NUT to MKV Converter
FileFlip converts NUT to MKV entirely inside your browser, using WebCodecs and FFmpeg compiled to WebAssembly. The file is never uploaded to a server: it is read off your disk, converted on your own machine, and saved back by the browser. That means no file-size limit, no queue and no account.
How do I convert a NUT file to MKV?#
- Drop your NUT file onto the converter at the top of this page, or click it to pick one from your computer. You can add as many as you like.
- Leave the target set to MKV and the conversion starts on its own. FileFlip usually downloads nothing, falling back to 30.8 MB of WebAssembly if your browser cannot encode these codecs itself, then runs WebCodecs and FFmpeg on your own machine.
- Save the MKV file. Your browser writes it straight to your downloads folder, because there was never a server holding it.
Does converting NUT to MKV lose quality?#
Sometimes. When the codecs inside the NUT file are ones the MKV container can hold, the streams are copied straight across and nothing at all is lost. When they are not, the file has to be decoded and encoded again, and that step does cost a little quality.
Is it safe to convert NUT files online?#
Yes, because nothing is uploaded. FileFlip converts NUT files inside the browser tab you already have open, using WebAssembly builds of the same engines a desktop converter would install. Your file is read from your own disk into the page's memory, and no request carrying it is ever made.
You do not have to take that on trust. Open your browser's network panel, run a conversion, and you will see the engine come down and your file go nowhere. There is no upload, no server-side copy, no retention window, and no account tied to what you converted.
How FileFlip works lists every engine, its version and its download size, and shows how to check for yourself that nothing leaves your machine.
What NUT to MKV actually does to the file#
Converting NUT to MKV runs on WebCodecs and FFmpeg, usually downloads nothing, falling back to 30.8 MB of WebAssembly if your browser cannot encode these codecs itself, copies the streams through when the codecs fit and re-encodes when they do not.
| Engine | WebCodecs and FFmpeg |
|---|---|
| Conversion path | One step: NUT → MKV |
| Downloaded to your browser | Nothing in most browsers, or 30.8 MB if yours cannot encode these codecs itself |
| Quality | Lossless when the codecs fit, lossy when they do not |
| You get back | One file |
| Where it runs | In a background worker, so the page stays responsive |
What survives the conversion
| What is in the file | This conversion | Why |
|---|---|---|
| Encoded streams | Sometimes | Survives for some files and not others. |
Worth knowing before you start:
- the FFmpeg fallback is an LGPL build, so a re-encode produces MPEG-4 Part 2, VP8, VP9, or Theora rather than H.264 or H.265
- whether the browser can do this without FFmpeg depends on the codecs it exposes through WebCodecs
- the encoded streams are copied when the file's codecs fit the target container, and re-encoded when they do not
What is a NUT file?#
A NUT file is a general-purpose media container created inside the FFmpeg and MPlayer projects, built as a simple, open, patent-free alternative to AVI and similar formats, and able to wrap any codec those tools support.
NUT is worth using only inside an FFmpeg-based pipeline, such as a test harness or a tool chain that already standardises on it.
Convert NUT to MP4 or MKV for anything that needs to open outside FFmpeg or a player built on its libraries, since almost no other software recognises the container.
What is an MKV file?#
An MKV file is a Matroska container built on the EBML binary format, and it can hold any number of video, audio, subtitle and attachment tracks, encoded with essentially any codec, in a single file.
Use MKV when a video needs multiple audio tracks, several subtitle languages, or a codec MP4 doesn't accommodate, and it's staying on software you control, like a media server or a desktop player.
Convert MKV to MP4 when a file needs to play on a device that never added MKV support, since that's usually a hardware or software gap rather than anything wrong with the codec inside.
NUT to MKV questions people ask#
Why would a file be in NUT instead of MP4 or MKV?
NUT usually turns up as the output of an FFmpeg-based tool or test pipeline rather than something a person chose deliberately, since it was designed as an open, patent-free container within the FFmpeg and MPlayer projects. Matroska ended up filling the same role for everyone else.
What opens a NUT file?
ffplay and mpv both open NUT files directly, since both are built on FFmpeg's own libavformat library, which is where the NUT container comes from. Outside that ecosystem, support is essentially nonexistent.
Is NUT related to Matroska or AVI?
NUT was designed around the same time as Matroska and for a similar reason, as a flexible, open alternative to AVI's limitations, but the two are independent formats with no shared structure. Matroska is the one that ended up widely adopted; NUT stayed inside the FFmpeg and MPlayer projects that built it.
Why won't my TV or game console play an MKV file?
MKV is just a container, so the refusal is almost always about the container itself rather than the codec inside it. A lot of hardware players, smart TVs, the PS5, some streaming boxes, never implemented support for parsing Matroska, even though they can decode the exact same H.264 or H.265 video when it's wrapped in an MP4.