When the sheet metal is put through the process of bending the metal around the bend is deformed and stretched. As this happens you gain a small amount of total length in your part. Likewise when you are developing a flat pattern you will make a deduction from your desired part size to get the correct flat size. Bend allowance is defined as the material you will add to the sum of the finished leg lengths of the part in order to determine the flat pattern’s length. It is important to distinguish between the leg and flange length. The leg length is the dimension outside of the bend radius while the flange length is the overall finished size of the piece.
The Bend Allowance (BA) formula takes into account the geometries of bending and the properties of your metal to determine the bend allowance. You will need to know your Material Thickness (MT), the Bend Angle (B<), the Inside Radius (IR), and the K-Factor (K). The material thickness will be measured in decimal form, not by the gauge number. It is important to convert from the included angle to the complimentary angle before performing any calculations. Another way of looking at bend allowance is how it relates to the Flat Pattern Length (FPL) and the Leg Length (LL) as shown in the second equation.
Shown below a part with flange lengths of 2” and 3” with an inside radius of .250” at 90° will have leg lengths of 1.625” and 2.625” respectively. The material thickness is .125” and we are assuming a k-factor of .33. Using the above formula we can calculate the bend allowance to be .457”. In order to develop the flat pattern we add .457” to 1.625” and 2.625” to arrive at 4.707”. The bend deduction can be determined easily by subtracting the finished flange lengths from this flat pattern length.