MKV, AVI, WMV, FLV and other video formats: what they are and what converting to MP4 keeps
2026-10-06
MP4 won. Phones, browsers, TVs and editing apps all expect it, and files in other containers are increasingly likely to be refused. Here is where the other formats come from and what happens when you convert them.
| Format | Where it comes from | Typical problem |
|---|---|---|
| MKV | Downloads, screen recorders such as OBS, disc backups | iPhones, many TVs and most upload forms will not open it |
| AVI | Windows software and cameras from the 1990s and 2000s | Large files; unsupported on phones and the web |
| WMV | Windows Media Player, old screen-capture tools | Does not play on Apple devices or in browsers |
| FLV | Flash-era websites and old downloaders | Flash is gone; almost nothing plays it |
| 3GP | Feature phones before smartphones | Low resolution; modern apps often reject it |
| MPG | DVDs, camcorders, TV capture cards | Plays, but files are large for their quality |
| TS | Digital TV recordings, streaming captures | Meant for broadcast, not for sharing |
| M4V | iTunes purchases and exports | Nearly identical to MP4; protected files cannot be converted |
What converting to MP4 does here
The video is re-encoded as H.264 and the audio as AAC, the combination that plays almost everywhere. Converters: MKV, AVI, WMV, FLV, 3GP, MPG, TS and M4V to MP4.
What is kept and what is dropped
| In the MP4 | |
|---|---|
| Picture | Kept, re-encoded at the quality you choose |
| Resolution and frame rate | Kept. An odd width or height is trimmed by one pixel, which H.264 requires |
| First or default audio track | Kept, re-encoded as AAC |
| Additional audio tracks | Dropped. Only one track is carried over |
| Subtitle tracks | Dropped |
| Chapters and attachments such as fonts | Do not rely on them surviving |
This matters most for MKV, which is often used precisely because it can hold several languages and subtitle tracks. If you need a specific audio language or the subtitles, a desktop tool that lets you pick tracks is the right choice.
A faster route for some MKV files
Many MKV files already contain H.264 video. A desktop program can move those streams into an MP4 container without re-encoding, which takes seconds and loses no quality. This site always re-encodes, which is simpler and works for every input, but takes about as long as the video itself and costs a little quality. For one or two short files that is fine. For a folder of full-length films, use a desktop tool.
How long it takes
Expect roughly one to two minutes of processing per minute of 1080p video on a laptop. Older formats such as 3GP and low-resolution AVI convert much faster because there are far fewer pixels. Keep the tab open and in front while it works.
Do not expect it to look better
Converting a 320 × 240 clip from an old phone to MP4 makes it playable, not sharper. The resolution and the compression damage in the original are carried over as they are. Choose High for old footage so that no further detail is lost; the settings are listed in the quality guide.
Files that will not convert
- Protected files. Purchased or rented films in M4V and WMV with DRM are encrypted and cannot be read.
- Damaged or partial downloads. If the file stops playing halfway in a media player, it will fail here too.
- Files over 2 GB. The whole file has to fit in the browser tab's memory.