One MOV becomes one MP4 in the browser
The file this page accepts
The picker accepts .mov and video/quicktime. That is the QuickTime movie an iPhone, a Mac, or a camera often hands you, and it is the only input here. Add one file. A second clip waits until the first encode finishes and you start again. If a download is named .mov but the browser reports a different type, the check is the extension or video/quicktime, and other containers are turned away. Rename a file only when it really is a QuickTime movie. A mislabeled MP4 or a project archive will not become a cleaner result by wearing a new suffix.
FFmpeg in the tab, core 0.12.6
The converter loads FFmpeg.wasm in the browser. The core is version 0.12.6, fetched from jsDelivr, and then the encode runs on this device. Your movie is not uploaded to our storage for the conversion itself. The first moment can be quiet while the engine arrives. After that, progress follows the transcode. Keep the tab open. A laptop fan is normal. A phone can try a short clip and may still run out of memory, which is a limit of encoding video inside a browser, not a missing button.
The encode arguments, in plain language
The command writes H.264 video with libx264, uses the ultrafast preset, encodes audio as AAC, sets movflags to +faststart, and names the result output.mp4. Ultrafast favors finishing the job over squeezing the smallest possible file, so the MP4 may be larger than a slow desktop export and it should still play in the players that expect H.264 and AAC. Faststart places the movie header where playback can begin sooner when someone streams the file. The download you save is that MP4. There is no second output, no watermark step, and no extra audio track invented if the MOV had none.
How large a clip can be
On a phone the cap is 100 MB. On a desktop it is 500 MB. Those are the device limits for this tool. A clip over the cap for the device you are using is refused before the encode. Short screen recordings and voice notes usually sit under the phone cap. A long 4K master often belongs on a computer, both because of the 500 MB desktop ceiling and because the wasm encode needs memory. If the tab reloads, try a shorter segment or a desktop browser rather than the same phone again.
What you should check in the MP4
Play the download from the start and from a moment in the middle. Confirm the picture is the take you chose, the audio is present if the MOV had audio, and the file opens in the app that rejected the original. AAC and H.264 are widely understood, which is why this conversion exists, and an exotic MOV audio layout can still surprise you. If the sound is missing, the source track may have been a codec this ultrafast path did not carry across. A desktop editor is the next stop for that rare file. For a normal iPhone clip, the MP4 is usually the file you can drop into a slide, a message, or a site that asked for MP4.
Privacy of the conversion itself
Because FFmpeg runs in the browser, the conversion does not place the movie in our storage. Close the tab when you have the MP4, and keep your own copy. The jsDelivr fetch is the engine, core 0.12.6, not your footage. One file goes in, output.mp4 comes out, and you decide where that MP4 lives. Do not use this page as a host. It is a converter. If you later upload the MP4 to a class site or a client folder, that upload is a separate choice, under that site’s rules.