SUB Converter
FileFlip converts SUB files into 6 other formats entirely inside your browser, using FFmpeg compiled to WebAssembly. The SUB file is never uploaded to a server: it is read off your disk, converted on your own machine, and saved back by the browser. There is no file-size limit and no account to create.
How do I convert a SUB file?#
- Drop your SUB file onto the converter at the top of this page, or click it to pick one from your computer.
- Choose the format you want out of it. FileFlip downloads the engine that conversion needs, once, and your browser caches it.
- Conversion starts the moment you pick the format, and the result is yours to download. The work happens in this tab, so nothing is uploaded and nothing is queued.
Is it safe to convert SUB files online?#
Yes, because nothing is uploaded. FileFlip converts SUB files inside the browser tab you already have open, using WebAssembly builds of the same engines a desktop converter would install. Your file is read from your own disk into the page's memory, and no request carrying it is ever made.
You do not have to take that on trust. Open your browser's network panel, run a conversion, and you will see the engine come down and your file go nowhere. There is no upload, no server-side copy, no retention window, and no account tied to what you converted.
How FileFlip works lists every engine, its version and its download size, and shows how to check for yourself that nothing leaves your machine.
What happens to a SUB file#
FileFlip converts SUB into 6 formats with FFmpeg, all of it inside the browser tab. Each conversion below names the engine it uses, what it downloads, and exactly what that conversion keeps or drops.
| Category | Subtitles |
|---|---|
| File extension | .sub |
| FileFlip support | Read only, so it is a source rather than a target |
| Conversions available | 6 |
| Engines | FFmpeg |
| Downloaded to your browser | 30.8 MB on the first conversion, then cached |
What is a SUB file?#
A MicroDVD .sub file is a plain-text subtitle track where each line names a start frame and an end frame instead of a timecode, in the form {start}{end}Text, so the same file displays differently depending on the frame rate of the video it's played against.
Recognize MicroDVD by its {start-frame}{end-frame}Text lines; it only makes sense alongside the exact video and frame rate it was timed against.
Convert MicroDVD to a time-based format like SRT as soon as the video might be re-encoded, retimed or played at a different frame rate, since that's what breaks its sync.
The SUB file format reference covers the specification, the programs that open one, and what each conversion keeps or drops.
SUB questions people ask#
Why does a .sub file mean two different things?
The .sub extension is shared by two unrelated formats: MicroDVD, a plain-text subtitle file timed by frame number, and VobSub, a bitmap subtitle format that always comes paired with an .idx index file and stores images of each caption rather than text. Check whether your .sub has a matching .idx file next to it; if it does, it's VobSub, not MicroDVD.
Why do my MicroDVD subtitles drift out of sync?
MicroDVD times every line by frame number rather than by clock time, so the file only stays in sync with the exact frame rate it was authored for. Re-encoding the video at a different frame rate, or opening the file against a different cut, shifts every line further out of sync the longer the video runs.
Can I convert a MicroDVD .sub file to another format?
A MicroDVD .sub file can be read and converted into a time-based format like SRT, which removes the frame-rate dependency entirely. It cannot be produced as an output here, since writing frame-accurate MicroDVD subtitles requires knowing the exact frame rate of a specific target video.