"Convert it to MP4 to save space" is advice you hear constantly, and it's mostly wrong. MP4 is a container — a box that holds video, audio and metadata — not a compression method. The thing that actually decides the file size is the codec inside it: H.264, VP9 or AV1. Rename a .webm to .mp4 and you've changed nothing about how big it is. So we measured how much the codec really matters.
How we measured it
We took two standard 720p test clips — Big Buck Bunny and Jellyfish — and encoded each across a full quality sweep in H.264, VP9 and AV1. Rather than trust the encoder's quality dial, we scored every output with VMAF, Netflix's perceptual quality metric, so we could compare file sizes at equal perceived quality instead of at equal settings. Every number below is the mean of the two clips.
The result: the codec is everything
At a target quality of VMAF 90 (visually indistinguishable from the source for most viewers):
- VP9 is about 40% smaller than H.264.
- AV1 is about 60% smaller than H.264.
Made concrete, the same 10-second 720p clip lands at roughly:
| Codec | Size (10s, 720p, VMAF 90) |
|---|---|
| H.264 | 3.3 MB |
| VP9 | 2.0 MB |
| AV1 | 1.3 MB |
Same picture, same perceived quality — and AV1 is less than half the size of H.264. Meanwhile the container extension does nothing on its own: a .mp4, .webm and .mkv holding the same H.264 stream are the same size. If you want a smaller file, you change the codec, not the extension.
The catch: encode time
Newer codecs buy their smaller files with CPU. AV1 and VP9 cost far more to encode than H.264 — often dramatically so. That reframes the choice as a trade:
- Encode once, stream many times (a video you publish): spend the CPU on AV1/VP9; the bandwidth savings compound over every view.
- One-off or fast turnaround (a clip you send once): H.264 is fine — the encode is quick and universally supported.
Reproduce it
The whole study is an FFmpeg quality sweep plus VMAF scoring, driven by a short script — no proprietary tooling. Point it at any clip and you get the same shape of result.
If you just need to convert something, the video tools — MP4 ⇄ MOV, WebM/MKV/AVI → MP4, GIF ⇄ video, and a trimmer — run entirely in your browser with ffmpeg.wasm, so nothing is uploaded.
Originally published on LK Forge.
Top comments (0)