Determining Patent Classification

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The U.S. Patent and Trademark Office (USPTO) assigns numbered classes and subclasses to inventions for the purpose of classifying patents issued on them and facilitating retrieval of these patents in the course of a patent search. To conduct a search for prior patents relevant to an invention, one must first determine its proper classification.

There are roughly 300 main classes, and an average of more than 200 subclasses under each main class. An invention will fall within at least one of the 66,000 separate classifications, and sometimes several.

Fortunately, the USPTO provides resources, available at the USPTO website (, to help a patent searcher find the correct classification(s):

  • The Office of Patent Classification. This is the main topic page for all classification sources and information at the USPTO.
  • Index of U.S. Patent Classification. All 66,000 categories (classes and subclasses) used to classify patented U.S. inventions are listed alphabetically. To conduct a patent search for prior patents relevant to an invention, it is useful to first determine the class and subclass within which the invention falls.
  • Manual of Classification. This manual lists by number the 66,000 classes and subclasses used by the USPTO to categorize inventions.
  • Class Definitions. This manual contains brief definitions of each classification and subclassification used to categorize patents. The manual helps a patent searcher determine the appropriate categories to search in.

Start by creating a series of keywords – terms that may apply to your invention or the various elements that comprise your invention. Search these terms in the USPTO database or at Google Patents. When you find patents that are similar or otherwise relevant, write down the classification numbers for those parents. That gives you a basic idea of classifications.

That’s an informal start. Next verify and fine-tune classification as follows:

  • Start by looking up your keywords – terms that may apply to your invention -- in the Index to the U.S. Patent Classification.
  • Check the accuracy of the classification(s) in the Class Schedule and Class Definitions in the Manual of Classification.
  • Search the patents and published patent applications in your list of relevant classes and subs. To do this go to the USPTO search page, click “Quick Search,” enter the first class and sub in the “Term 1” box, and hit Search. Do this for each relevant class and sub under Issued Patents and Patent Applications.
  • Carefully review each patent in the relevant classes and subclasses to see whether it is relevant, that is, does it come close to the hardware, steps, or purpose of your invention?
  • Write down the numbers, dates, and first Inventor of any patents that are relevant and obtain copies of them to study later.