So you just received an email with an attachment that someone sent you, but the extension on the file is .DAT. What exactly is a .DAT file and how to you open .DAT files? Those are two questions I’m going to try to answer as this is an issue that I’ve seen many times in my IT career!

The first thing to understand about .DAT files is that it indicates a file that has arbitrary data. That means it’s not associated with any one particular program or application. When you see a file with a .XLS extension, you know it’s referring to an Excel file, and so on. But with .DAT files, you have to figure out how to open it yourself and it may not be the same program each time.

file types

The best way to open a .DAT file is to use the program that created it. However, if you’re not sure, you can always try Notepad. Now when you open it in Notepad, you may be able to recognize some of the data, but the rest will most likely be junk spewed out by the program that created it.

You’ll normally only see this file format when receiving emails with attachments. Most common programs today do not generate .DAT files and only computer programmers use these files on any kind of regular basis.

So your first step would be to ask the person who sent you the email if they know which program was used to create the file. Now if they tell you that they actually sent a picture or a document and they are not sure why it’s a .DAT file, it could be that the file extension was changed in the process of having the email routed to you.

For some strange reason, some email programs automatically change the file extension on email attachments to .DAT. So if the person sent you a picture and now it’s a .DAT file, you first need to save it to your computer and then change the file extension to JPG or GIF or PNG or whatever you think it’s supposed to be. If they sent you a Word document, change it to .DOC, etc.

You can change the file extension on a file by first going to My Computer, clicking on Tools and then Folder Options.

tool folder options

Next click on the View tab and then scroll down to the option “Hide extensions for known file types” and UNCHECK it. In this way, we’ll now be able to see the file extension and change it to something else.

hide file extension

Now simply right-click on the .DAT file and change the file extension after the dot to the desired file format.

change file extension

You should also see the icon representing the file change to the appropriate program use to open that file type, i.e. Excel in the above picture.

So what do you do if you’re not sure which program it came from or what the original file extension was supposed to be? You can also try right-clicking on the file and then choosing Open With and try different programs. For example, I changed one of my Excel files to a .DAT extension and then right-clicked on it to open it with Excel and it worked!

open .dat file

Now I just chose Microsoft Excel from the list and Excel was able to read the file since the data was written by Excel, it just had a wrong file extension.

open file

You can also try other programs like Windows Media Player since it will open it if it happens to be a video, MP3, or similar media format. So hopefully you are now able to open your mysterious .DAT file using one of the above mentioned methods! If not, post a comment and I will try to help! Enjoy!