A few items to consider are (1) The filler metal minimum tensile strength, (2) The base metal yield point and tensile strength, (3) The orientation of the weldment, (4) and the effective area of the weldment. Once you have this type of information one can arrive at a value. However most specifications will dictate an allowable stress for the application and will usually show one how to calculate this value. The Structural Welding Code and the Manual of Steel Construction are two such references, and I am sure there are countless others.

Both Messrs. Arranaga and Clare have given you a highly valuable information on how to calculate precisely the allowable stress of a weld. However, if all that you want is a rough estimate and you`re using stick (covered) manual electrodes, then remember that the first two digits of the classification mean the minimum BREAKING stress of the electrode material measured in pounds per square inch (psi). E-6010, for example, means that the electrode material has a minimum breaking stress of 60,000 psi. Now, if you want to figure out the total ALLOWABLE stress of your particular weld, you must take into account two things:
1. the cross sectional area of it
2. a suitable safety factor
Remember, however, that this is a rough estimate and must be closely reviewed before using it.
Giovanni S. Crisi
Sao Paulo - Brazil