Convert MOV, MKV and WebM to MP4

Why converting to MP4 is usually instant

A video file is two separate things: a container and the codecs inside it. The container — MP4, MOV, MKV, WebM — is the filing system. It says where each chunk of picture and sound lives, how long the video is, which way up it should be played. The codecs are the compressed picture and sound themselves. Renaming the file changes neither, which is why renaming never works.

The useful consequence is that MOV, MKV and MP4 can hold the same codecs. A clip your phone recorded as a MOV is H.264 or HEVC video with AAC audio; an MP4 from the same phone is the same H.264 and the same AAC, filed differently. Converting between them therefore doesn't need to touch the video at all — the compressed picture can be lifted out of one container and dropped into the other, byte for byte. That operation is called remuxing, and it is thousands of times cheaper than re-encoding because no frame is ever decoded or compressed again.

That's the path this tool takes whenever it can. It reads the codecs the moment you pick a file and tells you which of two things is about to happen: a straight copy into a new container, which finishes in seconds and cannot change the picture, or a genuine re-encode, which takes real time and does. You see which one before you commit, along with the reason.

1. Choose a video, or drag it onto the box above. It's read from your disk, not uploaded.
2. Read the panel that appears. It names the codecs it found and says, track by track, whether each one will be copied or re-encoded.
3. Press Convert. A copy finishes almost immediately; a re-encode shows real progress. Download saves the MP4 next to your original, which is left exactly as it was.

Remuxing has no quality setting because there is no quality decision to make: the picture in the MP4 is the same compressed data that was in the MOV. Re-encoding is the only path where anything is lost, and it is never taken silently.

Which video codecs can be rewrapped, and which must be re-encoded

MP4 is a more permissive container than its reputation suggests — it will legally hold VP9 or AV1, and a file built that way passes validation happily. The problem is that legal and playable are different questions. We tested each of these codecs by copying it into an MP4 and then opening the result with Apple's own decoding stack, the one QuickTime and every macOS app use. Several files that were structurally perfect simply refused to open.

So the rule this tool follows isn't «what does MP4 allow», it's «what will still play once it's an MP4». Anything that survives that second test is copied. Anything that doesn't is re-encoded to H.264, because a converted file that only opens in the browser you converted it in hasn't been converted at all.

If your file is one of the unsupported three, you'll be told immediately — before you press anything. That check happens on your device, from the file's own headers, and costs nothing.

What happens to the soundtrack

Audio gets the same treatment on its own terms, and it's worth reading because the answer differs from the video answer for the same file: a MOV can easily end up with its picture copied and its sound re-encoded. When that happens the conversion is no longer instant, and the panel above says so rather than borrowing the fast path's wording.

AAC is copied, because MP4 and AAC are the pairing every player expects. Most other codecs are re-encoded to AAC — including a few, like FLAC and uncompressed PCM, that we measured copying successfully. That's a deliberate choice: those exotic MP4 audio boxes worked on the machines we tested, and we won't promise they work on yours. Re-encoding to AAC needs no such promise and costs a fraction of a second, at the price of one lossy pass on a lossless source.

Surround audio is preserved whenever the track is copied. On the re-encoding path it's mixed down to stereo, because browser AAC encoders refuse multichannel input outright — a downmix beats a failed conversion.

Converting MOV to MP4

This is the case remuxing was made for, and in practice it is close to a formality. MOV is Apple's container, and the two formats are so closely related that MP4 was standardised from it — which is why a MOV recorded on an iPhone or exported from a Mac almost always contains H.264 or HEVC video with AAC audio, exactly what an MP4 wants. Both tracks get copied, the whole thing takes a couple of seconds regardless of how long the video is, and the picture is not merely similar to the original but identical to it.

Two MOV variants behave differently, and both are worth knowing before you drop the file in. Screen recordings and camera footage from professional workflows are often ProRes, which copies fine but won't shrink — you'll get an MP4 the same size as what you started with, which is the honest outcome rather than a failure. And a MOV can carry Dolby audio, in which case the sound is copied rather than converted, for the reason in the table above.

Rotation survives. Portrait video shot on a phone is stored sideways with a flag telling players to turn it, and that flag is carried into the MP4 — the result won't come out on its side.

Converting MKV to MP4

MKV is the opposite of MOV in one respect: it is deliberately open-ended and will hold nearly anything, so what happens to your file depends entirely on what someone put inside it. The common case is still a straight copy — an MKV holding H.264 with AAC becomes an MP4 in seconds with nothing re-encoded. An MKV holding VP9 or an MP3 soundtrack takes the re-encoding path for that track. The panel tells you which before you start, so you know whether to expect two seconds or two minutes.

Multiple audio tracks come across. An MKV with separate language tracks keeps them all in the MP4, since MP4 has no practical limit on how many it will hold. They are all treated the same way, though: if the first track needs re-encoding, they all get re-encoded.

Subtitles do not come across. Embedded subtitle tracks are dropped, and this is a real limitation rather than a preference — MKV's subtitle formats have no MP4 equivalent worth writing, and the browser-side reader we use doesn't expose those tracks at all, so we can't even tell you a file had them. If your MKV has subtitles you need, extract them separately before converting, or keep the MKV.

Chapter markers and attachments (fonts, cover art) are also left behind. If an MKV is an archive of a disc rather than a single video, converting it to MP4 will flatten it to picture and sound.

Converting WebM to MP4

WebM is the one input where the fast path is almost never available, and it's better to know that up front. WebM exists to carry VP8, VP9 or AV1 with Opus or Vorbis audio — codecs built for the web and, in the case of the first two, unusable inside an MP4 anywhere outside a browser. There is nothing to copy here that would still play, so the video is genuinely re-encoded to H.264 and the audio to AAC. That takes real time, proportional to the length of the video, and it is one lossy generation of quality.

It's still worth doing, because it's the only thing that makes the file portable. A WebM won't open in most video editors, won't import into most phone galleries, and won't play on plenty of TVs and older devices — the properties that make it good for streaming are exactly the ones that make it awkward everywhere else. What you get back is a file that behaves normally.

Expect the MP4 to be larger than the WebM, sometimes considerably. VP9 and AV1 are newer and more efficient than H.264, so encoding the same picture in H.264 costs more bits. If size matters more than compatibility, the WebM you already have is the smaller file.

Because re-encoding is the whole job here, a long WebM takes about as long as compressing a video of the same length. Progress is measured, not guessed, and you can cancel at any point.

What this tool doesn't do

This converts to MP4 and nothing else, and the edges are worth stating plainly rather than leaving you to find them.

1. It only outputs MP4. There is no MP4-to-WebM, no GIF export, and no audio-only output on this page.
2. It doesn't make files smaller. A copy produces a file the same size as the original by design; a re-encode can even come out larger. Compressing is a separate job with its own settings.
3. It doesn't change the picture. Resolution, frame rate and rotation are all carried through as they were — nothing is scaled and nothing is cropped.
4. One file at a time. There's no batch queue.
5. Subtitles, chapters and attachments are dropped. Only picture and sound are carried over.
6. There's no size limit, but there is a disk check. The MP4 is written to local storage as it's produced, so memory doesn't grow with the file — but the space has to exist, and that's verified before anything starts rather than discovered halfway through.

Your original is never modified or moved. The MP4 is a new file you download, so deleting the source stays your own explicit decision.

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