InChI: How Chemical Structures Become Searchable

Updated 5 November 2026 · Standard formula, shown below

InChI, short for International Chemical Identifier, was designed to give chemical structures a consistent text representation that computers and people can use to identify and search for compounds. This guide shows how to create or find an InChI, use it in a chemical database, and interpret the result without assuming that every structure has only one possible representation.

What You’ll Need

A chemical structure, preferably as a structure file or a drawing in a chemistry program. A name, formula, or existing identifier can also help you locate it.
Access to a structure editor or chemical database that can generate, import, or search InChI. Availability and menu names differ by service.
For programmatic work, an InChI software implementation and a way to handle its output as plain text.

Step-by-Step

Choose the structure you want to identify. Start with the exact molecular structure, not just a familiar name. Check the atoms, bonds, charges, isotopes, and stereochemistry. A name can refer to different forms, and a molecular formula alone does not specify how atoms are connected.
Make the structure machine-readable. If you have a drawing, enter it in a chemical structure editor. If you have a supported structure file, import it instead. Confirm that the editor has interpreted the bonds and atom labels correctly. Pay particular attention to wedges or other stereochemical marks, formal charges, and isotopic labels.
Generate the InChI. Use the editor’s export, identifier, or structure-information function to request an InChI. The exact command depends on the software. A standard InChI commonly begins with InChI=1S/; the rest is a layered text string encoding structural details. Copy the complete string, not just its first line or a shortened display.
Keep the InChIKey separate. Some tools also provide an InChIKey, a shorter, fixed-length hash derived from an InChI. It is convenient for searching and indexing, but it is not a reversible encoding of the full structure. Save both identifiers with clear labels if you need them; do not paste an InChIKey where a database requests a full InChI.
Search a chemical database. Open a database that supports structure or identifier search, choose the appropriate search field, and paste the complete InChI or InChIKey. If there is no identifier field, try its structure-search tool or search by a reliable name, registry identifier, or formula as a fallback. Search behavior varies: some services accept only one identifier type.
Inspect the match before using it. Open the returned record and compare its displayed structure, stereochemistry, charge, isotope information, and any salt or mixture components with your intended compound. A matching text identifier is useful evidence, but context still matters: databases may store different forms or records for related substances.
Use the identifier consistently in your records or workflow. Store the full InChI as text and preserve its spelling, punctuation, and layer order. For a spreadsheet or database, use a dedicated field rather than editing the string. If you exchange structures between programs, retain the original structure file too, so the structure can be checked or processed again.
Record the generation details when reproducibility matters. Note whether the string is standard InChI or a non-standard variant, and record the software or version where practical. Different options can affect how a structure is interpreted or represented. This is particularly important in research pipelines and large-scale data cleanup.

Common Mistakes

Searching with the wrong identifier: An InChI and an InChIKey are different. Check the field label and paste the corresponding value.

Ignoring stereochemistry: Two structures with the same formula and connectivity can differ in stereochemistry. Make sure the source drawing or file actually includes the stereochemical information you need.

Treating a formula as a unique structure: Many compounds share a molecular formula. Generate an identifier from the full structure rather than guessing from the formula.

Changing the string manually: Removing separators, changing capitalization, or copying only part of an InChI can make it unusable or change its meaning. Copy it exactly.

Assuming every database match is interchangeable: Salts, tautomers, protonation states, mixtures, and stereoisomers may be handled differently by software and databases. Inspect the structure and record description.

Tips

Use a standard InChI when you want broad interoperability, unless your workflow specifically requires a different option.
Keep the original structure alongside the InChI. An identifier helps compare and search records, but a structure file is more useful for visual review and further editing.
Use an InChIKey as a compact lookup or indexing aid, not as a replacement for the full identifier when exact structural detail is needed.
When comparing strings from different tools, check whether both use the same InChI type and settings before concluding that the structures differ.
For bulk searches, preserve identifiers as plain text. Spreadsheet software may otherwise alter characters or treat values as formulas.

Troubleshooting

The database finds no result: Confirm that you copied the whole identifier and selected the right search type. Try the full InChI if you used a key, or the key if the database has a key field. If neither works, search by structure or a known name and compare the returned record.

The generated InChI looks different from a reference: Check for missing stereochemistry, charge, isotopes, or components, and verify whether the reference is standard InChI. Different software settings or structure normalization can also produce differences. Compare the actual structures, not only the text.

The editor rejects an imported structure: Check that the file format is supported, the file is complete, and atom and bond data are valid. If possible, open it in a compatible editor, save it in a supported structure format, and import the saved copy.

A search returns several related records: Review each structure and its record details. The search may be matching a broad or normalized form, or the compound may have different stereochemical, ionic, or tautomeric records. Select only the record that fits your intended structure.

You need to regenerate an identifier exactly: Use the same structure and, where available, the same software implementation and options used originally. Keep a note of those details instead of relying on a copied string without context.

Frequently Asked Questions

What does InChI stand for?
It stands for International Chemical Identifier, a text-based identifier intended to represent chemical structures consistently.

Is an InChI the same as an InChIKey?
No. The InChI is the layered structure string. The InChIKey is a shorter hash derived from it and cannot be decoded back into the complete structure.

Can I search by chemical name instead?
Often, yes, if the database supports name searches. Names can be ambiguous or vary between sources, so confirm any result against the structure.

Does one compound always have exactly one InChI?
Not necessarily. Structure interpretation, normalization, stereochemical detail, and generation options can affect the identifier. Use consistent settings when comparing results.

Can an InChI tell me a compound’s properties?
No. It identifies structural information; it does not by itself provide properties, safety information, or experimental data. Consult appropriate records for those details.

Why was InChI meant to make structures easier to find online?
A consistent text identifier can be indexed, copied, and matched across databases more easily than a drawing alone, helping computers connect records that refer to the same represented structure.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.