Y4M to MP4 Converter
FileFlip converts Y4M to MP4 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 Y4M file to MP4?#
- Drop your Y4M 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 MP4 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 MP4 file. Your browser writes it straight to your downloads folder, because there was never a server holding it.
Does converting Y4M to MP4 lose quality?#
Sometimes. When the codecs inside the Y4M file are ones the MP4 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 Y4M files online?#
Yes, because nothing is uploaded. FileFlip converts Y4M 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 Y4M to MP4 actually does to the file#
Converting Y4M to MP4 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: Y4M → MP4 |
| 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 Y4M file?#
A Y4M file holds YUV4MPEG2 data, raw uncompressed video frames in planar YCbCr with a short plain-text header, the format codec developers feed into encoders like x264, x265 and vpxenc so nothing but the codec under test touches the pixels.
Use Y4M when testing or comparing video encoders, where the point is to remove every other codec from the pipeline so the one under test is the only variable.
Convert Y4M to a compressed format like MP4 for anything other than encoder testing, since the raw size makes it impractical to store or share.
What is an MP4 file?#
An MP4 file holds video, audio, subtitle and chapter tracks together in the MPEG-4 Part 14 container, the structure ISO standardised from Apple's QuickTime format and the way most video is stored and shared today.
Choose MP4 when the file has to play somewhere you do not control, which covers anything uploaded, emailed or handed to another person.
Convert an MP4 to something else when the destination is not a player: pulling out the audio, making a GIF, or moving into Matroska to keep several subtitle and audio tracks in one file.
Y4M to MP4 questions people ask#
Why is a Y4M file so much bigger than the video it represents?
A Y4M file stores every pixel of every frame with no compression at all, unlike MP4 or MKV, which is exactly the point: encoder developers use it as clean, unaltered input so nothing but the codec being tested affects the result. A minute of 1080p footage can run to several gigabytes.
What is Y4M used for?
Y4M is the standard input format for testing and benchmarking video encoders such as x264, x265 and libvpx, because raw, uncompressed frames guarantee the encoder is working from an unaltered source rather than one already degraded by another codec.
Does a Y4M file have sound?
No, Y4M carries video only. It was designed purely as a way to pipe raw frames between video tools, with no provision for an audio track at all.
Is MP4 a video format or a codec?
MP4 is a container, not a codec. It is a wrapper holding video, audio and subtitle streams side by side, and the codec inside decides whether a device can actually decode it. Two MP4 files can behave completely differently: one holding H.264 plays everywhere, one holding H.265 or AV1 may not play at all on older hardware.