Your local library and technical colleges will both have books containing guides with bend allowance charts.

you can also google search "bend allowance" and get some hits.

Just remember that bend allowences vary not only from mild steel, stainless and aluminum. They vary in type of stainless and especially in type of aluminum. ie, a bend radius for a given thickness for 4043 aluminum will not be the same for 7075

Chillin, just a real quick shot here concerning the bend allowance nightmare if you will. In general most bends that fall into an angle type of bend require inside dimensions of the bent part to be added up. Example: if you were going to make a piece of angle iron out of 1/4" matl. and it was going to have equal legs that measured 2", thus a 2"x2" x 1/4" angle, the flat requirement of the material prior to bending would be the two ID. measurements added together or 1 3/4" + 1 3/4" and the product of this equation being 3 1/2" would be the flat length of the part prior to bending. If you are bending radiuses or rolling circles you would use the centerline of the matl. for your length calculation. Definitely, different materials will react to bending differently and may require different allowances. I noticed references to aluminum, there are heat treated and annealed grades of various types of aluminum, you have to watch very closely what you are bending because if it is a heat treated grade it will fracture or break when you try to form it. Example: Al 6061 T6, heat treated and can break if you try to form it, Al 6061 T1, annealed grade which is generally formable without breaking or fracturing. There are some grades of Aluminum that are considered as tooling plate, they are generally not formed, check the material descriptions carefully to ensure that they are suitable. Using punches and dies that have different opening widths and different nose radi will drastically effect the amount of material required to end up with specific sizes of finished bent parts. One of the general rules of thumb that you are maybe thinking that you have heard, refers to die sizing with relation to material thickness. Thickness of material x 8 is a good basic rule to go by for most bending, thus if I am bending 1/4" matl. a 2" opening on the bottom die will be a reasonable choice for bending that matl., you can vary from this rule, but the tonnage requirements for bending will vary with changes in this formula. Bending with openings less than the formula, to a point, will require much greater machine tonnage requirements and can cause die and machine dammage and possibly personal injury. As another poster suggested you can find technical books that cover this topic and there are other resources that you could tap into. What I'm trying to stress here is that bending of any sort of precision is an art in itself and those who are good at it are truly highly skilled. Good luck and regards, aevald