What is the SUB file format?
A .sub file is a plain-text file that contains video subtitles. Two dialects share the extension: MicroDVD, where each line encodes a start and end frame number in braces - e.g., {0}{50}Hello world - and SubViewer, which uses HH:MM:SS.hh timecodes instead. Both are human-readable and can be opened in any text editor.
The extension is also used by the binary VobSub format, but that variant always comes paired with a matching .idx index file and is not human-readable text.
Subtitle configuration
.sub subtitle files are simple and don't offer built-in styling or advanced text effects. However, many media players offer functions to apply effects to text, including:
- Font settings such as size and color,
- timestamp editing,
- subtitle offsets,
- subtitle reader functions.
.sub files are prepared for each video individually and are typically named the same as the related video but with a different file suffix, allowing players to auto-load them. Because MicroDVD timing is frame-based rather than time-based, a mismatch between the subtitle's assumed frame rate and the actual video frame rate will cause timing drift, which can be corrected with a subtitle editor such as Subtitle Edit. For modern projects, .srt or .ass are now more widely used alternatives.
Security & safety
RISK: LOWA text .sub is plain text and cannot execute code, so the file itself is safe to open. Two cautions: (1) keep your media player updated - years-old players had subtitle-parsing vulnerabilities (the 2017 'Subtitle Hell' research); (2) subtitles from sketchy sites can carry misleading filenames or spam URLs inside the text, so read rather than blindly trust. Save as UTF-8 if accented or non-Latin characters appear as garbage.
Format details
in a nutshell- VobSub subtitle (.sub + .idx) - Binary bitmap (image) subtitles ripped from DVD: a large .sub of pictures plus a text .idx of timecodes/offsets; not editable as text, OCR to .srt with Subtitle Edit.
- Subspace / SubStation niche subtitle scripts - A few other subtitle tools have emitted .sub variants; identify by the line format inside.
Programs that open SUB files
Windows4 apps
VLC media player
Open-source Play the video; if the .sub shares the video's name and folder it loads automatically, or use Subtitle > Add Subtitle File, or drag the .sub onto VLC. 
Subtitle Edit
Open-source Open the .sub to edit text/timing, fix MicroDVD frame-rate sync, and convert to .srt/.ass with a video and waveform preview. 
Notepad
Open-source Open a text .sub to read or quickly fix the lines; save as UTF-8 so accents display correctly. 
macOS3 apps
Jubler
Open-source Java subtitle editor: open the .sub to edit and convert between subtitle formats. 
Linux3 apps
Gnome Subtitles
Open-source Open the .sub to edit text/timing and convert to .srt with a synced video view. Technical details
deep spec| MIME type | text/plain (also text/x-microdvd for the MicroDVD dialect) |
| Text encoding | ANSI/Windows code page or UTF-8; legacy files are often single-byte encoded |
| MicroDVD timing method | Frame-number based: {startFrame}{endFrame}text on one line; timing depends on the video's frame rate |
| SubViewer timing method | Absolute timecodes in HH:MM:SS.hh,HH:MM:SS.hh format; timecode line followed by subtitle text on the next line |
| File signature / magic bytes | None - plain text; identified by line structure, not a binary header |
| Typical file size | 5 KB - 150 KB for a full movie's dialogue |
| SubViewer metadata header | Optional [INFORMATION] block at file start listing title, movie name, author, delay and CD track |
| Multi-line text within one subtitle | Pipe character | separates display lines in MicroDVD; [br] tag used in SubViewer |
| Frame-rate dependency | MicroDVD timing drifts if the player's assumed frame rate differs from the encoding frame rate; re-sync required |
| VobSub distinction | A .sub paired with a .idx file is an unrelated binary bitmap subtitle format, not this text-based format |
| Platform support | Windows, macOS, Linux, Android, iOS - any media player with subtitle support |
| Auto-load convention | Named identically to the video file (e.g. movie.avi + movie.sub) so compliant players load it automatically |
| Superseded by | .srt (SubRip) for basic use; .ass/.ssa for styled subtitles; .vtt (WebVTT) for web delivery |
| Released | late 1990s - early 2000s (DivX/MicroDVD era of PC video) |
| Latest version | N/A (stable de facto formats; no versioned spec) |
| Open standard | Yes · royalty-free |
| Specification | en.wikipedia.org |
SUB conversions
Community Q&A
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