Round steel - how can you calculate the shortening when bending?


When bending a round bar, one has to consider the change in length during the bend. How this can be calculated, and from which factors the change of the length depends, you can read in this article.

bend Deduction

Technically one speaks of the so-called "bending shortening". It can take on very different values ​​depending on the material, and you have to calculate them separately (even if you use a CAD program, because CAD software does not automatically take into account the occurring bending shortening of a material).
Depending on the bending reduction of the following factors:

  • Strength of the material
  • Coating of the material
  • the so-called ductility (deformation of the material during bending)
  • the texture of the steel (austenitic, ferritic or martensitic)
  • Diameter of the round steel
  • used bending technique
  • bending radius
  • bending angle

Due to the large number of factors that influence bending reduction, a technical calculation is never 100% accurate. With small deviations from the calculated result you must always expect.

Basic calculation

The length of the workpiece after bending would generally be calculated in the following way when bending a 90° angle:

Length of the first part + length of the second part + radians ½? (r + ½ t)

r... bending radius, t... material diameter

However, a shortening in the bent part would not be considered. In addition, the length of the workpiece is yes on its inside less than on its outside. This difference is not taken into account in this calculation, it is determined with the above-described way an average between inside and outside.

Approximately correction

With the mean value between inside and outside length you can definitely live in practice. Calculating the material shortening by means of correction factors can work approximately, but often does not provide a sufficient result in practice.

For constructions in the professional sector, therefore, exact tables are used which contain so-called deduction values. The angle must also be taken into account.

Differences due to the bending angle

The bending angle has a significant influence on the bending shortening. Basically, one can assume that the deviation is greater, the greater the angle. However, there is no linear relationship between angle and shortening. This makes the calculation all the more difficult.

Tips & Tricks

The more accurately a molded part has to be fitted, the more accurately the calculations must be made so that the blank can be made correctly before bending.


Video Board: Steel Cut Length for Circular Slab.