What’s a '.dat' file?

DAT files are used by many different applications. To know what to do with a DAT file, you need to know what application created it.

I have a “.dat” file on my computer, and I’d like to open it up and see what’s inside. What is a .dat file, and what application do I use to look at it?

Bottom line: there’s really no way to tell from just the filename.

The problem is that many applications use the file extension “.dat” to indicate a file that contains data.

But that’s exactly all it tells us. It doesn’t tell us what kind of data or how that data is formatted and represented.

In other words, it doesn’t give us a single clue as to what the file is or what program we should use to open it.

We need more information and there may be one way to get some.

File extensions

Whatever follows the last “.” in a file name is called the file or filename extension.

In general, it’s intended to tell you what type of information the file contains. .exe files, for example, contain executable programs, .jpg files contain images formatted using the JPEG compression algorithm, and so on.

On problem is that the use of extensions is an arbitrary convention. There’s no rules committee that says what gets to be called what, and there’s no requirement that any of the conventions be followed. It’s perfectly possible to store what would be an executable program in a file with a .jpg extension. In fact, in the past that was one trick that malware would use to infect your system.

While most extensions are somewhat descriptive, like the examples I just mentioned, other are less so.

Like .dat.

The .dat extension

… there is no way to know how to open a .dat file unless you know what program created it.
As I said, .dat simply means that the file contains data.

To me that’s almost redundant because all files contain data when you think about it.

The real problem is that so many different programs use .dat when they store their data that the filename, by itself, really doesn’t tell you anything about the file, what it contains, or what program it belongs to. There is no standard format and there is no standard way to interpret the contents. .dat is only a name and nothing more.

So I’ll be super clear: there is no way to know how to open a .dat file unless you know what program created it. Period.

A good example is that Yahoo Instant Messenger (YIM) apparently keeps its message history in a file that ends in .dat. The way Yahoo works, you can only open that .dat file using Yahoo Instant Messenger, and only of you are logged in to the account that created the file in the first place.

But if you don’t know what application created the file, there’s no way to know how to open it.

Fortunately, there may be one trick to identifying that application…

File signatures

In How do I figure out what kind of file I have – without the file extension? I discuss the concept of file signatures.

File signatures are nothing more than bytes with specific values at the beginning of a file that identify what the file contains.

For example, executable files begin with the bytes (represented in hexadecimal) 4D followed by 5A. That’s upper case MZ, the initials of the engineer that defined the original .exe file format.

JPEG image files generally start with the bytes FF, D8, FF. That means if you get a .jpg file, but it starts with something other than those values, you know there’s a problem – the type of file content doesn’t match what the filename extension implies.

How does this help our .dat file scenario?

Fairly simply: by examining the first few bytes of the data in that file, you may be able to determine what kind of file it is. (I have to stress may as there’s also no requirement that any file follow any particular convention.)

How do I figure out what kind of file I have – without the file extension? has more, including a link to a tool to examine those values, as well as a link to a fairly extensive list of file signatures.

Including the one I created, that begins with my initials, LN. :-)

This is an update to an article originally posted : July 28, 2004


  1. Trish Clark

    Thank you for the reply – next question can I use Crystal reports to generate reports from a .dat file? We use Oracle, sqlbase, sqlserver, access but I have never had to assist a customer with a system using .dat files so I am treding on new ground! Thank you in advance.


  2. lynne

    i have a few dat files that i want to open. tried opening them with notepad but that didn’t work. they are conversations that were automatically saved when chatting on yahoo messenger. how can i open these up?

  3. Leo

    Normally a .dat file should only be opened using the application that created it, or an application that is compatible with it. In fact here’s a note from the Yahoo messenger help page:

    “Please Note: Archives (.dat files located on your system) are encrypted and can only be viewed by the Yahoo! ID that created them. These archives are stored locally on your system, so the conversations held on that specific computer are only accessible from that computer. Once archives are deleted, they cannot be retrieved.”

  4. Carol

    I’m experiencing the same problem. My lab ran a research using a software called MEL, which produced .dat files as outputs. Because it is a DOS-based program, do you happen to know how I could crack the data through DOS?

    Thanks for your help in advance (much appreciated!!)


  5. Leo

    Only with the application that created it, or an application that’s designed to be compatible with that original application.

  6. Reginald Shepherd

    On several occasions people have emailed me Microsoft Word attachments that somehow have turned into “.dat” files when they reached me through Outlook Express, and Windows XP tells me that they are “Unknown File Format” and cannot open them. (This also happened once when someone tried to send me a file in Rich Text Format.) Why do the files change like this, and is there a way either to convert them back into Word documents or to open the “.dat” files that these Word documents have somehow become?

  7. Leigha

    “””Many applications use the file extension “.dat” to indicate a file that contains data. The problem is that so many different programs use “.dat” that it really doesn’t tell you anything about the file, what it contains, or what program it belongs to.”””….. – is that the entire article…. that’s all it shows and it doesn’t exactly answer much

  8. That’s because there is no answer beyond that. Without knowing the application that created the file, there’s precious little that can be done. Even then, it typically requires that appliction, and perhaps even more. Yahoo Messenger’s .dat file is a good example, you need yahoo messenger AND you need to be logged in as the account that created it.

    Sorry I can’t tell you more, but there’s just not more to be told.

  9. No way to tell. My recommendation: back them up somewhere (maybe burn them to CD), and then delete them. If something breaks as a result, put them back.

  10. tracy

    When I send email with attachments, (MS Word, Excel etc.) they are automatically changed to .dat files when received and cannot be opened. How can I stop this?

  11. sam

    I`m running a cd burner with NERO, so far all the CD`s have come out as .dat files. How can I resolve this???

  12. You need to review the instructions that come with Nero for how to burn a cd. Make sure you’re burning the correct type: data (for a computer date disk) or audio (for a music CD you can play in CD players).

  13. Ray


    What software program can merge .dat files associated with jpg file? My friend oversea send me this file “photos in nanning 1.jpg 1_3.dat” and I don’t know how to open this.

  14. I found out that if the .DAT file is a video file, you can open it by renaming it to a .AVI file. Works for me, should work for you.

  15. jo Williams

    Whenever I send a file from my Word program, it has a .dat extension. Why is that and how can I fix it?

  16. Valdir Silva

    In what website can I find .dat files? The original driver from HP for installation my HP printer does’n have some .dat files.

  17. Raf

    Hey Leo, I received a .dat file through e-mail from a Immo-office, they told me it should be opened with Acrobat Reader, but it doesn’t seem to work. I replied them with telling them that I couldn’t open it and then they just sent it again.
    Could you please help me?
    Thank you very much

  18. Leo

    An acrobat reader file is a “.pdf” file … I’ve not heard of them using “.dat”. Either the sender is doing something wrong, or their mail program is. Ask them to either send their mail as “plain text” with the attachment, or if they have some other way to get you the file.

  19. Kid

    Can u please tell me how to open an yahoo archived message that says dat file. and the symbol is in media player type….i have tried everything and they all say it does not support that kind of file. Need help fast. Please do the needful action in this regard reply me sooon


  20. Leo

    You need to open a Yahoo Messenger .dat file using Yahoo Messenger logged into the same account that created the .dat file.

  21. Hub

    I have found a lot of .dat files can be opened with Notepad. Sometimes you get clear text, sometimes programming symbols.

  22. Snert

    .dat files contain data. I surely hope so. .exe are executable, .com files contain commands, .bat files are batch files, .hic have been drinking and so on. Somewhere there’s a table with all the extentions and what they stand for but I don’t remember where I put it.

  23. Joe

    I’ve discovered that doing a Google search on just about any (full) file name will turn up links to explain what they are for and the program that created it.

  24. John Servis

    @ Snert, you probably placed it in a .dat file somewheres!
    As for these .dat files, I have few of those, I create a folder in Documents, “DAT Files” & place them all in there, Keeps them out of the way. Never had any problems with doing so. After 6 months I’ll delete the oldest up until the last month or so if it crosses my mind. Not that big a deal.

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