Converters Video converter
Convert MOV, MKV, AVI, WebM and MP4 in any direction, with the whole conversion running inside your browser.
Anything but Original forces a re-encode.
Quality, if a re-encode is needed
Drag files here, or
Converted to MP4 on your device. The converter itself is a 9.8 MB download, once.
Ready. Runs locally on your device.
Your files stay on your device. The tool works directly in your browser, using your device to process your files. Nothing is sent to our servers, and we never receive, store, or see your files or figures.
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Cite this page
Reembun. (2026, August 12). Video converter. https://reembun.com/video-converter
How to use it
Add your videos
Drop them in or use Choose videos. The whole batch queues up, and nothing is uploaded at any point.
Let the converter load once
The first conversion downloads the engine, which is around 10 MB. It is kept for next time, so every later run starts immediately.
Pick the output
MP4 plays everywhere, WebM is smallest for the web, MKV holds anything, MOV suits Final Cut, AVI is for legacy players, GIF makes a silent short clip.
Set resolution and quality
Original resolution keeps the frame size. Anything else forces a re-encode, which takes longer and is where the file size actually comes down.
Download the result
One video downloads on its own, a batch comes back as a ZIP.
What this converter does differently
Every other free video converter uploads your file. It has to: converting video is expensive, and the only cheap way to do it is on a server you pay for once and reuse for everybody. The consequence is a copy of your video on a machine you do not control, a queue to wait in, and a file size limit set by whatever the operator can afford.
This one runs FFmpeg compiled to WebAssembly inside your browser tab. Your video is read from your own disk, converted by your own processor, and written back to your downloads folder. Nothing is transmitted for the whole operation, because there is no server in the picture.
Containers and codecs are not the same thing
This is the single most useful thing to understand about video files, and it explains why some conversions take three seconds and others take three minutes.
A container is the wrapper: MP4, MOV, MKV, AVI, WebM. It holds streams and describes how they fit together.
A codec is how the picture itself is compressed: H.264, HEVC, VP9, AV1, MPEG-4.
They vary independently. An iPhone MOV and an Android MP4 usually hold exactly the same H.264 video and AAC audio inside different wrappers. Converting between them means rewriting the wrapper and copying the streams across untouched.
| Container | Usually holds | Comes from |
|---|---|---|
| MP4 | H.264 + AAC | Phones, cameras, everything online |
| MOV | H.264 or ProRes + AAC | iPhones, Final Cut, QuickTime |
| MKV | Anything at all | Downloads, rips, archives |
| AVI | MPEG-4 Part 2, DivX, Xvid | Old camcorders, files from the 2000s |
| WebM | VP8, VP9 + Opus | Browser recordings, YouTube downloads |
Copy first, re-encode only when forced
Every conversion here tries a stream copy before anything else. If the video codec is one the target container can legally carry, and the audio codec is too, both are copied over and the job finishes at disk speed with no quality lost whatsoever. The tool says so when it happens.
A re-encode is unavoidable in two cases: the codec cannot live in the target container (VP9 has no place in an MP4), or you have asked for something that changes the pixels (a lower resolution). Then the video is decoded frame by frame and compressed again, which is slow on a single core and does discard some quality.
Practical consequence: leave the resolution at Original unless you specifically want a smaller file. Changing it forces the slow path even when the fast one would have worked.
Choosing the output format
MP4 for almost everything. It plays on every phone, browser, TV and editor, and it is what every upload form expects.
WebM when the file is going on your own website and you want it as small as possible. Always requires a re-encode.
MKV for archiving, because it keeps subtitle tracks, chapters and multiple audio languages that MP4 will drop.
MOV when a specific Apple workflow has asked for it.
GIF for a short silent loop. Be warned that GIF is a 1987 format with a 256-colour palette, so a five-second clip can easily come out larger than the video it came from.
What the quality setting does
It only matters when a re-encode is actually needed, which is why it is labelled that way.
- High targets CRF 20, which is visually indistinguishable from the source on ordinary footage.
- Balanced targets CRF 24. This is the sensible default and roughly halves the file.
- Small file targets CRF 28, where softness starts to be visible on detailed scenes but the file is small enough to email.
Everything keeps working offline
Once the engine has been downloaded, the whole tool works with no connection. That is a real consequence of the architecture rather than a feature that was added: there was never a server involved, so there is nothing to be disconnected from.
Common questions
Is there a file size limit?
No limit is imposed by us, because nothing is uploaded and there is no server to run out of room. The real ceiling is your own device. The engine is a 32-bit WebAssembly build, so its address space caps out at 4 GB and the input, output and working buffers all live inside it. Files up to about 2 GB are fine on a desktop; on a phone with less memory to spare, expect trouble sooner.
Why is the first conversion slow to start?
The converter is a complete copy of FFmpeg compiled to WebAssembly, and it is a 9.8 MB download. It arrives once and is then cached for a year, so the second conversion starts immediately, and it keeps working with no connection at all.
Will converting reduce my video quality?
Often not at all. When the video inside your file can legally live in the target container, both streams are copied across untouched and the result is bit-for-bit identical picture. The tool tells you when that happened. A genuine re-encode is only needed when the codec cannot make the trip, such as VP9 from a WebM into an MP4, and that does cost a generation of quality.
Is there a watermark?
No. There is no watermark, no sign-up, no email, no queue and no paid tier holding features back. The conversion runs on your hardware, so it costs us nothing to run and there is nothing to gate.
Can it convert 4K video?
Yes, and a stream copy handles 4K as fast as anything else because it does not look at the pixels. A re-encode of 4K on a single core is genuinely slow, though. If you need to re-encode 4K, dropping the resolution to 1080p in the tool is usually both faster and what you actually wanted.
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