How to Open DAT File (and winmail.dat) on Windows, Mac & Phone

What is a DAT file?

A DAT file is a generic data file that a program saves for its own use. .dat is short for "data" — and unlike an extension like .png or .pdf, it was never standardized by anyone. It's a generic default that developers have reached for since the earliest days of computing whenever an application needs to save its own private data to disk, with no expectation that any other program will ever need to read it. That means two .dat files from different programs can be built on completely unrelated formats internally — one might be a structured database, another a simple list of numbers, another compressed binary data specific to one game engine.

Because there's no fixed structure, the only reliable way to know what a given .dat file actually is is to look at its raw bytes and see what patterns and headers show up. That's exactly what the tool on this page does — it doesn't guess from the file name, it reads the content.

winmail.dat: the .dat file attached to an email

If a .dat file reached you as an email attachment, it's almost certainly winmail.dat — sometimes renamed ATT00001.dat or similar by the receiving mail app. It isn't a virus and it isn't a mistake on your end: the sender wrote the email in Microsoft Outlook using the Rich Text message format, and Outlook or their Exchange / Microsoft 365 server packed the formatting and every attachment into one file in Microsoft's TNEF format (Transport Neutral Encapsulation Format, MIME type application/ms-tnef). Outlook unpacks that file automatically; Gmail, Apple Mail, Thunderbird and most phone mail apps don't — so you see one mystery file where a PDF or a photo should be.

Open winmail.dat on any device (fastest)

1. Save the winmail.dat attachment from the email to your device (download it on a computer, or use Save to Files / Download on a phone).
2. Drop it into the tool at the top of this page, or tap Click to choose a file and pick it.
3. Open the Attachments tab and click Download next to each file. The original names are restored (the tool reads the long filename Outlook stores, not just the shortened 8.3 name), and the email's text appears under Message text when it's stored in the winmail.dat.

The file is decoded entirely in your browser — nothing is uploaded. If the tool reports a checksum mismatch or a truncated file, the winmail.dat was damaged in transit and the only real fix is asking the sender to resend it (see the next section).

Windows

If you have the Outlook desktop app, open the message there (for example, by forwarding it to an account you read in Outlook): Outlook decodes TNEF itself and shows the normal attachments. Without Outlook, use the tool above or a free desktop extractor such as Winmail Opener. Double-clicking winmail.dat in File Explorer only brings up the "Select an app" prompt — Windows has no built-in TNEF reader.

Mac

Apple Mail doesn't reliably unpack winmail.dat, so the attachment often shows up as-is. Use the tool above in Safari or Chrome, or install a dedicated Mac app such as TNEF's Enough, which opens winmail.dat files and lets you save the attachments to disk.

iPhone and iPad

In the Mail app, touch and hold the winmail.dat attachment (or open it and tap the Share button), choose Save to Files, then open this page in Safari, tap Click to choose a file, pick Browse and select the saved winmail.dat. Extracted attachments download to the Files app (Downloads folder). The App Store also has several winmail.dat viewer apps if you'd rather not use a browser.

Android

In Gmail or your mail app, tap the download icon on the winmail.dat attachment so it's saved to Downloads. Then open this page in Chrome, tap Click to choose a file and pick it from Downloads. The real attachments are saved to Downloads too, where your Files app can open them.

Gmail, Yahoo and other webmail

Gmail on the web doesn't decode TNEF, so the message simply shows a winmail.dat attachment. Hover over it, click the download arrow, and drop the downloaded file into the tool above. There's no setting on the recipient's side that changes this — the fix happens on the sender's side.

Linux and the command line

Most distributions package the tnef utility (sudo apt install tnef on Debian/Ubuntu); run tnef winmail.dat in a folder to extract the attachments into it. On any system with Python, pip install tnefparse and then tnefparse -a winmail.dat does the same.

How the sender can stop sending winmail.dat

The permanent fix is on the sender's side: their Outlook (or their organization's mail server) must stop sending Rich Text / TNEF to people outside the organization. Send them this section.

In classic Outlook for Windows (the usual culprit)

1. Go to File > Options > Mail.
2. Under Compose messages, set Compose messages in this format to HTML (or Plain Text) instead of Rich Text.
3. Further down, under Message format, set When sending messages in Rich Text format to Internet recipients to Convert to HTML format.
4. Click OK and resend the message. When replying to an older Rich Text email, switch that one message to HTML with Format Text > HTML in the message window, because replies keep the original format.

If only one contact keeps getting winmail.dat, Outlook may have a cached preference for that address: deleting the address from Outlook's autocomplete suggestions (hover over it while typing and click the ×) and retyping it usually clears it. The new Outlook for Windows and Outlook on the web only offer HTML and plain text (there's no Rich Text option), so when they're involved the cause is almost always the server setting below or a Rich Text reply chain.

For Exchange Online / Microsoft 365 admins

Admins can stop TNEF for every external domain with the remote-domain setting, in Exchange Online PowerShell:

Set-RemoteDomain -Identity Default -TNEFEnabled $false

The same option is in the Exchange admin center under Mail flow > Remote domains. Allow a little time for the change to take effect, then ask the sender to resend the original message — emails that were already sent stay as they are.

Formats that commonly hide behind a .dat extension

Windows Registry hives

Core Windows Registry files — NTUSER.DAT, UsrClass.dat, and the system hives — are real binary databases in Microsoft's "regf" format, holding the entire tree of registry keys and values for a user or the whole system. The tool above reads the real regf header, including a genuine checksum verification.

Java serialized objects

Java applications that use ObjectOutputStream to save an object to disk very often name the result something.dat — commonly a saved game, cached computation, or application state. The tool above decodes the real class names and field values from the stream.

Minecraft-style world data (NBT)

Minecraft's own level.dat (and several other save files) are gzip-compressed NBT (Named Binary Tag) data — a simple, typed, big-endian tree format. The tool above decompresses and walks that tree for real.

BitTorrent client resume data

Clients like uTorrent and qBittorrent store per-torrent download progress in a bencode-encoded resume.dat — the same simple encoding (integers, byte strings, lists, dictionaries) used inside .torrent files themselves. The tool above includes a real bencode decoder.

Plain text or a renamed known format

Plenty of .dat files are just plain text (simple config or save data), or another well-known format saved under a .dat name by mistake or convention — the tool above also checks dozens of common file signatures (images, archives, executables, documents) before giving up.

Open DAT video files from a VCD (AVSEQ01.DAT)

If your .dat is large (tens or hundreds of MB) and came off an old disc, it's probably a Video CD movie track. VCDs store their video as MPEG-1 in a folder called MPEGAV, in files named AVSEQ01.DAT, AVSEQ02.DAT and so on — one per track. Copied to a computer, these files usually start with a small RIFF "CDXA" wrapper in front of the MPEG data; the tool above recognizes both the wrapped and the plain MPEG form.

• Play it: open it in VLC media player (Windows, Mac, Linux, Android, iPhone) — Media > Open File, or drag the file onto the VLC window. VLC reads VCD .dat files directly.
• Make other players accept it: rename a copy to .mpg. That works when the file is plain MPEG; a CDXA-wrapped file may still need VLC or a conversion.
• Convert it to MP4: in VLC use Media > Convert / Save with an H.264 + MP3 (MP4) profile, or open the file in HandBrake. Both re-encode the MPEG-1 video into a modern MP4 that phones and TVs play.
• Copying from the disc fails or stutters? VCD video sectors have less error correction than data CDs, so a scratched disc can make Windows report read errors. VLC can usually play straight from the disc (Media > Open Disc, then choose SVCD/VCD) and skip over bad spots.

DVDs use a different container — for VIDEO_TS folders, see the VOB file guide.

How to open DAT files on Windows, Mac, Android and iPhone

Every platform has the same limitation — there's no "DAT app", because there's no DAT format. What changes is which tools you have for looking at the file. Identify it with the tool above first, then:

Open DAT files on Windows 11 and 10

Windows has no built-in program to open DAT files, so double-clicking an unknown .dat brings up the app picker. To look inside as text:

1. Right-click the file and choose Open with > Choose another app.
2. Pick Notepad (scroll down or use Choose an app on your PC if it isn't listed) and click Just once. Choose Always only if every .dat you open should go to Notepad, which is rarely true.
3. If you see readable lines, it's a text data or config file. If you see a wall of symbols, it's binary: close Notepad without saving (saving can corrupt binary data) and use the tool above to find out what it is.

Notepad++ (free) copes with very large text .dat files better than Notepad, and a hex-editor plugin is available for it. For a winmail.dat, skip all of this and use the steps in the winmail.dat section.

Open DAT files on Mac

Control-click the file and choose Open With > TextEdit to check for readable text. For a quick, reliable identification, open Terminal and run file ~/Downloads/something.dat — macOS's built-in file command reads the same kind of magic bytes this page's tool does and prints a description such as "ASCII text" or "MPEG sequence". Linux has the same command.

Open DAT files on Android

The Files app can't preview .dat files, and tapping one usually gives a "can't open file" message or a list of unrelated apps. Open this page in Chrome instead, tap Click to choose a file and pick the .dat from Downloads — the tool runs on the phone itself. A winmail.dat is unpacked right there; a VCD video plays in VLC for Android.

Open DAT files on iPhone and iPad

The Files app shows a .dat as a generic document with no preview. Use this page in Safari: tap Click to choose a file > Browse, then pick the file. Text content appears in the Text preview tab and email packages in Attachments.

Best DAT file viewers and openers

A "DAT file opener" can only show what's really inside, so pick the tool that matches the kind of DAT you have:

Some online DAT viewers upload your file to a server and limit the free size to around 1 MB, and "DAT viewer" apps in app stores mostly show the file as text or hex. No app can turn an unknown program's private .dat into something readable; only the program that made it can.

Converting DAT to text, PDF, Excel or MP4: what actually works

Online "DAT to PDF" or "DAT to Excel" converters can't know what's inside your file any more than its name does — they either assume it's text or fail. What works depends on what the file really is:

Can I delete a .dat file?

It depends entirely on where it is:

• winmail.dat in Downloads or an email: yes, once you've extracted the attachments it has no other use.
• NTUSER.DAT, UsrClass.dat or anything in C:\Windows: no. These are your Windows profile's and the system's registry; Windows keeps them locked while you're signed in, and removing one breaks the user profile.
• Inside a program's or game's folder (Program Files, AppData, a game's save folder): only if you accept the consequences. Deleting the .dat typically resets settings, wipes a save, or makes the app re-download or rebuild it. If you're troubleshooting a crash, move it to the desktop instead of deleting it, so you can put it back.
• A random .dat you don't recognize in Downloads: identify it first with the tool above; if it isn't something you need (a stray email part, an old download), it's safe to remove.

If Windows says the file is open in another program, the app that owns it is still running — close it (or sign out, for profile files) before moving the file. If you're cleaning up disk space, .dat files are rarely the big win; temporary files are — see the TMP file guide.

DAT file won't open? Common problems and fixes

Windows says it doesn't know how to open the file

This is expected — Windows has no default program for .dat, since the extension carries no format information on its own. Use the tool on this page to identify what's actually inside, then open it with a program that matches (or the specific application that created it).

The file opens as garbled text/symbols

That means it's binary, structured data, not plain text — a text editor can't decode it correctly. Check the Overview tab on this page's tool for what format it actually is.

The tool finds winmail.dat but no attachments

Outlook sends a winmail.dat with every Rich Text email, even when nothing was attached — it then only holds formatting and message properties. If you expected a file, it may have been sent as a link, or a server stripped it. If the tool reports a truncated file or checksum errors, the winmail.dat was damaged on the way; ask the sender to resend in HTML format.

"This .dat file might be corrupted"

If this page's tool detects a known format (a Registry hive's checksum, for instance) and reports a mismatch, the file genuinely is damaged or was only partially copied. Re-download or re-copy it from the original source.

Related file extensions

Sources and technical references

Because a .dat file can be almost anything, each format this page's tool actually detects has its own authoritative reference:

• [MS-OXTNEF]: Transport Neutral Encapsulation Format (TNEF) Data Algorithm — Microsoft — Microsoft's published specification of the winmail.dat format: the attribute stream, checksums and MAPI property encoding this page's decoder implements.
• [MS-OXRTFCP]: Rich Text Format (RTF) Compression Algorithm — Microsoft — The LZFu compression used for the message body inside winmail.dat, which the tool decompresses to show the email text.
• How message format affects email messages — Microsoft Learn — Microsoft's explanation of when Outlook sends TNEF and the Outlook settings that turn it off.
• Windows registry file format specification — msuhanov/regf (GitHub) — The most complete public reverse-engineered reference for the regf Windows Registry hive format, since Microsoft has never published an official spec.
• Java Object Serialization Specification, Chapter 6: Object Serialization Stream Protocol — Oracle — Oracle's own specification for the exact byte-level protocol used by Java's ObjectOutputStream, which this page's parser implements.
• Winmail Opener — Eolsoft — free Windows program for winmail.dat files.
• How do I open a .dat attachment on my Windows PC? — Microsoft Q&A — Microsoft's community answer for .dat email attachments.
• NBT format — Minecraft Wiki — A detailed community-maintained technical reference for Minecraft's Named Binary Tag format, used in files like level.dat.
• BEP 3 — The BitTorrent Protocol Specification (bencoding) — bittorrent.org — The official BitTorrent Enhancement Proposal defining the bencoding format used by resume.dat and .torrent files.

Frequently asked questions