About Y4M Files
YUV4MPEG2
YUV4MPEG2 (Y4M) is a simple uncompressed video format that stores raw YUV pixel data with a minimal header. It is widely used as an intermediate format for video processing and quality benchmarking where no compression artifacts are acceptable.
Family
Video Files
Extension
.y4m
MIME Type
video/x-raw-yuv
Can Use As
HOW Y4M
CAME TO BE.
Y4M — YUV4MPEG2 — is a raw uncompressed video container used almost exclusively by codec developers. The format is brutally simple: a one-line ASCII header (`YUV4MPEG2 W1920 H1080 F30:1 Ip A1:1 C420 XCS=100`), then alternating frames prefixed with `FRAME\n` markers, then raw YUV pixel data. No compression, no timecodes beyond the framerate — a video codec’s perfect ground-truth input.
X264, x265, SVT-AV1, every video codec encoder on Earth accepts .y4m as test input. A 10-second 1080p Y4M weighs 600 MB uncompressed, which is the entire point: no prior codec decisions have been baked in, so codec-quality comparisons use Y4M as the source of truth.
CURIOSITIES &
TRIVIA.
Every video codec benchmark on the planet uses Y4M as ground-truth source — x264, x265, AOM AV1, SVT-AV1.
Y4M's header is one ASCII line — trivially parseable and human-readable.
10 seconds of 1080p Y4M is about 600 MB — codec developers work with terabytes of Y4M test data.
The `FRAME\n` marker between frames is deliberately simple — parsers can seek by scanning.
Y4M's simplicity is why codec papers always cite Y4M inputs: no ambiguity about pre-processing.
STRENGTHS &
LIMITATIONS.
Strengths
- Uncompressed raw YUV — codec benchmark truth.
- Dead-simple header.
- Universal codec development support.
Limitations
- Enormous file sizes.
- Development-only — not for consumption.
- No metadata beyond basic stream params.
Typical Sizes & Weights
10 sec 1080p Y4M
~600 MB
1 min 4K Y4M
~14 GB
Technical Specifications
- MIME type
- video/x-yuv4mpeg2
- Extension
- .y4m
- Pixel format
- YUV 4:2:0, 4:2:2, 4:4:4
- Header
- ASCII single line
CONVERT FROM
Y4M
Common Use Cases
Video quality benchmarking, uncompressed intermediate, codec testing
Related Formats
Popular comparisons
Y4M vs JPG
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Y4M vs WEBP
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Y4M vs GIF
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Y4M vs BMP
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Y4M vs MP4
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Popular Y4M conversions
The most-requested destinations when starting from Y4M.
Frequently Asked Questions about Y4M
Frequently Asked Questions
Y4M (YUV4MPEG2) is a video container format that bundles one or more video streams, audio tracks, and optional subtitles into a single file. The container format determines how metadata is organised and which codecs can live inside; the visual quality itself depends on the codec (H.264, H.265, VP9, AV1) rather than the Y4M wrapper. It is part of the video files family.
VLC, MPV and PotPlayer play nearly every Y4M file on desktop. Browser support varies: modern Chromium, Firefox and Safari play common containers via the HTML5 <video> tag, but niche Y4M variants may fail. If a device refuses your Y4M, convert to MP4 with our Y4M to MP4 converter for universal playback.
Upload your Y4M to KaijuConverter and pick MP4, MOV, MKV, WebM, or any other target. Our pipeline uses FFmpeg under the hood and stream-copies when codecs are compatible (no quality loss) or transcodes at high-quality defaults otherwise. Conversion runs server-side; both files delete within two hours.
Only when the target requires re-encoding. If the codecs inside Y4M match what the target container supports, FFmpeg stream-copies the streams and the output is bit-identical to the source. Transcoding uses transparent quality defaults (CRF 20–23 H.264) and produces output indistinguishable from the original at normal viewing distance.