Once you have determined your initial cost basis for a stock, you are not done! You need to adjust your total cost basis and/or cost basis per share whenever events such as the following occur:
These stock change events are commonly referred to as "corporate actions," "capital changes," "corporate reorganizations," or "corporate reorgs." These events are so common and numerous that your broker/dealer even has an entire department devoted to dealing with them, called the "reorg" department.
The pages on this website will help you understand how to adjust your cost basis for each type of change in your stock. The calculators will facilitate your computations.