Convert MOV, MKV, AVI, WebM or M4V to MP4 or WebM. Where possible only the container changes, with no loss of quality; otherwise your browser re-encodes. You see beforehand what will happen.
Good to know
A video file is a box (the container: MOV, MKV, AVI, MP4) holding the picture and the sound, each in its own encoding. If your MOV or MKV already contains H.264 or HEVC video with AAC audio, nothing needs to be re-encoded: the tool puts the same data into an MP4. That is the very same picture, bit for bit, with no loss of quality, and it takes seconds, even for a large file. If the encoding does not fit the target, such as VP9 in an MP4 that has to play everywhere or H.264 in a WebM, your browser re-encodes it. Before you start, you see for each track what is going to happen.
MOV and M4V from a phone or camera, MKV, WebM and AVI. The video may be H.264, HEVC, VP8, VP9, AV1 or Motion JPEG (older cameras); the audio AAC, MP3, AC-3, Opus, Vorbis, FLAC or uncompressed (PCM). The tool tells you honestly what your browser cannot decode: Xvid and DivX (MPEG-4 Part 2) from old AVIs, MPEG-2, Windows Media and DTS audio do not work in a browser. Re-encoding HEVC only works if your computer can play HEVC; converting to MP4 without re-encoding always works. If an MKV has several audio tracks, you pick one. Subtitles are not carried over.
The conversion happens in your browser; the video is not sent anywhere. A file of several gigabytes is read in pieces, so it does not have to fit in memory. Re-encoding takes time: expect roughly the running time, often less, and you can keep working in another tab meanwhile. You get an MP4 with its index at the front, so it can start playing on a website straight away. If the result is large, send it as a download link.