How do I convert an MP4 to WebM over an API?

Q: How do I convert an MP4 to WebM over an API?

POST the file to /api/ffmpeg/job/ with -c:v libvpx-vp9 and -c:a libopus, then poll the job until it reports success. WebM only accepts VP8/VP9 video and Vorbis/Opus audio, so the audio has to be re-encoded too — copying AAC into a .webm container fails.

Convert MP4 to WebM with an API

POST the file to /api/ffmpeg/job/ with -c:v libvpx-vp9 and -c:a libopus, then poll the job until it reports success. WebM only accepts VP8/VP9 video and Vorbis/Opus audio, so the audio has to be re-encoded too — copying AAC into a .webm container fails.

The ffmpeg command

What you would run locally, for reference.

ffmpeg -i input.mp4 \
  -c:v libvpx-vp9 -crf 33 -b:v 0 \
  -c:a libopus -b:a 96k \
  output.webm

The API request

The same flags, sent as JSON. Authenticate with Authorization: Token <your-api-token>.

POST /api/ffmpeg/job/

{
  "global_options": [
    "-y"
  ],
  "inputs": [
    {
      "input_source": "https://example.com/input.mp4"
    }
  ],
  "outputs": [
    {
      "filename": "output.webm",
      "options": [
        "-c:v",
        "libvpx-vp9",
        "-crf",
        "33",
        "-b:v",
        "0",
        "-c:a",
        "libopus",
        "-b:a",
        "96k"
      ]
    }
  ]
}

Worth knowing

-b:v 0 is not optional with VP9: without it -crf is ignored and you get constrained-quality mode instead of constant quality.
VP9 is markedly slower than H.264. Since jobs bill CPU time, the same clip costs more as WebM than as MP4 — worth knowing before you convert a library.

What it costs

Jobs bill in processing seconds — the CPU time this one actually uses — with a floor of 25 seconds per GB pulled out of storage to run it, so a stream-copy still counts. Uploading the output and downloading it afterwards are free. See the plans.

Related guides

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