What is the QT file format?
.qt is a legacy file extension for Apple QuickTime movies, carrying the QuickTime File Format (QTFF) - the same container used by the far more common .mov extension. The two extensions are structurally identical; .qt predates .mov as the original QuickTime extension introduced in 1991 and is now largely obsolete, found mainly in older archived video files.
A .qt file is an atom (box) tree. Older files begin directly with a moov or mdat atom; files produced by newer QuickTime tooling carry an ftyp atom at offset 4 with the major brand qt . The container typically holds an H.264/AVC or older Apple codec video track alongside an AAC or PCM audio track, though QTFF supports a wide range of codecs.
QTFF served as the architectural basis for both MPEG-4 and the ISO Base Media File Format, making .qt/.mov a direct ancestor of .mp4.
Do not install Apple QuickTime Player for Windows. Apple discontinued and stopped patching it in 2016, leaving unpatched security vulnerabilities. On Windows, use VLC to play .qt files. On macOS, QuickTime Player and most media applications handle .qt natively. For long-term storage, remuxing to .mp4 is usually lossless and produces a universally compatible file.
Security & safety
RISK: LOWA genuine .qt is media data, not executable, so it's safe to play. Cautions: do NOT install Apple's QuickTime for Windows (unpatched since 2016 - a known security risk); a file named 'clip.qt.exe' from an untrusted source could be malware in disguise (check the real extension); keep VLC updated since crafted media has occasionally exploited player bugs; and online .qt->MP4 converters upload your video to a third party - convert locally with VLC/HandBrake/ffmpeg for private footage.
Format details
in a nutshell- Quartz Composer composition (older) - Some Apple Quartz/QuickTime-adjacent data has appeared under .qt, but the video meaning dominates.
Programs that open QT files
Windows2 apps
VLC media player
Open-source Drag the .qt onto VLC - it bundles its own codecs and plays QuickTime files without QuickTime installed. The reliable choice on Windows. 
Media Player (Windows 11)
Built-in Double-click the .qt; the built-in Media Player plays common (H.264) QuickTime files. Use VLC for anything it refuses. 
macOS2 apps
QuickTime Player
Built-in Double-click the .qt to play in QuickTime Player - the native handler on macOS. 
Linux1 app
VLC media player
Open-source Open the .qt in VLC (or mpv) - plays QuickTime on any distro without extra codec packs. 
Android1 app
VLC for Android
Open-source Open the .qt in VLC; built-in Android players are unreliable with QuickTime. Technical details
deep spec| Container format | QuickTime File Format (QTFF) - structurally identical to .mov |
| Encoding | Binary |
| Byte order | Big-endian (QTFF/ISO Base Media convention) |
| File signature | ftyp atom at offset 4 with major brand qt (hex 71742020) in newer files; older files begin directly with a moov or mdat atom |
| Typical video codec | H.264/AVC or legacy Apple codecs (e.g. Sorenson Video, Cinepak) |
| Typical audio codec | AAC or PCM (Linear PCM uncompressed) |
| Atom tree structure | ftyp (brand declaration), moov (track metadata and table), mdat (encoded media data) |
| MIME type | video/quicktime |
| Relationship to .mov | Identical container; .qt is the older/rarer alias; .mov became the standard QuickTime extension |
| Relationship to MP4 | QTFF is the direct architectural predecessor of ISO Base Media File Format (MPEG-4 Part 12) and .mp4 |
| Audio sample rate | Typically 44.1 kHz or 48 kHz depending on the audio track codec |
| Windows support | QuickTime Player for Windows discontinued 2016 (security risk); use VLC as the safe alternative |
| Typical resolution | SD, HD, and 4K common; resolution limited by codec and encoding parameters, not the container |
| Released | 1991 (QuickTime 1.0); .qt was an early extension, later largely replaced by .mov |
| Latest version | QTFF as used by modern QuickTime/AVFoundation; .qt itself is a legacy alias |
| Specification | developer.apple.com |
QT conversions
Community Q&A
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