Where MOV files come from and why they cause trouble
MOV is Apple's QuickTime container, introduced in 1991 and still the default for iPhone cameras, Mac screen recordings, Final Cut exports and many professional cameras. MP4 was later standardised from the same file structure, which is why the two look alike to the operating system and are often confused.
The difference is in what each container is allowed to hold and what other software expects. A MOV from a recent iPhone usually carries HEVC (H.265) video, a Mac screen recording may hold H.264 at an unusual frame rate, and a camera or editing export can contain ProRes, a large editing codec that almost nothing outside Apple software plays. Windows Media Player, many smart TVs, older Android phones, web upload forms and some messengers either reject the .mov extension outright or play the sound with a black picture.
Typical reasons to convert:
- Uploading to a website, LMS or form that accepts only
.mp4. - Sending iPhone footage to someone on Windows or Android.
- Playing a clip on a TV or projector from a USB stick.
- Dropping footage into an editor or slideshow tool that chokes on HEVC or ProRes.
What the conversion actually does
Whatever is inside the MOV, the output video track is H.264 and the sound is AAC. H.264 is the codec with the widest hardware support: practically every phone, browser, TV and editor from the last fifteen years decodes it.
When the MOV already holds H.264, as screen recordings and older iPhone clips usually do, the browser copies the video track into the MP4 untouched, which takes seconds and loses nothing; AAC sound is copied the same way. HEVC, ProRes and everything else is re-encoded to H.264 at CRF 23. That is slower than a container swap, but a plain rewrap would keep HEVC inside the MP4, and that file would still fail on the same devices that refused the MOV.
CRF 23 is a constant-quality setting: the encoder spends more bits on busy scenes and fewer on static ones, aiming for the same visual quality throughout. At CRF 23 the difference from the source is hard to see in normal viewing. Resolution and frame rate are kept as they are, so a 4K 60 fps clip stays 4K 60 fps.
In the browser
By default FFmpeg runs as WebAssembly inside the page and the file never leaves your device. When re-encoding, the encoder uses the ultrafast preset so a long clip finishes in reasonable time on one CPU core. The trade-off is efficiency: the output is often larger than a carefully encoded file of the same quality. Audio that is not already AAC is encoded at 128 kbit/s.
On the server
The page also offers server processing with native FFmpeg. It always re-encodes, with the slower medium preset at the same CRF, which typically gives a noticeably smaller file for the same quality, and it writes the MP4 index at the start so playback can begin before the whole file is downloaded. Free use is one server job a day for files up to 500 MB; Premium removes the daily limit and accepts up to 4 GB. Job files are removed automatically within about an hour.
Why the MP4 can be larger than the MOV
People are often surprised that a 300 MB iPhone clip becomes a 450 MB MP4. This is expected in a few cases:
- HEVC sources. HEVC needs roughly 30 to 50% fewer bits than H.264 for the same picture. Converting to the more compatible codec costs space.
- Fast browser preset.
ultrafastskips many compression tricks. The server option usually closes much of the gap. - Very static footage. Screen recordings with little motion are already tiny; any re-encode has fixed overhead.
If size matters more than perfect quality, run the result through compress video afterwards, or cut unneeded parts first with trim video.
Practical notes
- HDR footage. iPhones record Dolby Vision HDR by default. The converter does not tone-map HDR, so after conversion colours can look flatter or slightly washed out on some screens. For sharing on ordinary displays this is rarely a problem; for archiving, keep the original MOV.
- Orientation. Portrait clips stay portrait. Phones store a rotation flag rather than rotated pixels, and the converter applies it, so the MP4 shows the same way up as the original did in your player.
- Memory. Browser conversion holds the input and output in memory at the same time. A 1.5 GB file on a phone with 4 GB of RAM may fail; a laptop or the server option handles it more reliably.
- Only video and audio. Chapter markers, timecode tracks and extra audio tracks from professional cameras are not carried over; only the main video and audio streams are converted.
If you only need the soundtrack, extracting it with MP4 to MP3 after conversion gives a small audio file you can play anywhere.