What is the IDX file format?
The .idx file extension is used by index files created by various applications and search tools. A single .idx file contains an index of records, files, or folders - for example, within a data catalogue or database. The exact internal layout is application-specific and may be binary or plain text.
.idx files can be generated by the search or indexing function of a given application. They can be quite large, and deleting such a file has no effect on the program's behaviour. In many cases, a deleted .idx file will be recreated automatically during the subsequent indexing process.
Index files are used by applications to speed up file search processes, in database queries, and as sorting algorithm subroutines. .idx files are usually found in databases - for instance, Visual FoxPro pairs .idx index files with a .DBF table - but many other programs also make use of them.
Because .idx is a generic extension, several distinct named formats share it:
- VobSub subtitle index - plain-text, paired with a .SUB data file for DVD picture subtitles
- Git pack index - binary, starts with bytes
FF 74 4F 63; ends with SHA-1 checksums - FoxPro / Visual FoxPro index - binary B-tree of sorted keys accelerating
.DBFqueries - LaTeX raw index - plain-text list produced by the
makeindexcommand
An .idx is almost always an auxiliary file: it makes its companion data file faster to search but holds no original content of its own. The application that created it is the right tool to open or repair it.
Security & safety
RISK: LOWA .idx is passive lookup data, not executable, so the file itself is not a malware vector. The practical cautions are operational: deleting or editing a binary index a program still needs can corrupt search or require a rebuild, so let the owning application manage it. Most application indexes regenerate automatically, making them safe to delete when you're sure which program owns them - but verify before removing one inside a live database or repository.
Format details
in a nutshell- VobSub subtitle index (.idx + .sub) - A plain-text index of timecodes and byte offsets pointing into a companion .sub bitmap-subtitle file; loaded by VLC/MPC-HC alongside the .sub.
- Git pack index (.idx) - Binary index (magic FF 74 4F 63) pairing with a .pack file in .git/objects/pack/ to locate objects quickly.
- LaTeX raw index (.idx) - Plain-text list of \indexentry terms LaTeX emits; processed by makeindex into a sorted .ind for the printed index.
Programs that open IDX files
Windows4 apps
Visual FoxPro
Paid For an xBase .idx, open the associated database/table in FoxPro; it uses or rebuilds the index. (FoxPro is discontinued/legacy.) 
VLC media player
Open-source If the .idx is a VobSub subtitle index, keep it beside the matching .sub and same-named video; VLC loads the pair as subtitles. 
The application that created it
Built-in Open the parent program (Visual FoxPro, your mail client, the CAD/GIS app) - it reads or rebuilds its own .idx automatically; you rarely open the .idx directly.
Notepad++ / a hex editor (HxD)
Open-source Open the .idx to inspect the first bytes and decide if it's a text index (VobSub/LaTeX) or a binary app index. Don't edit a binary index by hand. 
macOS1 app
The application that created it
Built-in Open the originating program; it manages its own .idx. Use a hex/text editor (Hex Fiend) only to identify the type. 
Linux1 app
The application that created it
Built-in Let the parent app (database engine, Git, LaTeX/makeindex) read or rebuild its .idx; inspect with 'file'/a hex editor to identify it. Technical details
deep spec| Format type | Companion index/lookup file - sorted keys or offset pointers mapping record IDs or search terms to byte positions in a separate data file |
| Encoding | Binary or plain text - determined entirely by the producing application; no universal encoding |
| Byte order | Little-endian for most Windows binary variants (e.g. FoxPro); Git pack index is big-endian |
| Magic bytes | No universal file signature; Git pack .idx starts with FF 74 4F 63 (the string tOc preceded by 0xFF); all other .idx variants must be identified by context or origin folder |
| MIME type | application/octet-stream for binary variants; text/plain for plain-text variants such as VobSub and LaTeX |
| Internal structure | Sorted keys, B-tree nodes, or flat offset tables; Git pack v2 uses a fanout table + sorted SHA-1 list + offset array |
| Compression | None - indexes are kept uncompressed so individual records can be located with fast random-access reads |
| Encryption | None in typical use; some application indexes inherit access protection from the parent data store |
| Typical file size | A few KB to hundreds of MB, scaling with the size of the indexed dataset |
| Companion file | Always paired with a primary data file it indexes (e.g. .dbf, .dat, .pack, .sub); the .idx file is meaningless without it |
| Regeneration behaviour | If deleted, the application usually recreates the .idx automatically on the next launch or scheduled indexing run |
| Checksum / integrity | Application-dependent - Git pack .idx ends with two SHA-1 hashes; most other application indexes include no checksum |
| Platform | Cross-platform convention used on Windows, macOS, and Linux; no OS-specific dependency |
| Known named variants | VobSub subtitle index (.idx + .sub), Git pack index (v1 and v2), FoxPro/Visual FoxPro index, LaTeX raw index |
| Content ownership | Auxiliary only - holds no original content; all original data resides in the companion data file |
| Indexing models used | B-tree (FoxPro), flat offset table (many mail stores and search caches), fanout + SHA-1 sorted list (Git pack v2) |
| Released | 1980s-1990s (the 'index' convention predates and spans dBASE, FoxPro, mail clients, LaTeX, etc.) |
| Latest version | N/A (no single specification - defined per application) |
IDX conversions
Community Q&A
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