The free online sheet metal air bending radius and k-factor calculator! No strings attached!

This free K-factor calculator was designed by sheet metal professionals who actually make sheet metal parts. This calculator is free because we hope to inspire and educate a new generation of makers and fabrication professionals who are newly discovering the joy of building something from raw materials.

In addition to this tool, we offer many other services (not all of them as free as this tool, but we gotta get groceries somehow):

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Free K-factor calculator diagram of an example of free k-factor calculator and its application for air bending sheet metal parts for fabrication

More Free K-factor calculator

This free K-factor calculator works with CAD programs like Autodesk Inventor or SolidWorks and it returns the k-factor and actual bend radius you should use in your design. It works very well for the parts we design daily. This is mostly thin gauge stainless and mild steel. Additionally, we plan on expanding this part of the website to include some generic design tables and the UI’s we use with them to make them easy for anyone to navigate.


K-Factor and what it means

For a further explanation and quick example, please read below:

K-factor is based on a model searching to find a “neutral axis” in a part that is bent or formed. This neutral axis model determines how large the flat blank of a formed part will be. Furthermore, it also determines where all the holes and other geometry are placed when processing the flat blank on raw materials with machines such as turret presses or lasers.

The k-factor number is actually a percentage of the material thickness that a designer will offset his or her punch side geometry in order to create a length that will result in “perfect” geometry after bend or forming processes are applied. For instance, in the illustration above, the k-factor for 304 stainless steel is .42 and the material thickness is .048″. Multiply .048″ by 42% and you get .020″. This is your neutral axis and therefore, the actual arc you will be calculating your blank length on will be about .095″.


Let the math begin

For instance, let’s say that you have a bent piece of angle with 2 flanges of 1.000″ each overall and want to find a flat length for this part with dimensions like the illustration shown. First, we can find the length of each flange from its tangent point to the end of the part. Of course, this number doesn’t change regardless of the k-factor – the actual bend radius dictates these lengths.

These lengths equal the added overall flange lengths minus two times the die side radius of the part. In this case, the outside radius equals the material thickness (.048″) plus the actual inside bend radius (.075″) or 0.123″.

Accordingly, our equation will be 1.000 (flange 1) + 1.000 (flange 2) – (2 x .123″) or 2.000 – .246 or 1.754.

Now, all we have to do is calculate the arc length, which is really easy for 90 degree bends. It’s just a quarter of the circumference of the neutral axis arc. Here’s what that formula looks like:

[.095 (neutral axis arc) x 2 (to find diameter) x 3.14 (to find circumference)] / 4 (360 degrees divided by 90 degrees) or

.5966 / 4 or


Finally, we add our arc length to our flat flange length and we have our blank length.

.1491 + 1.754 or

1.903 (rounding to .001″ is more than acceptable for most fabrication demands)

There you have it, a quick run-down of what k-factor is and how to apply it to your design.

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