MP4, MKV, MOV, AVI — What's the Actual Difference?

Convert an MKV to MP4 and it finishes almost before you've let go of the mouse. Convert an old AVI and you might be waiting minutes. Same source video, same destination format, wildly different experience.

The reason is the single most useful thing to understand about video files, and almost nobody explains it.

These aren't video formats

MP4, MKV, MOV and AVI are containers. Think of each one as a box. Inside the box is a video track, one or more audio tracks, and possibly subtitles, chapter markers and metadata.

The video track itself is encoded with something else entirely — a codec. H.264 is the common one. H.265 (HEVC) and AV1 are newer. The codec is what actually compresses the picture; the container just holds it, labels it, and keeps the audio in sync with it.

Which means the box and its contents are independent. An MKV and an MP4 very often contain exactly the same video. Not similar video — the identical encoded bytes, in a differently-shaped wrapper.

Once you see it that way, "convert MKV to MP4" turns out to be a slightly misleading phrase. Most of the time you aren't converting the video at all.

The four boxes

MP4 is the one everything agreed on. It isn't the most capable container and it wasn't the first, but it's supported by effectively every phone, TV, browser, console and editor made in the last fifteen years. When you have no particular reason to choose something else, this is the answer.

MKV (Matroska) is the flexible one. Open, unencumbered, and able to hold almost anything you throw at it — several audio tracks in different languages, multiple subtitle tracks, chapters, attachments. It's why archived films and TV are so often MKV. The catch is hardware: plenty of smart TVs and phones simply don't support it.

MOV is Apple's, and it's MP4's close relative — the two share a common ancestor and usually the same codecs inside. It's excellent while you're editing on a Mac and awkward the moment the file leaves that world.

AVI dates to 1992, and it shows. It predates most of what modern video expects, handles modern codecs awkwardly where it handles them at all, and carries more overhead than the others. You mostly meet it in old camcorder footage and old downloads — which is exactly when you need to get out of it.

Remuxing vs re-encoding — the bit that matters

There are two completely different things a converter can do, and they feel nothing alike.

Remuxing means lifting the video track out of one box and setting it down in another. Nothing is decoded. Nothing is compressed again. Not one pixel changes. It's limited by how fast your drive can move bytes, so it's very quick, and the quality loss is exactly zero — because there is no quality step at all.

Re-encoding means decoding every frame back to raw pixels and compressing the whole thing again from scratch. It's slow, it's heavy on the processor, and because the codecs involved are lossy, it costs a little quality every single time you do it.

So the speed difference isn't really about which container you picked. It's about whether the codec inside your file is one the destination container accepts. MKV and MOV usually hold H.264, which MP4 takes happily, so those are typically a straight repack. When the codec inside isn't compatible, everything has to be rebuilt — and that's where the waiting comes from.

Which is why AVI is unpredictable

AVI is the interesting case, because you can't tell from the extension what you're dealing with.

A genuinely old AVI — a 2004 camcorder file, something downloaded twenty years ago — is likely to hold a codec from that era. Nothing modern accepts those, so converting means a full re-encode: slow, and lossy. That's also why the MP4 usually comes out dramatically smaller, since H.264 is far more efficient than what it's replacing.

But an AVI made recently might have perfectly modern video inside it, in which case it repacks as quickly as any MKV would.

Same file extension, two completely different jobs. This is the practical payoff of understanding containers: the name on the file tells you about the box, and the box is the part that matters least.

What can get lost in the move

Repacking is lossless for the video, but the extras are another matter. MKV can carry things MP4 handles poorly or not at all — a stack of subtitle tracks, half a dozen audio languages, chapter markers, attached fonts. Move that into an MP4 and some of it may be quietly dropped or land in a form your player ignores.

For a normal file with one video track and one audio track, nothing is lost. For a carefully assembled archival MKV, check the result before you delete the original.

So which should you use?

Sending a file to someone, or playing it on a TV, phone or console — MP4, every time. If it won't play, that's usually the fix: MKV to MP4 or MOV to MP4.

Archiving something with multiple audio or subtitle tracks — MKV, and accept that it won't play everywhere.

Editing on a Mac — MOV while you work, MP4 when you export.

Anything still in AVI — convert it. It's a museum piece, and it's costing you storage for the privilege.

The bottom line

Most of the time you aren't converting video. You're changing the box it travels in, and the video rides along untouched. That's why it's usually instant and free — and why the exceptions, when a codec has to be rebuilt from scratch, are worth recognising before you start.