Diquan,
The allowable stress is a design stress considered by the code safe for the jurisdiction of that particular code. It is the result of the material tensile strength divided by the safety margin, say 3.
Depending what code you are following, the allowable stress for the materials under the code jurisdiction are listed within the code, without your need to calculate them. However, some codes provide a calculation method of allowable stresses for other materials acceptable, but not listed within the code.
If you could provide some details of what you are trying to do / achieve, perhaps more help could be provided.
gr2vessels