.

You can input your data manually into a matrix of the bend angle and bend radius. If you are not sure of the bend allowance value, you can run some tests.

You need a piece of the exact same sheet metal you will use to manufacture your part, and then you bend it using the same processes you will use during your machining. Simply take some measurements before and after bending, and based on the same information, you can adjust the necessary bend allowance.

Bend Deduction

Another method that Solidworks uses is the bend deduction method.

The formula is as follows:

  • Lf = D1 + D2 – BD
  • BD = Bend Deduction

The flattened length of the parts, Lf, equals D1 plus D2 minus the bend deduction.

Like bend allowance, bend deduction comes from the same sources: tables and manual testing.

As you can see, it is easy to understand how these values are related to each other based on the information provided by these formulas.

  • L1 + L2 + BA = D1 + D2 – BD


Another method for calculating bend allowance uses the K-factor.

K represents the neutral axis offset.

The general principle of this formula is as follows: the neutral axis (shown in red in my diagram) does not change during the bending process. During the bending process, the material inside the neutral axis will compress, and the material outside the neutral axis will stretch. The neutral axis will be closer to the inside bend (indicated in blue in the diagram). The more the part bends, the closer the neutral axis will be to the inside of the part.

The bend allowance calculation formula with the K-factor is shown below:

BA = 2πA(R+KT)/360

  • π=3.14
  • A=Angle (degrees)
  • R=Bend Radius
  • K=Neutral Axis Offset (K-factor) t/T
  • T=Thickness of Material
  • BA=Length of Bend Allowance

The K-factor equals t, which is the offset distance to the neutral axis, divided by big T, which is the thickness of the material.

In this formula, the bend allowance equals 2 times pi multiplied by A (the angle) multiplied by the sum of R (the bend radius) and the K-factor multiplied by T (the thickness of the material). Then, you divide all of this by 360.

In theory, the K-factor can be anywhere between 0 and 1, but for practical purposes, it is typically between 0.25 and 0.5.

  • K-Factor = 0 – 1 (in theory)
  • K-Factor = 0.25 – 0.5 (practical)

For example, hard materials like steel have a higher K-factor, such as 0.5, while soft materials like copper or brass will have a lower K-factor closer to 0.

And don’t worry, this is the last formula we will be walking through in this lesson. It might seem a little confusing now, but with some practice, it will become second nature.

K-Factor Example

One last point: let’s take a look at the example. There is a hem on this part that has a K-factor of around 0.3. On the other hand, a soft bend, such as the gradual bend on the other side of this part, has a higher K-factor of about 0.5. And this concludes our lesson on bend allowance.

Further reading:

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1 thought on “Mastering Sheet Metal Bend Allowance in Solidworks”

  1. Brandon Rivera

    Hello, I’ve been designing parts and following your bend allowance calculator but I don’t think I am using it correctly. I use a program called Shapr3D.

    For example, when I design a part such as a 90 degree bend in a 1/4” thick L-bracket, with 4” long flanges. I input the interior radius as 1/4”, angle at 90 degrees, and the exterior radius is calculated for me at .5537”.

    The problem I run into is when I sent this part out as a .step file for flattening, I’ve been told that the other CAD programs like fusion or solid works, the flatten process errors and there lies and issue with the radii.

    Is this something you can assist with?

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