Patent Classifications Explained: IPC and CPC

Every patent ever published has been categorised — assigned to one or more classification codes that describe what the invention does, what technology area it belongs to, and how it relates to other inventions. These classification codes are the most powerful prior art search tool most inventors never learn to use.

Keyword searches find patents that use the same words you use. Classification searches find patents that solve the same problem — regardless of what words the inventor used to describe them. A "pressure-actuated shut-off valve" and a "flow-responsive closure device" are the same thing, but a keyword search finds only one. A classification search finds both, because a patent examiner categorised them into the same technology class.

If you are conducting prior art searches using only keywords, you are missing relevant prior art. This guide teaches you how classification systems work, how to find the right codes, and how to use them to search with the precision of a professional patent searcher.

Why Classifications Matter

Patent offices receive millions of applications per year. Examiners cannot read every patent in every field — they specialise. Classification systems organise the entire body of patent literature into a structured hierarchy so that examiners can find relevant prior art efficiently, and so that inventors, attorneys, and researchers can do the same.

Two classification systems dominate global patent searching:

IPC (International Patent Classification): Administered by WIPO. Used by virtually every patent office worldwide. The IPC has approximately 70,000 subdivisions covering all areas of technology. It is updated annually.

CPC (Cooperative Patent Classification): Jointly administered by the USPTO and the EPO. The CPC is a more granular extension of the IPC — it includes all IPC codes plus approximately 250,000 additional subdivisions that provide finer-grained categorisation. The CPC is the primary classification system used by the USPTO, the EPO, and increasingly by CNIPA, KIPO, and other major offices.

In practice: If you are searching for prior art in a specific technology area, use CPC codes. They are more detailed than IPC codes and are the classification system most commonly used by the world's two largest examining offices. IPC codes are useful for searching offices that have not adopted CPC — particularly some Asian and developing-country offices.

How the Classification Hierarchy Works

Both IPC and CPC use the same hierarchical structure, moving from broad to specific:

The Hierarchy

Section (1 letter) → Class (2 digits) → Subclass (1 letter) → Group (1–3 digits) → Subgroup (2+ digits)

Example: A Water Filtration Device

Full CPC code: C02F 1/28

A water filtration device using activated carbon to remove contaminants would be classified under C02F 1/28. Every other patent in this subgroup describes a technology that uses sorption-based water treatment — regardless of what words the inventor used in the title or description.

The Eight Sections

Most inventor patents fall into sections A, B, C, F, G, or H. Section G covers an enormous range of modern technology including computing, AI, sensors, and measurement devices.

How to Find the Right Classification Code

Method 1: Start From a Known Relevant Patent

This is the fastest and most reliable method.

1. Find a patent that is relevant to your invention — through a keyword search, a competitor's product, or a citation in your prior art research
2. Open the patent on Google Patents, Espacenet, or USPTO Patent Center
3. Look for the "Classifications" section — it lists every CPC and IPC code assigned to that patent
4. Click any code to see all patents in that class
5. Browse the sibling codes (same level in the hierarchy) to find related technology areas you may have missed

Example: You find a competitor's patent for a solar panel mounting bracket. Its CPC codes include F24S 25/00 (solar heat collectors — supports and mounting arrangements). Clicking that code reveals hundreds of other solar mounting patents — including designs you would never have found by searching "solar panel bracket" as keywords.

Method 2: Browse the Classification Tree

Both CPC and IPC have browsable hierarchical trees:

• CPC: Available at worldwide.espacenet.com/classification (Espacenet's CPC browser)
• IPC: Available at ipcpub.wipo.int (WIPO's IPC publication)

Start at the section level, drill down through class, subclass, and group until you reach the codes that describe your technology area. This takes longer than Method 1 but gives you a systematic understanding of how your technology fits into the broader classification landscape.

Method 3: Use WIPO's IPCCAT Tool

WIPO provides an AI-powered classification suggestion tool (ipccat.wipo.int) that takes a text description of your invention and suggests the most relevant IPC codes. Enter your invention's title and abstract, and the tool returns ranked classification suggestions. Use these as starting points, then verify by examining patents in the suggested classes.

Using Classification Codes in Patent Searches

In Google Patents

Enter CPC codes directly in the search bar:

• CPC:C02F1/28 — all patents classified under sorption-based water treatment
• CPC:C02F1/28 activated carbon — combining classification with keywords
• CPC:C02F1/28 assignee:Brita — all Brita patents in this class
• CPC:C02F1/28 before:priority:20200101 — patents in this class with priority before 2020

In Espacenet

Use the "Advanced search" and enter CPC or IPC codes in the classification field. Espacenet's classification-based searching is particularly strong for European and Asian patent documents.

In USPTO Patent Center

Use the "CPC" field in the search form. USPTO's full-text search combined with CPC codes produces highly targeted results for US patents.

In CNIPA (China)

China has adopted CPC codes alongside its own classification system. Searching by CPC code on CNIPA's patent search system (pss-system.cnipa.gov.cn) captures Chinese patents that keyword searches in English would miss entirely.

A Worked Search Using Classification Codes

Invention: A self-heating food container that uses an exothermic chemical reaction to heat the contents without external power.

Round 1 — Keyword search: "self-heating food container exothermic" on Google Patents returns 47 results, mostly US patents from military ration manufacturers.

Round 2 — Extract classification codes: The most relevant result (a US Army MRE heater patent) is classified under:

• B65D 81/26 — Containers with means for heating contents
• C09K 5/18 — Materials not otherwise provided for, for chemical heating

Round 3 — Classification search: CPC:B65D81/26 returns 312 results — six times more than the keyword search. The results include Japanese patents from Nippon Steel for self-heating sake containers, Chinese patents for self-heating hot pot packaging, and German patents for chemical heating elements integrated into food packaging. None of these used the phrase "self-heating food container" — they used entirely different terminology — but they are all directly relevant prior art.

Round 4 — Explore the chemical heating side: CPC:C09K5/18 surfaces patents for the exothermic reaction chemistry itself — calcium oxide/water reactions, iron oxidation heating, and proprietary reaction formulations. These may not mention "food" at all, but the underlying chemistry is relevant prior art for the heating mechanism.

Result: The keyword search found 47 patents. The classification search found 312+ patents across two complementary technology areas (container design and heating chemistry), in multiple languages and from multiple countries. The classification approach revealed an entire landscape that keyword searching could not access.

CPC vs. IPC: When to Use Which

Practical guidance: Use CPC for your primary searching — it is more granular and covers the two largest examining offices. Use IPC when searching offices that have not adopted CPC, or when you want to ensure you have not missed a broad technology category.

Classification Pitfalls

A patent can have multiple classification codes. A self-heating food container has codes for both container design and chemical heating. Search all relevant codes, not just the first one you find.

Classification codes evolve. The CPC is updated continuously — new codes are created, old codes are reorganised. A code that was current in 2015 may have been reclassified. Use the current CPC tree, and when searching older patents, check whether their codes have been mapped to current equivalents.

Classification is done by humans. Examiners classify patents based on their reading of the claims and description. Occasionally, relevant patents are misclassified or classified under unexpected codes. Classification searching is powerful but not infallible — combine it with keyword searching for comprehensive results.

Subgroup depth varies. Some technology areas have deeply granular classification (electronics, chemistry, automotive) with hundreds of specific subgroups. Others are less developed (emerging technologies, novel materials). If your technology area has limited classification depth, broader group-level searches may be necessary.

Sources

1. Espacenet CPC Classification Browser — Interactive browser for navigating the Cooperative Patent Classification hierarchy
2. WIPO IPC Publication — Official International Patent Classification scheme with full hierarchy and definitions
3. WIPO IPCCAT — AI-powered tool that suggests IPC classification codes from a text description of an invention
4. Google Patents — Patent search supporting CPC code queries, combining classification with keyword and assignee filters
5. USPTO - Cooperative Patent Classification — Information on CPC development and usage at the USPTO