How do I copyright an Excel application that I created?
Optional Information: Country relating to Question: United States
Copyright exists automatically the moment you save your code on a disk. You can copyright more formally by filling out a form at the Copyright Office webpage. The cost is roughly $30, and you get some small advantages if you end up litigating with the copyright. That said, remember that copyright stops people from copying the creative parts of your work, but does not stop people from taking the idea, and does not stop people from copying purely functional parts. It might be, then, that you want to use contract law to protect your code as well. You could ask buyers to promise by contract that they will not duplicate the code or use it without paying. (You also could protect your code by keeping it as secret as is possible. Much computer software, for instance, is written in ways that are intentionally confusing and hard to understand, so that buyers decide to buy rather then copy/re-create.)
Hope that helps. Good luck.
Experience: Professor of Law at Top-Tier Law School, specializing in patent & copyright