It can be somewhat tricky to accurately estimate when an issued US patent should expire. Nowadays, there are plenty of software solutions that automatically perform this calculation so you don’t have to. For example, AcclaimIP provides the calculated patent expiration date. But if you do not have access to such a solution, fear not — you need not calculate the expiration manually. The USPTO provides a Microsoft Excel spreadsheet called “Patent Term Calculator” with associated instructions — you just need to find the indicated information and input it into the spreadsheet to have the expiration date calculated.
According to the USPTO:
The United States Patent and Trademark Office does not calculate expiration dates for patents. In response to patent owner and public inquiry, the USPTO is providing a downloadable patent term calculator as a resource to help the public estimate the expiration date of a patent. The calculator can be used to estimate the expiration dates of utility, plant, or design patents. The calculator contains prompts to enter specific information related to the patent in order to help in estimating expiration dates. [Link]