Containers and codecs: what “converting” really means
A video file has two layers, and knowing the difference saves a lot of time.
- The container is the box: MP4, MOV, MKV or WebM. It decides the file extension and how the parts are packed together.
- The codecs are what is inside the box: the compressed picture (H.264, HEVC, VP9, AV1) and the compressed sound (AAC, Opus, MP3).
Most “MOV to MP4” or “MKV to MP4” jobs only need a new box. The H.264 video and AAC audio inside are already fine for MP4, so this tool copies them across untouched. That is called remuxing. It is fast (often a few seconds for a long video), and the result is identical in quality to the original.
When the codec does not fit the target, the stream has to be re-encoded: decoded to raw pictures and compressed again. That takes longer and loses a little quality, which is why the tool only re-encodes what it must.
What happens to each common file
| Your file | Convert to MP4 | Convert to WebM |
|---|---|---|
| MOV from an iPhone or camera (H.264 + AAC) | Copied, lossless | Video and audio re-encoded |
| MOV from a recent iPhone (HEVC + AAC) | HEVC re-encoded to H.264* | Re-encoded |
| MKV (H.264 + AAC) | Copied, lossless | Re-encoded |
| MKV (H.264 + AC-3 or DTS) | Video copied; audio only if your browser can decode it | Re-encoded |
| WebM screen recording (VP9 + Opus) | Re-encoded to H.264 + AAC* | Copied, lossless |
| MP4 (H.264 + AAC) | Copied | Re-encoded to VP9 + Opus |
* With Maximum compatibility switched off, HEVC, VP9 and AV1 video and Opus audio are copied into MP4 as they are. That is lossless and fast, but some players, older phones and video editors will not open the file.
Before you convert, the tool reads the file and tells you which path it will take.
Worked example: a 4-minute iPhone video
Say you have a 4-minute MOV from an older iPhone: 1080p H.264 at about 16 Mbps with AAC audio, roughly 480 MB.
- To MP4: both streams are copied. The new file is also about 480 MB and is ready in seconds, because nothing is decoded.
- To WebM: every frame is decoded and encoded again as VP9. At 30 frames per second that is 7,200 frames to process. On a laptop with a hardware decoder this typically takes a few minutes; on a phone it can take longer. The size depends on the quality setting.
If your goal is a smaller file rather than a different format, use the Video Compressor instead. It lets you choose a target size and lower the resolution.
Which format should you pick?
- MP4 is the safe choice for sharing, phones, TVs, PowerPoint, and editors like CapCut, Premiere and DaVinci Resolve.
- WebM is open and royalty-free, plays in every modern browser and is smaller for the same quality when it holds VP9. It suits websites and web apps.
Tips and edge cases
- Quality only matters when re-encoding. Copied streams keep their exact original quality whatever you choose.
- Surround sound. Audio codecs such as AC-3 (Dolby Digital) and DTS, common in MKV films, cannot be decoded by most browsers. If the browser cannot decode the audio, the video is still converted and you are told the sound was left out.
- HEVC needs support. Decoding HEVC depends on your browser and device hardware. Safari, and Chrome or Edge on most recent computers, can do it.
- Very large files are held in memory while they are written. Over about 2 GB, phones and older laptops may run out of memory. Cut long recordings into parts with Trim Video first.
- Need just the sound? MP4 to MP3 pulls the audio out of any video. For sound files, use the Audio Converter.
- Batches are converted one file at a time, and you can download everything together as a ZIP.
Privacy and browser support
Everything happens on your device using the browser’s built-in WebCodecs engine. Nothing is uploaded, so the tool is safe for private or work videos. It works in Chrome, Edge and Opera 94 or newer, Firefox 130 or newer on a computer, and Safari 16.4 or newer. In older browsers you will see a message explaining what to use instead.