BAK File Documentation
Summary
A BAK (backup) file is a saved copy of some other file, kept as a fallback before a program overwrites the original. It is not a format of its own: the bytes inside are simply a copy, so a .bak’s real type is whatever the original was — a database, a CAD drawing, a config file, a document. Its MIME type is application/octet-stream. To use one, find out which program made it and either restore it there or rename the copy back to the original extension.
Technical details
| Feature | Value |
|---|---|
| Full name | Backup file (generic) |
| File extension | .bak |
| MIME type | application/octet-stream |
| Format type | Not a format — a copy of another file, kept as a fallback |
| Category | Backup files |
| Developer | None — a convention used by many programs (AutoCAD, SQL Server, editors) |
| Introduced | Predates Windows; a long-standing PC software convention |
| Real type | Whatever the original file was |
| Magic number | None of its own — the first bytes are those of the copied file |
| Typical creator | An app about to overwrite a file (drawing, database, settings, save game) |
| Typical location | Same folder as the working file it copies |
| Safe to delete | Usually yes, once the working file is confirmed intact |
| How to identify | Inspect the header, or note which program created it |
| Related extensions | .old, .tmp, .sav, .~, plus the original’s type (.dwg, .mdf) |
What is a BAK file?
A BAK file is a backup copy, and that is the whole story: .bak is a naming convention, not a file format. When a program is about to overwrite or substantially change a file, it often saves the previous version first with a .bak extension, so you can recover if the new version is wrong or the save fails. The convention predates Windows and has been used by PC software since the 1980s.
Because a .bak is just a copy, it has no structure of its own. Its real type is whatever the original file was. A .bak made by a text editor is plain text; a .bak made by AutoCAD is a full CAD drawing; a .bak made by Microsoft SQL Server is a database backup. This is why there is no universal “BAK format” and no single program that opens every .bak. The right tool depends entirely on which application created the file.
Identify the program that made it
The first step with any .bak is to work out what it is a backup of. Two clues help. The obvious one is context: a .bak sitting next to drawing.dwg in a CAD folder is almost certainly the drawing’s previous version, and a .bak in a SQL Server backup directory is a database dump. The reliable one is the file’s header. Because the bytes are a copy of the original, the first bytes are the original’s signature, and inspecting them reveals the true type.
| First bytes | Underlying file | Made by |
|---|---|---|
54 41 50 45 (TAPE) | Microsoft Tape Format (MTF) database backup | SQL Server |
41 43 31 30 (AC10…) | A DWG drawing | AutoCAD |
| Readable ASCII / UTF-8 | A text, config or code file | A text editor or app settings |
On Windows you can open the file in a text editor or a tool like 7-Zip to peek at the first bytes; on macOS and Linux the file something.bak command detects the real type from the header and tells you what to open it with.
The rename-back trick
The most common everyday recovery is simply to restore the original extension. Once you know what program made the .bak, make a copy of it (so you keep the backup), then change the copy’s extension back to the original: drawing.bak becomes drawing.dwg, notes.bak becomes notes.txt, and each opens normally in its own application. This works because a .bak is usually a byte-for-byte copy; nothing about the data changed, only the extension that tells the operating system which program to hand it to. Always copy first, so a failed guess does not cost you the only backup.
The SQL Server case
The one .bak you cannot simply rename is a Microsoft SQL Server backup. These are written in Microsoft Tape Format (they begin with the ASCII TAPE marker) and can contain one or more full or differential database backups. You do not open such a file by hand; you restore it into a server. In SQL Server Management Studio the path is Databases → Restore Database → Device, then select the .bak, which runs a RESTORE DATABASE operation. The database has to be reconstructed on a running server instance, so renaming the extension achieves nothing here.
Can I delete BAK files?
Usually yes, but only after confirming the working file is intact. A .bak is a safety copy, so deleting it is safe once the current version opens correctly and you do not need to roll back. Keep it if the original is missing, corrupted, or newer than you want, because in those situations the .bak may be your only route back to a good version. Folders that accumulate many .bak files (CAD project directories are notorious for this) can be cleaned once the live files are verified.
Frequently asked questions
How do I open a BAK file?
Find out which program created it, then either restore it in that program (SQL Server, AutoCAD) or use the rename-back trick: copy the .bak, change the copy’s extension to the original’s (.dwg, .txt, .docx…) and open it normally.
How do I recover an AutoCAD drawing from a BAK file?
An AutoCAD .bak is already a DWG. Copy it, rename the copy to .dwg, and open it in AutoCAD; it is the previous saved version of the drawing.
References
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