Format
MOV, the QuickTime container.
A MOV is a box that holds video and sound. What matters is what is inside it, because that decides how a MOV behaves everywhere else.
Flex Convert reads MOV and writes MP4. It never writes MOV.
Facts
The record.
Extension
.mov.
What it is
The QuickTime file format, introduced by Apple in 1991. The MP4 format grew out of it, which is why they are close relatives.
What it holds
H.264, HEVC and ProRes video among many others, with AAC or uncompressed sound, plus timecode and extra tracks.
Who writes it
iPhones and iPads, many cameras and Apple's editing software.
Who reads it
Apple software natively. Windows needs a player or the right codec, and TVs and web forms are hit and miss.
In Flex Convert
Read when the browser can decode what is inside. ProRes is refused with a message.
Measured examples
What we measured.
Two generated 20-second 1080p clips, one with H.264 and one with HEVC, both in MOV files, converted to MP4.
| Test file | Size | Change | Time | Result |
|---|---|---|---|---|
| 20 s clip, 1080p, H.264Chrome | 30 MB → 30 MB | Same size | 190 ms | Video stream identical · Copied, no quality lost |
| 20 s clip, 1080p, HEVCChrome | 25 MB → 32 MB | 25 percent larger | 3.3 s | Video converted |
Measured on November 7, 2026 in Google Chrome 157, on Apple M1 Pro. Times run from pressing Convert to the result being ready, with the page already loaded. Likeness is PSNR against the original: higher is closer, and above 40 dB the difference is hard to see. Every test file was generated for these measurements, none is anyone else's work, and each is described on the About page.
The H.264 clip was copied into an MP4 in 193 milliseconds, and its video stream came out identical. The HEVC clip was converted to H.264: 3.3 seconds, and the file grew from 25 MB to 32 MB.
Good to know
What is inside the box.
A MOV is only as awkward as the codec it holds.
A container is not a codec
A MOV can hold any of a dozen video formats, which is why two MOV files can behave so differently. With H.264 and AAC inside, moving the streams into an MP4 takes a fraction of a second and loses nothing. With HEVC or ProRes inside, it is a different job.
The iPhone case
An iPhone set to High Efficiency records HEVC in a MOV. Windows programs, TVs and web forms often refuse it. Converting makes H.264, which they all take, and the file gets bigger. Settings, Camera, Formats, Most Compatible makes new recordings H.264 from the start.
ProRes
ProRes is a professional editing codec with very large files. A browser cannot decode it, so Flex Convert refuses ProRes files with a message instead of writing nothing.
Choosing
When to pick it, and when not to.
A format is a trade. These are the trades this one makes.
Keep MOV when
you edit in Final Cut, Premiere or DaVinci Resolve. They open it directly and converting buys nothing.
Convert to MP4 when
the video goes to a Windows PC, a TV, a website or someone who might not have QuickTime.
Compress as well when
the result has to fit an email or a chat. The compress page sets a size.