Defining your total market is not done in a day. You establish a “Potential Sales Dollars” field in your database and continually update it. You find out these numbers through primarily your sales people. They ask their buyers how large their market is or what percentage of business is given to your company. You find out your customers’ SIC codes and try to locate all the other companies that are classified in that industrial code to get a listing of new companies to call on. When customers exceed your sales guess, you modify the sales amount in your system. Some purchases may be a percentage of a company’s overall sales. If you can get an estimate of the company sales (sales/head using employee count), you may be able to make a fairly good guess at their customer potential. Try to enter all potential customers (no sales yet) into your sales system so you can track how many of your sales people are calling on them.