SUB to WebVTT Converter
FileFlip converts SUB to WebVTT entirely inside your browser, using FFmpeg compiled to WebAssembly. The file is never uploaded to a server: it is read off your disk, converted on your own machine, and saved back by the browser. That means no file-size limit, no queue and no account.
How do I convert a SUB file to WebVTT?#
- Drop your SUB file onto the converter at the top of this page, or click it to pick one from your computer. You can add as many as you like.
- Leave the target set to WebVTT and the conversion starts on its own. FileFlip downloads 30.8 MB of WebAssembly the first time and nothing after that, then runs FFmpeg on your own machine.
- Save the WebVTT file. Your browser writes it straight to your downloads folder, because there was never a server holding it.
Does converting SUB to WebVTT lose quality?#
No. Subtitle cues are text, and converting SUB to WebVTT rewrites the same timings and the same lines in the target's syntax. Anything WebVTT has no syntax for, such as positioning or styling, is dropped rather than degraded.
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 SUB to WebVTT actually does to the file#
Converting SUB to WebVTT runs on FFmpeg, downloads 30.8 MB of WebAssembly the first time and nothing after that, rewrites the cues as text in the target's syntax, keeps cue durations.
| Engine | FFmpeg |
|---|---|
| Conversion path | One step: SUB → WebVTT |
| Downloaded to your browser | 30.8 MB, once, then cached by your browser |
| Quality | Text preserved, unsupported styling dropped |
| You get back | One file |
| Where it runs | In a background worker, so the page stays responsive |
What survives the conversion
| What is in the file | This conversion | Why |
|---|---|---|
| Cue durations | Kept | Carried through into the converted file. |
Worth knowing before you start:
- MicroDVD times cues by frame number, so a frame rate has to be assumed and the result drifts if it is wrong
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.
What is a WebVTT file?#
A VTT file is a WebVTT text track: a WEBVTT header followed by timed cues, each with a start and end timecode and one or more lines of text, optionally carrying position, size and CSS-style settings that place and format the text on screen.
Use WebVTT for subtitles served to an HTML5 <video> element, where the browser reads the track natively and honors its cue positioning and styling.
Convert WebVTT to SRT when the destination is a player or device outside the browser that only expects plain, unpositioned captions.
SUB to WebVTT 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.
What is a WebVTT file used for?
A WebVTT file supplies the captions or subtitles for an HTML5 video, loaded through the <track> element so the browser displays them without any extra plugin. It's the format browsers expect natively, the way SRT is the format most desktop players expect.