What is the difference between MP4 and MOV?

Both are containers with a similar structure. MOV was made by Apple, while MP4 is an international standard with wider compatibility, so MP4 is usually better for sharing.

Is MKV better quality than MP4?

Not the box itself: quality is determined by the codec and bitrate. MKV is just more flexible for holding multiple audio tracks and subtitles.

Why does an MP4 file sometimes not play?

Most often because of the codec. If it contains HEVC or AV1, an older device may not support it. Re-encoding to H.264 usually solves it.

Is WebM better than MP4 for a website?

WebM can be smaller, but MP4 with H.264 works more widely. Many sites offer both and let the browser choose.

MP4 vs WebM vs MOV vs MKV: Video Formats Explained

A file extension like .mp4, .mov or .mkv does not tell you how the video is compressed or whether it will play on your device. To understand why one file plays and another does not, you need to separate the container from the codec. This guide explains it simply and suggests what to choose.

Container vs codec: what is what

A video file is like a box (the container) holding several parts: a video stream, an audio stream and sometimes subtitles. MP4, MOV, MKV and WebM are types of box. A codec is the method used to compress the picture or sound, for example H.264, H.265 (HEVC), VP9 or AV1 for video, and AAC, MP3 or Opus for audio.

That is why two MP4 files can behave differently: one with H.264 will open almost anywhere, while another with a rarer codec may cause problems. When a file will not play, the codec inside is more often to blame than the box.

MP4: the universal choice

MP4 with H.264 video and AAC audio is the most widely supported combination. It plays on iPhone, Android, Windows, Mac, TVs and every browser, and social networks accept it. If you are unsure what to pick, pick MP4 with H.264.

MP4 can also hold newer codecs such as H.265 or AV1, which give smaller files but do not play everywhere.

WebM, MOV and MKV

WebM was built for the web. It typically uses VP8 or VP9 video (or AV1) with Vorbis or Opus audio. It works well in Chrome, Firefox and Edge, and newer Safari versions support it too, but older devices and apps handle it less reliably.

MOV is Apple's QuickTime format. iPhone recordings are usually .mov with H.264 or HEVC. It works well on Apple devices, but on Windows or in some players it may need extra codecs. MKV (Matroska) is a flexible container that can hold multiple audio tracks and subtitles, so it is popular for movie collections, although phones and TVs accept it inconsistently.

Comparison table

Format	Typical codecs	Strength	Weakness
MP4	H.264, H.265, AV1 + AAC	Works almost everywhere	Codec inside still matters
WebM	VP8, VP9, AV1 + Opus	Web-oriented, open	Weaker on older devices
MOV	H.264, HEVC, ProRes	Great on Apple devices	Not universal
MKV	Almost anything	Multiple audio tracks, subtitles	Limited phone and TV support

Devices and social networks

iPhone and iPad reliably play MP4 and MOV with H.264 or HEVC. MKV and older AVI usually need converting. Android phones play MP4 and often WebM, while MKV support depends on the player app.

Social networks and video platforms (YouTube, Instagram, TikTok, Facebook) generally work best with MP4, H.264 video and AAC audio. Each platform changes its exact size and length limits, so check their help pages for current numbers.

When converting is worth it

A video will not open on a TV, phone or in email: convert to MP4 (H.264).
You need a small video for a website: use WebM or MP4.
An MKV or AVI movie will not play on a device: convert to MP4.
You want a short animation for a message or chat: turn MP4 into GIF, but only for a brief clip, because GIFs grow quickly.

What to expect from conversion

When only the container changes and the codecs are kept, quality stays the same and the process is quick. If the codec changes, the video is re-encoded and quality drops slightly; the process is also slower, especially for long recordings or weak devices.

Converting in the browser means the file is not uploaded to a server, but large videos can take a while to process. Keep the original until you are happy with the result.

What to do when a video will not play

If a file will not open, do not guess. Follow a simple sequence. First look at the extension, but remember it does not tell you which codec is inside. Then try another player, since sometimes the player is the problem. If it still fails, convert to MP4 with H.264 video and AAC audio, which works almost anywhere.

Typical situations: an iPhone MOV with HEVC will not open on an old Windows PC, an MKV movie is not shown on a TV via USB, or a downloaded WebM will not open in a video editor. In all three cases MP4 (H.264) is the most reliable fix.

1. Check that the file is not corrupted or only partly downloaded.
2. Try a different player.
3. If needed, convert to MP4 (H.264 + AAC).
4. Convert large videos patiently, since it can take a while.

Decision checklist and size tips

A video's size is determined not by the container but by its length, resolution and bitrate. For example, a one-minute 1080p clip in H.264 is often a few tens of megabytes, while the same clip in 4K is considerably more. To reduce size, lower the resolution or bitrate rather than only changing the format.

GIF is not a video format in the usual sense: it has no sound and is very inefficient, so even a few seconds can take several megabytes. Use it only for short animations.

Sharing and social networks: MP4 (H.264 + AAC).
Archiving from Apple devices: MOV or MP4 without converting.
A movie collection with several audio tracks: MKV.
A website: MP4, plus WebM if you want a smaller file too.
A short animation without sound: GIF or WebM.