Well, if you have an AISC Manual of Steel Construction book available, all the allowable loads and stresses are available either thru tables or equations.
Unfortunately, there is no easy answer, since the allowable load depends on the span, slenderness ratios, and support conditions.
You can download Enercalc, it will calculate columns, as well as most other standard structural items. The program costs about $1000, but you can download a 30 day free trial. The only limit to the trial is when you print the results, a watermark is placed which states it is a demo.
Of course, if you are not comfortable doing the calculations yourself, maybe you should hire a structural engineer. This way you protect both yourself, and your clients.
If you still need this question answered, let me know what your application is and I will quickly size the beam for you. Simply give the end conditions, height of beam and load weight. If this is solely in compression, this will take 1 minute.
