BAK (backup) file
Also known as: .bak file, backup file, bak extension
A BAK file (.bak) is a backup copy of another file, created automatically by an app before it saves or changes the original. It exists as a safety net, and once you are sure the current version is fine, the .bak is usually safe to delete.
- A backup copy made before a file is changed
- Acts as a safety net for undoing or recovering
- Usually safe to delete once the original is fine
Why .bak files appear
Many programs make a backup before overwriting your work, renaming the previous version with a `.bak` extension. If a save goes wrong or you want to undo a change, you can recover by renaming the .bak back to the original extension.
The .bak extension is a convention, not a single format — a .bak might be a copy of a document, a database, or app settings. What is inside matches whatever the original file was.
Is it safe to delete?
Usually yes. A .bak is a duplicate of an earlier state, so deleting it only removes your ability to roll back to that point. Once the live file is working and you do not need the older version, the backup is reclaimable space.
Old .bak files can quietly accumulate next to documents and project files. Before deleting, confirm it is truly a backup of something you still have, not the only copy of important data.