Sheet Metal Bend Allowance; Terms and Calculations of Sheet Metal

First of all, sheet metal is a very important type of manufacturing. And, there are lots of kinds of manufacturing processes that depend on sheet metals. In sheet metal bending processes, allowance calculations and other terms about sheet metal bending are very important. The illustration of sheet metal bending on technical drawings is also very important. So, you will see that they are very basic to understand.

What is Sheet Metal Bend Allowance?

Bend allowance in sheet metal bending operations.
Image Source: FREDERICK E. GIESECKE, SHAWNA LOCKHART, Technical Drawing With Engineering Graphics 15th Ed., Pearson, Pg.536

Firstly, the bending allowance is a very important term for sheet metal bending. And, bending allowance value is the required additional length to the open state of the sheet metal bent. So, you need to add the bending allowance value to the total dimensions of the bent sheet metal shape. Because bent sections of sheet metals take a portion of length and dimensions which shortens the total dimensions. To obtain the required dimensions, we need to add sheet metal bending allowance to the open state of sheet metal.

How to Calculate Sheet Metal Bend Allowance?

Above all, the calculation of the bend allowance is very simple. And, there is a empirical formula for the sheet metal bend allowance that we use in the calculations;

Bend allowance formula in sheet metal bending process
Image Source:

In this formula, the ‘Radius’ value is the radius of the bend that we calculate according to the inside surface of the sheet metal bend. The unit of the radius value is inches or millimeters. So, ‘Thickness’ is the total thickness of the sheet metal bending which has the unit of inches or millimeters. Also, ‘Degrees’ is how many degrees that you obtain this sheet metal bending.

So, as you understand from the formula above, with the increasing radius of the sheet metal bending, the total bend allowance increases. Also with the increasing thickness of the sheet metal, the total bending allowance increases. And with the increasing bending degrees, the bending allowance increases.

Bend Allowance Calculator

We prepared a calculator about sheet metal bending allowance. You can use this calculator to calculate it. You can deal with complex calculations of a total process in a manufacturing site and you may need to use bend allowance calculation many times. So, this calculator will be a very big companion. You can save lots of time with a bend allowance calculator. You do not need to use hand calculations for it.

Sheet Metal Bend Allowance Calculator

The use of the bend allowance calculator is very simple. You just need to enter these values;

  • Radius(in or mm)
  • Thickness(in or mm)
  • Degrees(degrees)

After that, click on the ‘Calculate!’ button to see the bend allowance value. If you want to make further calculations, you just need to click on the ‘Reset’ button. After that, re-enter the values.

Inner Mold Line and Outer Mold Line

These parameters are two important parameters in sheet metal bending operations. The inner mold line is the intersection of the extensions of the inner surfaces of the sheet metal bend. Also, the outer mold line is the intersection of the outer surface extensions.

Neutral Axis

In the sheet metal bending operations, the outer surface undergoes tensile stresses because it stretches. And the inner surface undergoes compressive stresses. And there is a section inside the thickness of the sheet metal that does not undergo any stress. So, the total stress on that axis is zero. This is the neutral axis of the sheet metal bend.

The neutral axis is every time the thicknesses of sheet metal bends.

Also, the bending allowance is equal to the length of the neutral axis.


Above all, these are the very important terms for sheet metal bending operations. If you calculate the bend allowance of a sheet metal bend, you will obtain the total length and dimensions of sheet metal that you need.

Mechanicalland does not accept any responsibility for calculations made by users in calculators. A good engineer must check calculations again and again.

You can find out much more calculators like this in Mechanicalland! Take a look at the other engineering calculators available in Mechanicalland!

Finally, do not forget to leave your comments and questions below about the bending allowance in sheet metal bends and terminology about the sheet metals.

Your precious feedbacks are very important to us.

FAQs About Sheet Metal

Which materials are used in sheet metal bending operations?

In sheet metal bending operations, the material must be malleable and have the ability to plastic deformation. Also, this material must have good mechanical strength. Considering these facts, low-carbon steels are the best materials for sheet metal bending operations.

Which parts are produced in sheet metal bending operations? 

Mostly, the structural parts are produced in sheet metal operations. Wall attachments, automotive car bodies, and structural parts of car seats are the most common applications of sheet metal bending operations. Metal grooves and other structural elements such as shelves are also other important applications of sheet metal bending operations.


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