How to convert MKV, AVI, MOV and other videos to MP4

You download a film as an MKV, record a clip as MOV, or find an old AVI on a hard drive, and your phone, TV or editing software refuses to play it. MP4 is the format almost every device understands, so converting to it is the usual fix. Understanding two ideas makes the whole process quicker and keeps quality high.

Container versus codec

A video file is like a box with things inside:

The container is the box: MP4, MKV, AVI, MOV and so on. It holds the video, the audio, subtitles and some information about them.
The codec is how the picture or sound inside is compressed: H.264, H.265 (HEVC), VP9, AAC, MP3 and many others.

A device that fails to play a file may be rejecting either the box or what is inside it. This is why the same “.mkv” plays on one TV and not another.

Remux or re-encode?

That distinction gives two ways of converting:

Remux (fast, lossless). If the video and audio inside are already something MP4 supports well, such as H.264 video and AAC audio, the converter simply moves them into an MP4 box. It takes seconds, even for a large file, and the quality is exactly the same because nothing is re-compressed.
Re-encode (slower, adjustable). If the contents are not MP4-friendly, or you want a smaller file or a different size, the video is decompressed and compressed again. This takes longer and loses a little quality each time, so you control it with quality, bitrate and resolution settings.

The converters on this site look inside your file first and choose for you. If a remux will work, “Fast remux” is selected and the page says so. You can still switch to re-encode.

Convert a video to MP4

Open the page for your format, for example MKV to MP4, AVI to MP4 or MOV to MP4. There are pages for MKV, AVI, MOV, WebM, FLV, TS, WMV, MPEG, 3GP, M4V, MOD, VOB, MTS, OGV, RM, MXF, DV, F4V and SWF.
Choose your file. Large files are fine because the video is read straight from your disk.
Wait for it to be analysed. The page lists the video and audio streams and recommends a setting.
Click convert, then download your MP4.

Not sure of the format, or you want a different target such as WebM or MKV? The Video Format Converter handles twelve output formats.

Controlling quality and size

When re-encoding, a few settings decide the result:

Setting	What it does
Quality (CRF)	Lower numbers mean better quality and larger files. Around 18 to 23 is a good range for H.264.
Bitrate	Sets the size directly, in kilobits per second. Use it when you need a specific file size.
Resolution	Halving the width and height cuts the file size by a large margin. 720p is plenty for a phone screen.
Frame rate	Usually keep the original. Lowering it saves a little space but makes motion less smooth.

If your goal is simply a smaller file to send, use the MP4 compressor, which offers Light, Medium and Strong levels or a target size such as “under 25 MB for WhatsApp or email”.

Honest limits of converting in a browser

Speed. A browser uses one processor thread for re-encoding, so a long HD movie can take several times its running length. Use the Start and End boxes to convert only a part, or keep to remux whenever possible.
Memory. The finished MP4 has to fit in the browser’s memory, which is roughly 1 to 2 GB. Very long, high-bitrate files may be too large.
Some codecs cannot be encoded in this setting, such as H.265 and VP9. They can be copied into an MP4 in some cases but not re-encoded, and AV1 video cannot be decoded at all. The page tells you when that applies.
Copy-protected files such as commercial DVDs and iTunes purchases are encrypted and cannot be converted by any tool.

Other small jobs

Need just the sound? Video to MP3 extracts audio. Want a short looping clip? MP4 to GIF turns a moment into an animated GIF.

Your video is not uploaded to a server. The conversion runs on your own device. To see how that works, read Are online file converters safe?.