A BAK file is a backup copy that software creates automatically when you edit or save a document

When you open a file in most programs — a spreadsheet, a document, a configuration file — and make changes, the software often creates a BAK file as a safety measure. This backup copy sits in the same folder as your original file and holds the version from before you made your latest edits. If something goes wrong during your save, or if you realize you made a mistake you want to undo, the BAK file is there to fall back on.

The name comes from the word "backup" shortened to three letters. The file itself is usually identical to your original — same content, same format — but with a .bak extension added to the filename. So if you edit a file called budget.xlsx, your program might create budget.xlsx.bak or sometimes budget.bak depending on which software you are using.

Key Takeaways

  • BAK files are automatic backups created by your software before you save changes to a document or configuration file.
  • They sit in the same folder as the file you are editing and use the same format as the original.
  • Most programs replace the BAK file each time you save, keeping only the most recent previous version.
  • You can safely delete BAK files if you no longer need them, but keeping them costs almost no storage space.
  • Some programs let you turn off BAK file creation in settings if you prefer not to have them.

How BAK files get created and replaced

The process is straightforward. You open a file that already exists on your computer. Your software reads it into memory and displays it on screen. You make changes — edit text, adjust numbers, modify settings. When you click Save, the program does not overwrite your original file right away. Instead, it first renames the current version to filename.bak, then writes your new changes to the original filename.

This means the BAK file always contains the version from your last save, not from when you first opened the file. If you save the same document five times in one session, the BAK file gets replaced five times. It only ever holds one previous version — the one when ready before your most recent save.

Some programs, like text editors and code editors, create BAK files by default. Others, like Microsoft Word and Google Docs, use different backup systems instead. A few programs let you turn BAK file creation on or off in their settings, usually under Save Options or Backup Settings.

When a BAK file actually helps you

The main reason BAK files exist is protection against data loss during the save process itself. If your computer crashes, loses power, or the program freezes while writing to disk, the BAK file is still intact. You can recover your previous version and lose only the changes from that one session.

BAK files also help when you make a mistake and want to undo changes that go beyond what your program's undo function can reach. If you saved a document, closed it, reopened it later, and then realized you deleted something important, the undo history is gone — but the BAK file still has the previous version. You can rename the BAK file back to the original name and recover your work.

For configuration files and code, BAK files serve the same purpose. If you edit a settings file and accidentally break something, you can restore the BAK version and start over. Many system administrators and developers keep BAK files around for exactly this reason.

Whether you should keep or delete BAK files

BAK files take up very little storage space — usually the same size as your original file, sometimes slightly smaller. On a modern computer with hundreds of gigabytes of storage, keeping BAK files costs you almost nothing. Most people never notice them.

If you are running low on storage space, or if you are certain you will not need to recover a previous version, you can safely delete BAK files. They are not system files and deleting them will not break anything. You can delete them one at a time, or search your computer for all .bak files and remove them in bulk.

The trade-off is straightforward: keep them if you want the safety net of a recent backup, delete them if you need the space. There is no wrong answer. If you delete them and later wish you had not, you can usually recover deleted files using recovery software, though success depends on how long ago you deleted them and what else your computer has written to disk since then.

How BAK files differ from other backup types

BAK files are not the same as version history or cloud backups. Microsoft Word, for example, does not create .bak files by default. Instead, it uses AutoRecover, which stores temporary copies in a hidden folder and lets you recover unsaved work if the program crashes. Google Docs saves versions automatically to the cloud and lets you see the full history of every change.

A BAK file is simpler and older — it is just a single previous version sitting on your hard drive. It does not track who made changes, when they were made, or let you jump back to any point in time. It is one step back, not a full history.

For serious work, you should use version control software like Git, or cloud storage with version history like OneDrive or Dropbox, in addition to whatever BAK files your program creates. BAK files are a safety net, not a complete backup strategy.

Where to find BAK files on your computer

BAK files sit in the same folder as the file you edited. If you edited a document on your Desktop, the BAK file appears on your Desktop. If you edited a file in your Documents folder, the BAK file is there too.

On Windows, you can search for all .bak files by opening File Explorer, clicking the search box, and typing *.bak. On Mac, open Finder, click the search icon, and type the same thing. Both will show you every BAK file on your computer.

Some programs hide BAK files by default so they do not clutter your folders. If you cannot see them, you may need to turn on the option to show hidden files. On Windows, go to View and check "Hidden items". On Mac, press Command+Shift+Period to toggle hidden files on and off.

Frequently Asked Questions

Can I open a BAK file directly?

Yes. A BAK file is usually the same format as your original, so you can open it with the same program. You may need to rename it first — remove the .bak extension — or you can right-click it, choose Open With, and select the program you want. The content will be identical to what you had before your last save.

Will deleting BAK files speed up my computer?

No. BAK files are so small that they have no measurable impact on performance. Deleting them will not make your computer faster. If you are running out of storage space, look for larger files first — old downloads, duplicate photos, or unused programs will free up far more space than BAK files.

Why does my program create multiple BAK files with different names?

Some programs create numbered backups like filename.bak1, filename.bak2, and so on, keeping several previous versions instead of just one. This gives you more recovery options. Check your program's settings to see how many backups it keeps and whether you can change that number.

Is a BAK file the same as a temporary file?

No. Temporary files (often with .tmp or .temp extensions) are created while a program is running and deleted when you close it. BAK files are permanent — they stay on your computer until you delete them. They serve different purposes: temp files are working space, BAK files are backups.

What if I accidentally deleted my original file but the BAK still exists?

You can recover it. Rename the BAK file by removing the .bak extension — so filename.bak becomes filename.xlsx — and you have your file back. It will be the version from your last save, not the current state, but it is better than nothing.