MP4 vs WebM vs MOV: which video format to use

The file extension on a video tells you which box it is in, not what is inside the box. That one distinction explains almost every “won’t play” and “won’t upload” problem, and it decides whether a conversion will take seconds and lose nothing or take minutes and cost quality. Here is how the three everyday formats relate, what plays where, and when converting is worth it.

Containers and codecs

A video file has two layers. The container is the outer structure: it holds one or more streams (video, audio, sometimes subtitles), stores their timing, and keeps an index so a player can jump to any point. MP4, MOV, WebM, MKV and AVI are containers. The codec is the method used to compress the pixels inside a stream: H.264 (also called AVC), HEVC (H.265), VP8, VP9 and AV1 for video; AAC, Opus, MP3, Vorbis and uncompressed PCM for audio.

A player has to understand both layers. It must be able to open the box and it must have a decoder for what it finds inside. Most compatibility problems are codec problems wearing a container’s name: an “MP4 that won’t play” is almost always an MP4 whose video is HEVC or AV1 on a device that only decodes H.264.

This also explains the two kinds of conversion. Changing the container while keeping the streams is called remuxing: the compressed data is copied into a new box, byte for byte, in seconds, with no quality loss. Changing the codec is transcoding: every frame is decoded and encoded again, which takes real time and is lossy by nature. A good converter always remuxes when it can and transcodes only when it must.

MP4, WebM and MOV in turn

MP4 (MPEG-4 Part 14) is the general-purpose box. It most often contains H.264 video and AAC audio, the pair that every phone, computer, browser, TV and chat app made in the last fifteen years can play. It can also hold HEVC and AV1, which is where compatibility starts to fray. A well-made MP4 puts its index at the front (“fast start”) so it can begin playing before it has fully downloaded.

MOV is Apple’s QuickTime container and the direct ancestor of MP4; the two share the same ISO base structure. Cameras and iPhones write MOV. An iPhone MOV from 2017 onwards usually carries HEVC video (Apple’s “High Efficiency” setting), which is where Windows users run into trouble, as the HEVC guide explains. A MOV with H.264 and AAC inside is an MP4 in all but name and can be repackaged in seconds.

WebM is a restricted form of Matroska created for the web. It allows only royalty-free codecs: VP8, VP9 or AV1 video and Vorbis or Opus audio. Browsers write WebM when you record your screen or camera with a web app, and those recordings have a quirk: because the browser does not know how long the recording will be while it is writing, the file often has no duration and no seek index, so some players show 0:00 and refuse to scrub. Putting a WebM into an MP4 almost always means transcoding, because its codecs do not belong in an everyday MP4.

What plays where

The table is about typical defaults. A Windows PC with the paid HEVC extension installed, or a Linux machine with extra codecs, will play more than this; a smart TV from 2016 will play less.

Read the table by codec and the pattern is clear: H.264 plays everywhere, HEVC plays on Apple devices and recent phones, VP9 plays in browsers and on Android, and anything in MKV or AVI depends on the player rather than the device.

When converting actually helps

Converting helps when the receiving side cannot play what you have, and it helps most when it can be done by remuxing. Three common cases:

• A camera MOV with H.264 and AAC that an Android phone or a website upload form rejects by extension. Remux to MP4: seconds, no loss, identical picture.
• A screen recording in WebM or MKV that your video editor or a chat app will not open. This needs a transcode to H.264 and AAC, plus a proper duration and index. It takes a while, but it is the only route to a file that opens everywhere.
• An iPhone HEVC MOV for a Windows PC or an Android phone that will not decode HEVC. Transcode the video to H.264; the AAC audio can be copied unchanged.

Converting does not help when the problem is size (use the compressor), when the file is protected by DRM (nothing will), or when the codec is one no browser engine decodes at all, such as Xvid, DivX, MPEG-2 or Windows Media. For those, a desktop converter does the one-time job and ClipHeron can take it from there.

The Convert to MP4 tool shows which path it will take on the file card before you click: “just repackaging” (remux), “audio converted” (video copied, only the audio re-encoded) or “video converted to H.264” (a real transcode). Reading that line tells you how long it will take and whether any quality will change.

Which format to use

Sending to people: MP4 with H.264 and AAC, every time. It is the format that needs no explanation on the other end.

Keeping originals: whatever the camera wrote. Do not convert for storage; conversion only loses information, and the HEVC originals from a phone are already smaller than an H.264 copy of the same quality.

Publishing on a website you control: MP4 as the baseline, optionally with a WebM (VP9 or AV1) version for browsers that take it, because the files are smaller.

Editing: whatever your editor prefers, which is often MP4 or MOV with H.264, or an intra-frame format such as ProRes for heavy editing. If an editor refuses a MKV or WebM screen recording, converting it to MP4 first solves it.

Just the sound: M4A, which is an MP4 container holding only the audio track. The extract audio tool makes one by copying the AAC frames out, with no re-encoding at all.

Convert a video to MP4

This guide explains how things work in general terms. Platform limits and codec support change; the figures were checked on the date shown above. Spotted something out of date? Email errors@clipheron.com and we will check it against the source.