Convert a video to MP4
The video to MP4 tool takes a MOV, WebM, MKV, or AVI file and re-encodes it as an MP4 with H.264 video, the combination that every phone, browser, TV, and editing program can play. It runs ffmpeg compiled to WebAssembly inside the browser, with the same x264 encoder that desktop ffmpeg uses, and the file is never uploaded.
Typical jobs: a MOV screen recording from a Mac that a Windows colleague cannot open, a WebM download that an iPhone refuses, an MKV for a smart TV, or an old AVI that a modern editor will not import.
The CRF setting
There is one option, CRF, which stands for constant rate factor. It runs from 0 to 51 and defaults to 23.
CRF tells x264 how much quality to hold rather than what bitrate to hit, so the encoder spends bits where the picture needs them and saves them on static scenes. Lower numbers mean higher quality and bigger files. Each step of about 6 doubles or halves the file size.
23 is x264's own default and looks close to the source for most footage. Use 18 to 20 when the MP4 is a master you will edit again, or when the source is already very clean. Use 26 to 28 for a clip that will be sent over chat or embedded on a web page, where a smaller file matters more than the last bit of sharpness. Values above 30 start to show blocking in dark or fast-moving scenes. 0 is lossless and produces enormous files; there is almost never a reason to use it here.
What happens to resolution and audio
The tool does not change the frame size or frame rate. A 4K source stays 4K. If you want a smaller picture as well as a smaller file, use compress video, which has the same CRF option plus a maximum height.
The audio track is carried into the MP4 along with the video. If you only need the sound, video to MP3 extracts it on its own.
Dropping an MP4 on this tool
MP4 is also an accepted input. The tool re-encodes it with the CRF you choose and names the result with "-converted" so it does not overwrite the original. This can fix an MP4 that was made with an unusual codec and will not play on some device, because the output is always H.264. For plain shrinking, compress video is the better choice since it can also cap the height.
Output name and related tools
The output keeps the base name and swaps the extension, so "recording.mov" becomes "recording.mp4".
If you need the open WebM format instead, for example for a site that prefers VP9, use video to WebM. For a short silent loop that has to be an image file, use video to GIF.
Long files take a while because every frame is decoded and encoded again on your own processor. Keep the tab open until the download appears.
Frequently asked questions
Which formats can I convert to MP4?
MOV, WebM, MKV, and AVI. You can also drop an MP4 to re-encode it with a different CRF.
What CRF should I use?
The default of 23 looks close to the source for most footage. Use 18 to 20 for a file you will edit again, and 26 to 28 for something sent over chat.
Does the resolution change?
No. This tool keeps the frame size and frame rate. To scale the picture down as well, use the compress video tool.
Is the audio kept?
Yes. The audio track is carried into the MP4 along with the video.
Why does my output have '-converted' in the name?
That happens when the input is already an MP4. The suffix keeps the new file from colliding with the original.
Why would an MP4 not play somewhere?
MP4 is a container, and some MP4s hold video in codecs that older devices cannot decode. The output of this tool is always H.264, which plays everywhere.