You can look up patents by number on the United States Patent and Trademark Office, or USPTO, website. The website is located at USPTO.gov, and you can find the USPTO Patent Search tool in the Patents tab under the home page, under the Application process subsection.
The USPTO's Patent Search tool is a search engine that allows users to search for a variety of different trademark and patent records using a number of different queries and filters. You can search for applications and registered by a number of parameters, such as patent number, applicant name, application location, issue date and keywords. No matter how you search, the patent number is included in the search results, located to the left of the title of the patent application.
When you click on the hyperlink of each result, you are taken to the full patent application, which provides you the application number directly under the Related US Patent Documents section. More than one application number may be available for US patents, depending on if a patent was reissued due to a discrepancy. All application numbers are unique. The application number is not the same as the patent number, but both can be used to locate a certain patent. One patent number can have different application numbers.