Understanding the Bend Allowance and consequently the Bend Deduction of a part is a crucial first step to understanding how sheet metal parts are fabricated. 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 trying to develop a flat pattern you will have to make a deduction from your desired part size to get the correct flat size. The Bend Allowance is defined as the material you will add to the actual leg lengths of the part in order to develop a flat pattern. The leg lengths are the part of the flange which is outside of the bend radius. Unfortunately sheet metal bending is not always going to be the same in every shop. The largest variations come from the materials themselves. Protective coatings, variations in the alloy and thickness as well as many other small factors all add up to give you Bend Allowances unique to your operation. This chart will get you close enough for most applications and may not require fine tuning on your part.