How to Restore a File from a .BAK Backup

You lost or corrupted a file but found a .bak next to it. That is the backup. Here is how to restore your work from it.

Filename or extension

Category

Safe to delete?

Opens with

Restore it in three steps

Make a copy of the .bak first so you never touch the only backup. Work on the copy.
Rename the copy to the original extension. If it was report.docx.bak, rename to report.docx. If you only see report.bak, add the extension the source program uses (.docx, .psd, .dwg, .sql…).
Open it in the matching program. If Windows hides extensions, turn on File name extensions in File Explorer's View tab so the rename actually changes the type.

When you don't know what made it

Open the copy in a text editor and read the first line — many formats announce themselves (e.g. %PDF, SQLite format 3, PK for Office/ZIP). That header tells you which extension to rename to.

Database and game .bak files

SQL Server .bak files are restored from inside the database tool (RESTORE DATABASE / SSMS → Restore), not by renaming. Game and app .bak files usually restore by copying them back into the save folder with the backup suffix removed.

FAQ

How do I open a .bak file?

Copy it, rename the copy to the original file's extension, then open it with the program that created it. If unknown, inspect the header in a text editor to identify the type.

Will renaming a .bak damage it?

No — renaming only changes the label, not the data. Always rename a copy so the original backup stays intact in case the guess is wrong.

How do I restore a SQL Server .bak?

Use SQL Server Management Studio → right-click Databases → Restore Database → Device, or run RESTORE DATABASE. Do not rename it.

Is it safe to delete a .bak file?

Yes, once the current file works and you no longer need the older copy it preserves.