File extensions · Data and system files · Published
How to open DAT files on Windows 11
DAT is not a file format. The extension only says that a file holds data, and every program is free to use it for anything: email attachments, video, game saves, license keys, caches and databases. That is why Windows does not know what to open a DAT file with, and why a "DAT file opener" that promises to open all of them does not exist.
The useful question is where the file came from. Three cases cover almost everything people try to open, and each one has a different answer: an email attachment called winmail.dat, a video file from an old Video CD, and a data file that belongs to an installed program.
.dat at a glance
- Full name
- None: .dat simply means "data"
- Type
- Generic extension used by many unrelated programs
- MIME type
- Depends on the content; email apps usually send application/ms-tnef for winmail.dat
- Most common kinds
- winmail.dat and ATT00001.dat email attachments, Video CD AVSEQ01.DAT files, app and game data
- Opens in Windows 11 by default
- No; Windows has no program associated with .dat
winmail.dat or ATT00001.dat from an email
If the DAT file arrived as an email attachment, it is almost certainly a TNEF file. Microsoft explains that Outlook packs a message into Transport Neutral Encapsulation Format when it is sent as Rich Text. Mail apps that do not understand TNEF, which includes most apps outside Outlook and Exchange, show the plain text of the message plus an attachment named winmail.dat, or a generic name such as ATT00001.dat. The real attachments, such as PDFs, photos or Word documents, are packed inside it.
- Open the winmail.dat opener on this site and drop the DAT file into it. It reads the TNEF structure in your browser and lists the files inside.
- Download the attachments you need, one by one or all as a ZIP. If the message was sent as Rich Text, the formatted message body is offered as an .rtf file too.
- Ask the sender to switch Outlook to HTML format for future messages. Microsoft recommends HTML or plain text when the recipient does not use Outlook; Microsoft 365 admins can also turn off TNEF for external domains.
Microsoft also notes that a winmail.dat contains the path of the sender's Outlook data file and their sign-in name. That is one more reason for senders to avoid Rich Text when writing outside their company.
| Byte offset | Bytes | What it means |
|---|---|---|
| 0 | 78 9F 3E 22 | TNEF stream: a winmail.dat or ATT00001.dat file from Outlook or Exchange. |
| 0 and 8 | "RIFF" … "CDXA" | Video CD track, such as MPEGAV\AVSEQ01.DAT on a VCD. |
| any | readable text | Plain text, CSV or configuration data that a program saved with the .dat extension. |
AVSEQ01.DAT from a Video CD
Video CDs from the 1990s and 2000s store their films as DAT files in a folder called MPEGAV, named AVSEQ01.DAT, AVSEQ02.DAT and so on. Inside is ordinary MPEG-1 video wrapped in a RIFF header with the type CDXA.
VLC and MPC-HC play these files directly: open the DAT file, or open the disc itself. To keep the video, copy the DAT file to your PC first; then HandBrake or Shutter Encoder converts it to MP4, which plays on phones and TVs that do not know the Video CD layout.
DAT files that belong to a program
Most DAT files on a PC are not meant to be opened. Games, drivers and apps keep their settings, saves and caches in DAT files with a structure only that program understands. Examples are files in a game's install folder or in AppData. Opening them does no harm, but editing, renaming or deleting them can break the program or corrupt a save.
To see whether a program's DAT file is readable, open a copy in Notepad++. If you see words, numbers or CSV columns, it is text and you can read it. If you see random symbols, it is binary: open it in a hex editor such as HxD or ImHex and look at the first bytes, which often name the real format. The file type checker does that check for you in the browser and recognizes TNEF, RIFF, ZIP, PDF, SQLite and common media formats.
Can you convert a DAT file?
Only after you know what it is. A winmail.dat is not converted but unpacked: the files inside come out exactly as the sender attached them. A Video CD track can be converted to MP4 like any MPEG-1 video. A program's data file cannot be meaningfully converted at all, and changing its extension to .pdf, .docx or .mp4 does nothing except hide it from the program that owns it.
Be careful with websites that offer to convert any DAT file to PDF or Word online. They cannot know what is inside a generic DAT file, and uploading a winmail.dat sends the email's attachments and the sender's Outlook details to a stranger's server.
Free programs that open .dat files
-
Notepad++ Text and Code Editor
Shows whether a DAT file is text: settings, logs, CSV or exported data open as readable lines.
-
HxD Hex Editor
Free hex editor that shows the first bytes of any DAT file, which often reveal the real format.
-
ImHex Hex Editor
Open-source hex editor with pattern files for analyzing unknown binary data formats.
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VLC media player Media Player
Plays AVSEQ01.DAT and other Video CD tracks directly, without renaming.
-
MPC-HC Media Player
Light player that also opens MPEG-1 Video CD DAT files.
Free browser tools for .dat files
These tools run in your browser. Files are processed on your own computer and are not uploaded.
Frequently asked questions
Is it safe to open a winmail.dat file?
The winmail.dat itself is a container and does not run anything. The attachments inside deserve the same caution as any email attachment: if one is a program or a macro-enabled document from someone you do not know, do not open it.
Why do I get winmail.dat instead of my attachments?
The sender's Outlook sent the message in Rich Text format, which packs attachments into TNEF. Your mail app does not understand TNEF, so it shows the package instead. Extract the files with a TNEF reader, and ask the sender to use HTML format.
Can I just rename a DAT file to open it?
Renaming does not change what is inside, so it rarely helps. A winmail.dat stays a TNEF package under any name, and a program's data file stays unreadable to other apps. Find out the real format first, then open the file with a program for that format.
Can I delete DAT files on my PC?
Do not delete DAT files inside program, game or Windows folders: they are part of those programs. Use the program's own uninstaller or cleanup option instead. A winmail.dat you saved from an email can be deleted after you have extracted what you need.
Sources
- Microsoft Learn: How message format affects email messages (TNEF and Winmail.dat)
- Microsoft Learn: Prevent Winmail.dat attachments for external recipients
- Microsoft: [MS-OXTNEF] TNEF Data Algorithm
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