To do a prior art search, break your invention into its core technical concepts, build a keyword list with synonyms for each, run those keywords across several free patent databases and non-patent sources, then narrow and expand your results using classification codes until you can confidently compare what already exists to what you plan to claim. The goal is to find anything publicly available before your filing date that discloses your invention or something close enough to block it.
Know What You’re Looking For
Prior art is any information made available to the public, anywhere in the world, before the effective filing date of your patent application. Under 35 U.S.C. § 102(a)(1), you cannot patent something that was already patented, described in a publication, in public use, on sale, or otherwise accessible to the public before you filed.1Office of the Law Revision Counsel. 35 USC 102 – Conditions for Patentability; Novelty
That “otherwise available to the public” language reaches further than most inventors expect. A YouTube video demonstrating a mechanism, a product listed in an old catalog, an archived forum post, a master’s thesis on a university library shelf — all qualify if they were publicly accessible before your filing date. The reference does not need to be in your industry, your language, or your country.
Prior art matters because it decides two questions. Novelty asks whether any single reference already discloses every element of your claim. Non-obviousness, under 35 U.S.C. § 103, asks whether the differences between your invention and the prior art would have been obvious to someone with ordinary skill in your field.2Office of the Law Revision Counsel. 35 USC 103 – Conditions for Patentability; Non-obvious Subject Matter Your search has to be good enough to test both.
One boundary worth knowing before you start. Your own public disclosure of your invention within the year before you file does not count against you in the United States, under 35 U.S.C. § 102(b)(1)(A).1Office of the Law Revision Counsel. 35 USC 102 – Conditions for Patentability; Novelty Most other countries do not offer that grace period, so if international protection matters, file before you disclose anything publicly. And the grace period only shields disclosures traceable to you; if an independent third party publishes the same concept before your filing date, it still counts.
Break the Invention Into Searchable Concepts
The quality of your search depends on the words you feed it. Start by listing the functional components of your invention and every term someone might use for each one. Include technical jargon, brand-neutral descriptions, and British spelling variants.
An invention involving a “fastener” might also be described as a clip, clamp, coupler, latch, or connector. If you only search “fastener,” you miss the rest. Do this exercise for every meaningful component before you touch a database. A weak keyword list is the single most common reason a search misses the reference that later kills the application.
Search the Free Patent Databases
You do not need paid tools to run a meaningful search. Four free databases cover most of what you need, and each has strengths worth using.
- USPTO Patent Public Search. The U.S. Patent and Trademark Office’s own tool searches the full text of U.S. patents and published applications. It supports Boolean and proximity operators, so it handles detailed queries well. Best starting point for U.S.-focused searches.3United States Patent and Trademark Office. Patent Public Search
- Espacenet. The European Patent Office’s free database contains over 150 million patent documents from around the world, updated daily, with built-in machine translation and a classification search tool. Strong for international searches.4European Patent Office. Espacenet – Patent Search
- PATENTSCOPE. WIPO’s database covers over 126 million patent documents, including published international (PCT) applications. Fills gaps that U.S.-only databases miss.5World Intellectual Property Organization. PATENTSCOPE – Search International and National Patent Collections
- Google Patents. Covers over 120 million patent publications from more than 100 patent offices, with U.S. records back to 1790. The interface is the most intuitive, and it links to cited and citing patents so you can follow chains of related inventions.
No single database catches everything. Run the same core searches across at least two or three, because each indexes documents differently and covers different jurisdictions more thoroughly.
Search by Classification Code
Keyword searches miss patents that describe the same technology using different words. Classification systems fix this by grouping patents into technology categories with standardized codes, regardless of the language or phrasing inside the document.
Cooperative Patent Classification (CPC)
The Cooperative Patent Classification system, developed jointly by the USPTO and the EPO, is the primary scheme for U.S. and European patent searching.6United States Patent and Trademark Office. Patent Classification Each code represents a specific technical area. For example, CPC code A61B covers diagnostic and surgical instruments.7United States Patent and Trademark Office. CPC Scheme – A61B Diagnosis; Surgery; Identification
The practical method: start with the USPTO’s CPC scheme browser or Espacenet’s classification tool. Find a known relevant patent, note the CPC codes it carries, then search within those codes to surface similar patents, including foreign-language ones you would never reach through English keywords. This iterative loop often reveals entire clusters of relevant art.
International Patent Classification (IPC)
The International Patent Classification, maintained by WIPO, divides technology into eight main sections with roughly 75,000 subdivisions and is used by over 100 countries, covering nearly 110 million patent documents.8World Intellectual Property Organization. International Patent Classification (IPC) CPC is an extension of IPC with finer subdivisions, so CPC codes are usually more precise. When you search patent offices outside the U.S. and Europe, IPC may be the only classification available.
Search Non-Patent Literature
Patent databases capture one slice of the prior art universe. Technical knowledge published outside the patent system, called non-patent literature, is equally valid prior art and often harder to find. It includes academic journal articles, conference proceedings, technical manuals, industry standards, university theses, product data sheets, and archived websites.
Academic databases like PubMed, IEEE Xplore, and Google Scholar are essential in fields where research moves faster than patent prosecution. A conference paper describing your approach, published two years before your filing date, is devastating even if no patent ever covered it. Academic writing uses its own vocabulary, so your keyword list needs a research-side version, not just the patent-side terms.
The Wayback Machine at archive.org deserves a place in every search. Product pages, blog posts, and technical documentation that have disappeared from the live web often still exist in cached form. Patent examiners and litigation opponents use those snapshots, so you should too.
Use Operators to Sharpen Your Queries
Well-constructed queries beat volume. A handful of techniques do most of the work.
Boolean and Proximity Operators
Basic Boolean operators (AND, OR, NOT) combine terms logically. “Fastener OR clip OR clamp” captures all three variants. Adding AND narrows: “(fastener OR clip) AND rotating AND shaft.”
Proximity operators require terms to appear near each other, which filters out documents that mention your keywords in unrelated contexts. In USPTO Patent Public Search, the ADJ operator requires terms to appear adjacent, while NEAR followed by a number (NEAR3, for example) requires them within a specified word distance.9United States Patent and Trademark Office. Patent Public Search – Search Overview Quick Reference Guide “Solar NEAR5 panel” is far more targeted than the same terms joined with AND.
Truncation and Wildcards
Truncation captures word variations automatically. A truncation symbol after “compress” retrieves “compression,” “compressor,” “compressed,” and “compressible” at once. The specific symbols vary by database, so check the help documentation for whichever tool you use.
Combining Keywords With Classification Codes
The most effective strategy layers keyword queries on top of classification searches. Identify the relevant CPC or IPC codes for your technology, then run your keywords restricted to patents within those codes. A search for “rotating fastener” across the whole database might return thousands of irrelevant results; the same search limited to the CPC subclass for shaft couplings returns a manageable, highly relevant set.
Analyze What You Find
Finding prior art is half the work. Deciding what it means for your invention is the harder half, and it turns on two distinct legal concepts.
Anticipation (Novelty)
Anticipation is an all-or-nothing test. A single reference anticipates your claim only if it discloses every element of that claim. Miss one element and the reference does not anticipate, though it may still contribute to an obviousness rejection. For each promising reference, map every element of your claim against what the reference discloses. Every element matched means the claim lacks novelty.
Obviousness
Obviousness is more flexible and more dangerous to applicants. Unlike anticipation, an obviousness rejection can combine multiple references. The question is whether someone with ordinary skill in your field, looking at the existing prior art, would have found it obvious to combine known elements to reach your invention.
If your search turns up references that together cover all elements of a claim, the analysis is not automatically over. Secondary considerations can rebut an obviousness finding: commercial success, a long-felt but unsolved need in the industry, the failure of others who tried to solve the same problem, or unexpected results your invention achieves.
Claim Charts
The practical way to organize the analysis is a claim chart: a two-column table with each element of your claim on the left and the corresponding disclosure (or absence) from the reference on the right. Building one chart per relevant reference forces you to see exactly where your invention overlaps with existing technology and where it diverges. The gaps in the right column are where your patentable contribution lives, and they often suggest how to narrow or restructure your claims to design around the prior art.
Disclose What You Find
Once you find prior art, you have a legal obligation to tell the USPTO about it. Under 37 C.F.R. § 1.56, every inventor, patent attorney, and person substantively involved in preparing or prosecuting an application must disclose all information they know to be material to patentability, and the duty continues as long as any claim is pending.10eCFR. 37 CFR 1.56 – Duty to Disclose Information Material to Patentability
Information is material if it would establish a case for unpatentability of a claim, alone or combined with other information, or if it contradicts a position you take during prosecution. If you found a reference that undermines any of your claims, disclose it, even if you believe the claim survives.
You submit references through an Information Disclosure Statement (IDS). Timing controls the cost. File the IDS within three months of your application’s filing date or before the examiner’s first action on the merits, and no fee is required. File after that window but before a final action or notice of allowance, and you owe $280 for a large entity ($112 small entity, $56 micro entity), or you can submit a statement that the references came from a foreign patent office communication within the previous three months. After a final action or allowance notice, you need both the fee and the statement.11United States Patent and Trademark Office. Information Disclosure Statement12United States Patent and Trademark Office. USPTO Fee Schedule
Do not bury inconvenient prior art. If someone later proves you intentionally withheld material information, a court can declare the entire patent unenforceable, not just the affected claims. Overdisclosing is the safer strategy.
Doing It Yourself Versus Hiring a Professional
A DIY search using the free databases above costs nothing but time, and spending a few days searching before committing to professional help is a reasonable first step. You may find a dead-on reference that settles the question immediately.
Professional searches by patent attorneys or specialized search firms typically run from $500 to $3,000 or more, depending on the complexity of the technology. A straightforward consumer product sits at the lower end; a complex software algorithm or biotechnology invention pushes toward the higher end. The search itself is usually the cheaper part. The more valuable piece is the patentability opinion that follows: a written analysis of how the prior art affects your specific claims and whether filing is worth pursuing. That opinion shapes your claim strategy and can save you from filing doomed applications.
One Search This Is Not: Freedom to Operate
A patentability search asks whether you can get a patent. A freedom-to-operate search asks whether making, selling, or using your product would infringe someone else’s active patent. An invention can be fully patentable, genuinely novel and non-obvious, and still infringe an existing patent when it falls within the claims of a broader, active one. If you plan to commercialize, you may need both searches, and the prior art search described here does not answer the infringement question.