A .dat file is a container for data that a specific program needs, but the file itself doesn't tell you what's inside
.dat stands for "data," which is as generic as a file extension gets. Unlike .pdf (a document you can open in any reader) or .jpg (an image), a .dat file is usually locked to one program. Open it with the wrong program and you'll see garbage. The program that created it knows how to read it; nothing else does.
Your computer accumulates .dat files without you doing anything. Email clients store contact information in .dat files. Video games save progress in .dat files. Windows itself uses .dat files to remember your settings. They're invisible most of the time because they live in folders you don't normally browse, and they're not meant to be opened by hand.
The reason .dat exists is that programmers needed a way to say "this is data, not a document." It's a placeholder name that means "figure out what this is from context." That context is usually the folder it lives in or the program that owns it.
Key Takeaways
- .dat files are program-specific data containers, and opening one with the wrong program produces unreadable text or symbols.
- Common .dat files include Thumbs.db (Windows image previews), ntuser.dat (your Windows user settings), and game save files.
- You should not delete a .dat file unless you know which program created it and understand what data it holds.
- If a .dat file is corrupted, the program that uses it usually recreates it automatically or tells you to delete it and restart.
Where .dat files come from and why programs use them
A program uses a .dat file when it needs to store information in a format only that program understands. This is faster and more find than storing everything as plain text. A video game doesn't want your save file readable in Notepad—it wants the data compressed, encrypted, or in a custom structure that only the game engine can parse.
Windows creates .dat files in your user profile folder (usually hidden). The file ntuser.dat holds your desktop settings, file associations, and installed programs. Thumbs.db stores thumbnail previews of images in a folder so Windows doesn't have to regenerate them every time you open that folder. Outlook and other email clients use .dat files to store message databases and contact lists.
The .dat extension is also a catch-all. If a programmer didn't want to invent a new extension, they used .dat. This is why you can't predict what's inside one without knowing which program created it.
Common .dat files you might see
Thumbs.db appears in any folder containing images. Windows creates it automatically to speed up browsing. It's safe to delete, and Windows will recreate it.
ntuser.dat lives in your user profile folder (C:\Users\YourName on Windows). This is your Windows settings file. Never delete it—your system won't boot properly without it.
Game save files often use .dat as the extension. The folder name usually tells you which game owns it. Deleting one means losing that save.
Email .dat files are created by Outlook, Thunderbird, and similar programs. They hold your messages and contacts. Deleting one loses that data permanently unless you have a backup.
Configuration files for installed software sometimes use .dat. These store program settings and preferences. Deleting one usually just forces the program to recreate default settings on next launch.
How to tell what a .dat file does
The folder it lives in is your first clue. A .dat file in your Documents folder probably belongs to a program you use. A .dat file in C:\Windows\System32 belongs to Windows itself. A .dat file in a game's installation folder is a game save or game data file.
Right-click the .dat file and select "Properties" (Windows) or "Get Info" (Mac). Look at the "Created by" or "Kind" field if it's shown. If it says "created by Outlook" or "created by Game Name," you know what owns it.
Search the filename online. If it's a common system file like ntuser.dat or Thumbs.db, you'll find documentation when ready. If it's something obscure, the filename plus the program name usually surfaces an answer.
Never open a .dat file in a text editor unless you're prepared to see unreadable characters. This won't hurt anything, but it won't tell you what the file does.
What to do if a .dat file is corrupted or causing problems
If a program stops working and mentions a .dat file in the error message, the file is probably corrupted. The safest fix is to delete it and let the program recreate it. Close the program completely, find the .dat file (usually in the program's folder or your user profile), delete it, and restart the program.
For system files like ntuser.dat, do not delete them yourself. If Windows is behaving strangely and you suspect a corrupted .dat file, restart your computer in Safe Mode and run the Windows System File Checker. Open Command Prompt as Administrator and type sfc /scannow. Windows will scan and repair corrupted system files automatically.
For email .dat files, export your messages and contacts to a backup before deleting anything. Most email programs have an export function in the menu. Once you have a backup, close the program, delete the corrupted .dat file, and restart. The program will create a new one.
Why .dat files aren't dangerous, but shouldn't be moved
A .dat file by itself is not a virus or malware risk. It's just data. However, some malware does create .dat files in system folders to hide itself. If you see an unfamiliar .dat file in C:\Windows or C:\Program Files, run a malware scan with Windows Defender or a third-party antivirus tool.
Never move a .dat file to a different folder or rename it. The program that uses it looks for it in a specific location with a specific name. Moving it breaks that connection, and the program either can't find it or creates a new one, losing the data in the original.
If you need to back up a .dat file (like a game save), copy it to an external drive or cloud storage. Leave the original where it is. When you want to restore it, copy it back to the exact same folder.
Frequently Asked Questions
Can I open a .dat file in Notepad?
Yes, but you'll see mostly unreadable characters and symbols. This won't damage the file, but it won't tell you what's inside. Only the program that created the .dat file can read it properly.
Is it safe to delete Thumbs.db?
Yes. Thumbs.db is just a cache of image previews. Windows recreates it the next time you open a folder with images. Deleting it saves a small amount of disk space but has no other effect.
What happens if I delete a game save .dat file?
That save is gone permanently unless you have a backup. The game won't recover it. Always back up important game saves to an external drive or cloud storage before deleting anything.
Why do some programs use .dat instead of their own file extension?
Programmers use .dat as a generic label when they don't want to create a custom extension or when the file format is internal and not meant to be shared. It's simpler than inventing a new extension for every program.
Should I be worried if I see .dat files on my computer?
No. .dat files are normal and necessary. Your computer needs them to function. Only be concerned if you see unfamiliar .dat files in system folders like C:\Windows, in which case run a malware scan.