Video and audio formats that work in your browser
A filename such as MP4 or WebM describes a container. The audio and video inside use codecs, and your browser must understand those codecs before CaptionKite can hear the file.
Formats that usually work best
CaptionKite accepts these common video and audio files plus other formats your browser can decode. Chrome or Edge is the best first choice when compatibility matters.
Why the extension does not tell the whole story
Two files can both end in .mp4 while using different internal audio codecs. One may open immediately and the other may fail. This does not necessarily mean the file is damaged; the browser may simply lack a decoder for that combination.
If a file plays in a desktop app but not in the browser, convert a copy to a common MP4 or extract its audio as WAV, M4A, or MP3. Transcribing audio alone is often faster and uses less memory.
Local file and direct URL limits
Local files are read incrementally, so a high-resolution video does not need to fit entirely in memory. The extracted audio and speech model still consume memory, so practical capacity depends on the device and browser. For long recordings, close other tabs or split the file. A fetched file is written into your browser's private storage as it downloads rather than held in memory, so a direct URL now has the same ceiling as a local file. Download time and your connection are the practical limit instead.
A web page URL is not always a media URL
A direct URL points to the video or audio file itself and allows your browser to fetch it. A normal YouTube watch page, a sign-in screen, or a protected streaming page does not. Authorized Vimeo or Zoom download links can work when they resolve to accessible media and permit cross-origin browser access.
CaptionKite does not bypass passwords, permissions, disabled downloads, or a provider's access controls. Protected playback will be handled by the planned browser extension only after the user opens and plays content they are allowed to view.
If your file will not load
- Try the latest Chrome or Edge.
- Confirm the file plays normally on your computer.
- Use a local file instead of a remote link.
- Convert a copy to MP4, MP3, WAV, or M4A.
- For a long video, extract or split the audio into shorter sections.
- Read the complete transcription troubleshooting guide.
Working out what you actually have
When a file will not load, the useful first question is not “what format is this?” but “what is inside it?” The extension answers neither.
An .mp4 is a container — a box. What matters is the codecs in the box, and two files with the same extension can hold entirely different things. One opens instantly; the other needs a decoder your browser does not have.
The quickest test costs nothing: drag the file into a new browser tab. If it plays there, the browser can decode it and the tool will work. If it does not, no setting in CaptionKite will change that, and converting the file is the fix rather than trying again.
For captions, audio alone is usually the better input
If all you want is the words, feeding in a video is doing unnecessary work. Extracting the audio first and transcribing that is faster, uses far less memory, and sidesteps most codec problems in one step — video codecs are where compatibility trouble lives, and audio ones are comparatively boring.
It also makes long recordings practical. A two-hour video is a demanding thing for a browser tab to hold; two hours of audio is much less so.
The trade is that you lose the picture, so if you need timings to match a specific edit you should extract from that exact export rather than an earlier one. Timings are only valid for the cut they were made from.
What actually happens when you paste a URL
A pasted link only works if it points at a media file rather than a page containing one. A YouTube watch page, a sign-in screen, or a share link is a web page — the video inside it is fetched separately, often with protection, and there is nothing at that address for a browser to decode.
Even a genuine media link can be refused. Your browser is fetching it directly from whoever hosts it, and if that host has not allowed cross-origin requests, the browser blocks it before CaptionKite sees anything. That is a decision by the host, not a limitation we can configure around.
When a link will not load, download the file yourself and select it locally. That always works, because at that point it is simply a file on your disk.
Converting a copy, and checking that it worked
If a file has to be converted, convert a copy and keep the original untouched until the result is verified. Conversion is lossy and irreversible, and an original is the only thing that lets you try again with different settings.
MP4 with H.264 video and AAC audio is the safest target — it is the combination essentially everything can play. For audio, MP3 or M4A. Avoid exotic settings; the point of converting is compatibility, and unusual choices reintroduce the problem you were solving.
Then check the converted file before doing anything else with it, and check the end rather than the beginning. Truncated conversions are common and they always look fine for the first few seconds.