Every tire company has a size chart for all their tires. It is one hell of a lot easier to just look at one of those charts! For example, click on the orange box here: http://us.coopertire.com/Tires/Passenger/COBRA-RADIAL-G-T.aspx You'll note that the tread width is sometimes on the chart too. Wheel size too. (Different manufacturers tires don't vary too much these days either.)
For a really complete analysis of tire sizes, go here: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46