A patent classification search is a type of patent search done by searching patent classification schemes in patent databases. Patent applications are classified into classification schemes based on their technical content. These patent applications are assigned classifying symbols or codes that make it easier to search for other patents.

By searching with classification symbols, you are able to do a more precise search of existing patents than if you just searched using keywords. Most systems separate patents into classes, known as the original references, and then smaller subclasses. All of these classes organize patents according to their function, composition, manufacture, and process.

The Origins of Classification Searching

Classification is the original method of patent searching. Long before the days of online databases, copies of U.S. patents were organized by class and subclass and stored in "shoe boxes" at the United States Patent and Trademark Office (USPTO) headquarters in Alexandria, Virginia, and Patent and Trademark Depository Libraries across the country. Back then, people needed to browse the subclasses by hand or through microform.

While technology has evolved, the basic idea behind classification searching remains.

Reasons to Search Patents with Classifications

A classification search is a good way to search through patents. This helps inventors:

  • Improve their patent applications. A classification search unearths patents in your invention's class. Seeing the way other people in your field wrote patents can inspire you.
  • Understand other people's work. You have much to learn from your peers. Browsing through their patents can help you improve your own work. 
  • Gain information about your competitors. A patent classification search can teach you valuable lessons about the technology behind your competitor's inventions and how they plan to market them. 
  • Learn more about your industry. You'll also learn a lot more about your field of interest which you can apply to your own work. 
  • Avoid patent infringement. A thorough patent search is the best way to avoid infringing on someone else's patent. An invention may not be original, even if you haven't seen it before. If you find patents for inventions very similar to yours, you can abandon the project or brainstorm ways to make yours differently enough for approval. Note that your own preliminary patent search is a good starting point, but no substitute for a professional patent search. You should always get a professional patent search before submitting a patent application.
  • Focus on an area of interest. The USPTO patent database contains more than 7.5 million documents. A patent classification search helps you manage this vast volume of information. 
  • Search more effectively. Classification codes are less ambiguous. For example, a patent's title might contain the word "valve." The classification code makes it clear if this invention is mechanical or electrical. People may not get all the search results they want because they aren't sure what keyword to use or a keyword has many synonyms. Some old patents may use obsolete terms modern searchers wouldn't use. Some inventors even use unusual language to make their work stand out, which makes finding it very difficult. Even spelling errors or regional spelling variations can cause problems when searching for keywords. Also remember that digital text data only dates back to 1976, so a keyword search along won't retrieve earlier patents.

Understanding Patent Classification Systems

Every patent has at least one patent classification code. This shows anyone reading the document what type of invention the patent is for.

Classification codes help people search for similar invention patents. Inventors and businesses might do this to make sure their own inventions don't infringe on other patents. Examiners also need to search patent classification codes to make sure the patent applications they're assessing are for original inventions.

Most countries use the Cooperative Patent Classification (CPC) system. The U.S. Patent and Trademark Office (USPTO) and European Patent Office (EPO) created this system together. Prior to its introduction, the major classification systems used around the world were:

  • United States Patent Classifications (USPC)
  • European Classifications (ECLA)
  • International Patent Classification (IPC)
  • Japanese Classification Systems (F-Term and F-Index)

The USPTO and EPO based their CPC system on the ECLA system. This was a more comprehensive version of the IPC system. USPTO and EPO examiners can modify the CPC system by redefining its groups and subgroups as technology advances. This may see CPC classifications changing as the system evolves. They might merge subclasses, further divide them, or make brand new subclasses.

The USPTO and EPO CPC databases are kept identical as both group's file services synchronize regularly. The USPTO and EPO websites also have the latest information about CPC codes.


The United States Patent and Trademark Office (USPTO) started transitioning from the US Patent Classification (USPC) system to the CPC system in November 2011. The transition has been slow, with the official CPC launch delayed until January 1, 2013. U.S. utility patent documents still included USPC codes until June 1, 2015. After that date, utility patents in the United States only featured CPC codes.

CPC classification is now available for all U.S. patent documents dating back to 1836, just as old European documents now feature CPC codes. In addition, U.S. utility patents issued between 1790 and the end of 2014 still have searchable USPC codes. U.S. Plant and design patents continue to carry USPC codes at the time of writing.

USPC (sometimes called USNCL) has 460 classes and 150,000 subdivisions, while CPC has around 250,000 subdivisions. That means many USPC codes match two or more CPC codes, so there is no clear correlation between USPC and CPC codes. Since there are more subdivisions, the CPC system creates smaller groups of inventions that are more closely related. 

Finding Relevant CPC Codes

CPC codes are printed on recently printed patent documents. Electronic databases have additional CPC codes for patents published after the CPC's introduction. You can find these codes using subscription-based search tools or free searchable databases available through most patent offices. Some good tools include:

Read through each resource's information for information about the countries it covers.

  • If you have a patent document, search for its CPC code. If you can't find a CPC code, try a citation search. You could also search for another classification system to reveal patents with similar technology that have CPC codes.
  • Without a patent document, text searching should help you find the patent for a similar invention that has a CPC code. 
  • Search the CPC system directly via Espacenet. Click "classification search," then enter your keywords. You'll get a selection of definitions and codes. Clicking a code will present subcategories with more detailed information.

Many search tools accept CPC codes. You can use them alone for an overview of all patent documents in a particular area. That's because the online patent databases combine the CPC codes from the USPTO and EPO given to each family member into a single, searchable record. That means you need only the CPC to search for United States and European patents, as well as patents from other other nationsusing the codes. Alternatively, you can use CPC codes with keywords for a more refined search. The USPTO's 7-Step Method is a great template for search practices.

  • Brainstorm keywords. Think about the invention's function, purpose, usage, features, and composition.
  • Search for the keywords to find the right class. Browse the U.S. Patent Classification's index and note down the relevant classes and subclasses.
  • Check relevancy. Consult the Manual of Classification's Classification Schedule to see whether the classes and subclasses you've found relate to your invention. Focus on the bold, capitalized titles.
  • Read the definitions. Read each class' and subclass' definitions. This will confirm each group's scope. You should also look at the "see also" references. 
  • Read the full texts. Conduct a "current U.S. specifications" search via the Issued Patents and Published Applications databases. Read through any relevant patents you find. You may also like to search Free Patents Online and Google Patents.  
  • Review the patents. Pay attention to the claims, specifications, and illustrations in the patents you've retrieved. Assess how relevant they are to your own work.
  • Check references. View each relevant patent's references. Note the "U.S. Cl." and "field of search" sections for more searchable classes.

Most patent databases update their files every week so you see the most current information.

General Tips for Patent Searching with CPC Classes

  • Document your search. Clear documentation makes sure you don't repeat your searches and waste time. Add your search notes to your inventor's notebook, the notebook you're using to document your invention's development. Any bound or glued notebook will work.
  • Consider the CPC coverage. One database may have more CPC classifications than another. You should verify a database's CPC coverage before searching with these codes. 
  • Use multiple search tools. No database is likely to have complete CPC coverage. Several different search tools will give you the most comprehensive results.
  • Filter classifications and subclassifications by examiner. USPTO examiners often work in a particular area where they have expertise for a long time. If you filter your search results by examiner, you're likely to find similar inventions that examiner assessed.  
  • Filter classifications and subclassifications by most referenced. This will show you the patents that a high number of other patents referred to. If other inventors and examiners found them relevant, they're likely to be useful for you. 
  • Beware of paper patents. Electronic databases may also include CPC codes that aren't listed on a printed publication. This is especially common for patents issued before CPC's official 2013 launch. For this reason, you shouldn't rely solely on a printed patent document.

How to Search CPC Classes With AcclaimIP

AcclaimIP is a leading patent search and analysis program. This software:

  • Searches CPC in a variety of ways
    • Hierarchical CPC class searches
    • Hierarchical CPC class filters
    • Hierarchical CPC class charting
    • Single- and multi-series charts
  • Embeds descriptive tooltips for CPC codes
  • Links to CPC class schedules

The program uses the following search codes:

  • CPC: all CPC classes
  • CPCI: inventive CPC classes (marked in bold on patent PDF and AcclaimIP document details window)
  • CPCA: additional CPC classes, formerly non-inventive CPC classes (marked in standard weight type on patent PDF and AcclaimIP document)
  • CPCF: first CPC class (should be the best match)

AcclaimIP has a number of functions to help users search for patent classifications.


These are used throughout the program near class code references.

  • Hover your mouse to view the tooltip detailing the class' titles and hierarchical relationship.
  • Right-click the class code to freeze the tooltip in place.
  • Left-click the green link to see the classification schedule including titles, definitions, and cross-reference information. There are links for USPC, CPC, and International Patent Classification (IPC) schedules.

Special classification tooltips show the fully exposed class titles and CPC class hierarchy position.

Hierarchical CPC Searching

The CPC is divided using a hierarchy as follows:

  • High-level section
  • Class
  • Subclass
  • Main group
  • Further dots below this as deep as 12 dots

Patents may receive classifications at any level from the main group and below. Searching the CPC hierarchy is the only way to isolate patents in a particular tech area.

Here's how it works in AcclaimIP. To find the top patentees for implantable prostheses concerning joints, you'd use the A61F2/30 two-dot subclass. This detailed subclass contains 916 additional subclasses. Typing in 916 codes would be tedious, but the hierarchy can save you the trouble.

  • CPC:A61F2/30 searches within the subclass. It finds 257 unique global families published in the last 20 years.
  • CPC:A61F2/30+ searches within the subclass and 916 children classes below it. It finds 13,695 unique global families published in the last 20 years. 

Children classes are viewable at any level of the CPC hierarchy. Searching for them by adding a plus sign gives more than 50 times the number of results.

Hierarchical Filters

AcclaimIP offers CPC at five classification hierarchy levels:

  • Section
  • Class
  • Subclass
  • Main group
  • Complete

Each level evaluates your results set more deeply than the previous one.

Hierarchical Charts

Charts let you visualize your search results. Radio buttons in the Refine Chart panel lets you view results at a higher or deeper level

How to Search CPC Classes with STN's Databases

STN is another popular patent search firm. It was quick to add CPC search and display functions, including a CPC thesaurus, to its databases including:

  • CA
  • CAplus
  • USPAT2

You can still search for in-computer only (ICO), European Classification System (ECLA), and USPC codes in these databases.

With the launch of CPC, STN programs added a number of new search fields, including:

  • /CPC: search and choose a CPC code
  • /CPC.KW: search with CPC code metadata like:
    • CPC codes for a patent's inventive aspects
    • CPC codes initially assigned to the patent
    • The patent body that assigned the CPC code
  • CPC.TAB: CPC codes and each code's available metadata
  • CPC.UNIQ: Unique lists of CPC codes for patent family (only CA/CAplus, and USPAT databases)
  • CPC.HIT: HIT display for searched code (only CA/CAplus, and USPAT databases)

The CPC thesaurus has a similar structure to STN's IPC and ECLA thesauri. The thesaurus is updated monthly to include the latest CPC codes. 

STN database users can set up alerts or Selective Dissemination of Informations (SDIs). CPC SDIs don't always work as CPC codes aren't available for the earliest patents. For the most complete SDIs, users should also use the closest corresponding IPC code with their CPC code.

How Patent Classifiers and Examiners Search for Patents

The USPTO website has a refined search engine for its patent classifiers and examiners. The University of Massachusetts' Center for Intelligent Information Retrieval developed the system, modeling it on Inquery. They wanted to create a user-friendly system to allow searches for patent text, patent images, and trademark images without knowing any special codes. 

The system automates two key tasks performed by patent office personnel: searching for prior art and assigning patent classes and subclasses. Its key features include:

  • A large, distributed collection. The system boasts around 1.5 million patents, equalling 55 GB of data spread across 40 collections.
  • Collection selection. This technology selects the best collection for a query, reducing search time.
  • Indexed fields. There are 50 indexed fields, making it easier for users to search by inventor or assigned company, for example.  
  • Choice of query syntax. Users can search with natural language, Boolean and field operators, form fill-ins, or any combination of these.
  • Choice of text sources. Users can enter queries directly into the form, by selecting text from an ASCII file, or by selecting a patent. The system searches and classifies queries.
  • Phrase help. Users can see a list of phrases and terms related to their query. They can then add these items to the search.
  • Phrase and compound handling. Automatic query processing which understands compound words and common phrases.

The DWPI Classification System

DWPI is a less common classification system used for all patents created by Thomson Reuters subject experts. It allows people to search through Thomson Reuters patents to find those dealing with a particular field of technology.

The DWPI system uses a combination of letters and numbers. It divides patents as follows:

  • Chemical (A-N)
    • Polymers and Plastics (A)
    • Pharmaceuticals (B)
    • Agricultural Chemicals (C)
    • Food, Detergents, Water Treatment, and Biotechnology (D)
    • General Chemicals (E)
    • Textiles and Paper-Making (F)
    • Printing, Coating, Photographic (G)
    • Petroleum (H)
    • Chemical Engineering (J)
    • Nucleonics, Explosives, and Protection (K)
    • Refractories, Ceramics, Cement, and Electro(in)organics (L)
    • Metallurgy (M)
    • Catalysts (N)
  • Engineering (P-Q)
    • General (P)
    • Agriculture, Food, Tobacco (P1)
    • Personal, Domestic (P2)
    • Health, Amusement (P3)
    • Separating, Mixing (P4)
    • Shaping Metal (P5)
    • Shaping Non-metal (P6)
    • Pressing, Printing (P7)
    • Optics, Photography; General (P8)
    • Mechanical (Q)
    • Vehicles in General (Q1)
    • Special Vehicles (Q2)
    • Conveying, Packaging, Storing (Q3)
    • Buildings, Construction (Q4)
    • Engines, Pumps (Q5)
    • Engineering Elements (Q6)
    • Lighting, Heating (Q7)
  • Electronic and Electrical Engineering (S-X)
    • Instrumentation, Measuring, and Testing (S)
    • Computing and Control (T)
    • Semiconductors and Electronic Circuitry (U)
    • Electronic Components (V)
    • Communications (W)
    • Electric Power Engineering (X)

The 21 sections are divided further into classes. There are 138 chemical classes, 103 engineering classes, and 50 electronic and electrical engineering classes. Each class code features the section letter then two digits. Patents that fall into multiple classes are included in each relevant class.

Search for DWPI codes in combination with keywords to search effectively.  For example, if you searched the DWPI code X22, for automotive electrics, and the keywords "warn" you could find patents for automotive warning devices.

Thompson Reuters includes an abbreviation of the equivalent of International Patent Classification/s (IPC) on its website. This should be used as a guide only as some categories don't have direct equivalents.

Frequently Asked Questions

  • What inventions get utility patents?

Inventions are patentable when they have new, useful, and not obvious processes, machines, articles of manufacture, or composition. You may also get a patent if your invention improves on these factors on its competitors. 

Patents are new when they are different from prior art. Prior art is found in several places, including:

  • U.S. patents and patent applications
  • Foreign patents and patent applications
  • Articles from journals and magazines
  • Books, catalogs, and manuals
  • Online databases and other websites
  • Conference proceedings
  • Scientific essays
  • Literature packaged with products
  • Other public documents

Classification codes are a good tool for searching patent applications for prior art.

  • Who assigns patent classification codes?

In the United States, the USPTO patent examiners classify approved patents. Serco Inc., under the guidance of the USPTO, classifies local patent applications, or pre-grant publications.

In Europe, EPO patent examiners give EPO patent application publications, EPO granted patents, and non-EPO published patent documents their classification codes. 

The USPTO and EPO can both add additional CPC codes to a patent after it's published. Only the group that gave a specific CPC code can remove it. Either group can recommend a CPC code's removal though.

  • Are patent classification codes reliable for patent searching and landscaping?

Even similar patents can have very different patent classifications. This is caused by classification variation, a variation in how patent classifications are assigned rather than variation in the inventions. Patent classification codes can be a good starting point, but other tools can help you accurately search and group patents, like Ambercite's similarity filter, AmberScope.

  • Why aren't patents always in the classification I think they should be?

This can occur for a number of reasons. As subclassifications are created, merged, and deleted over time, an old patent may have a different classification than it would get today. Emerging technologies may get a new class as the technology becomes more common. The examiner has the final say, and he or she may put a patent in a different class than you would.

  • How are USPC codes written?

USPC codes have the broad class number followed by a slash, then a second number for the subclass.

  • How can I find good keywords?

A classification search works best with a keyword search. However, many people are unsure what keywords to use. Answer the following questions to find good keywords:

  • What am I patenting?
  • What is the invention's purpose?
  • How does my invention function?
  • What is my invention made of?
  • What does my invention achieve?
  • What category is my invention in?

The USPTO's Patent Full-Text and Image Database (PatFT) and Application Full-Text Database (AppFT) let you connect keywords with the Boolean operators "and" and "or." You can also put your search in parentheses to make two searches appear in the same document. For example, an auto inventor might search for wheel and (tire or tyre). This search would present all the patents containing the words wheel, tire, or the European spelling tyre.

  • What other codes does the USPTO site recognize?

You can use a variety of search codes for more effective searching including:

  • ABST/: abstract
  • SPEC/: specification or description
  • CCL/: current U.S. classification
  • AN/: assignee or owner's name
  • IN/: inventor's name
  • Have we seen the end of the USPC system?

USPC codes are not printed on US utility patents or applications published after June 1, 2015. However, they will continue on design and plant patents, as the CPC has no provision for these. 

Some people will still use USPC when searching old patents for its original or primary class. This is the patents most descriptive class as chosen by USPTO examiners. The CPCF search should give a similar result in time, but it's not reliable on older patents where CPC classes were mapped with a concordance.

  • Why did we change to CPC?

CPC is a better system because it's more descriptive and detailed. It's also used throughout the world. Along with Europe, where it replaced the ICO, ECLA, and IPC systems, CPC is used in China, Russia, Korea, and Brazil. Using a system people around the world can understand makes sense. CPC should become more powerful and functional than the USPC system was. 

  • Are there any other good patent search systems?

You could also use PatBase, Qweb, QPAT, MicroPatent Patent Web, or Thomson Innovation. All these systems have useful features like classification analysis that would benefit anyone searching for patents.

If you've found patents that concern you during your classification search, it's wise to speak to a patent attorney. UpCounsel lists a number of excellent patent attorneys that can help you search and secure your own patent.

  • How big is the U.S. patent database?

There are well over 5 million patents published in the United States. Each one contains between 100 and 2000 GB of text. In addition, the U.S. database features many more than 40 million pages of bitmap images, taking up more than 4 TB of data.

Roughly 1.5 million searchable U.S. patents date back to between 1980 and 1996. These patents equal roughly 100 GB: 55 GB of text and 45 GB of indexes. As of 1999, these patents accounted for a quarter of all United States patents. 

Another key part of the database is a collection of roughly 220,000 U.S. patents published in 1995 and 1996. This smaller collection's text and indices takes up 16 GB. These patents are in Greenbook format, a format which tags hundreds of fields at two levels.

These fields include small pieces of numerical information like the:

  • Patent number
  • Application number
  • Application and issue dates
  • Number of figures

Slightly larger fields include text information like the:

  • Author name(s) and addresses
  • Assignee name(s) and addresses
  • Patent examiner name
  • Patent attorneys

Each patent in this small collection ranges from just a few kilobytes to half a megabyte.

If you need help with a patent classification search, you can post your question or concern on UpCounsel's marketplace. UpCounsel accepts only the top 5 percent of lawyers to its site. Lawyers on UpCounsel come from law schools such as Harvard Law and Yale Law and average 14 years of legal experience, including work with or on behalf of companies like Google, Menlo Ventures, and Airbnb.