What does it cost to convert MKV to MP3?

$0.003 per call, paid in USDC on Base via x402.

How large can the MKV file be?

Up to 100 MB (104,857,600 bytes), and at most 60 min (3,600 s) of media. Past a limit the call answers too_large (413).

What happens if the conversion fails?

The tool answers with a typed JSON error from the errors table. Payment is settled only after the tool has produced its result; a call that fails inside the tool is never settled.

catalogue / tools / Audio & video / MKV to MP3

Convert MKV to MP3

To convert MKV to MP3, send one POST to /v1/audio-convert with the MKV source and "to": "mp3" in the body. "mp3" is the schema default of "to", so the field may also be left out. Each call costs $0.003 in USDC on Base via x402, with no account and no API key. The source file can be up to 100 MB and 60 min (3,600 s) long.

Endpoint: POST /v1/audio-convert
Price: $0.003 per call
Service: audio-convert
Docs: /docs/audio-convert

01Limits

Limit	Value
Largest source file	100 MB (104,857,600 bytes)
Longest input	60 min (3,600 s)
Timeout	94 s
Source formats	AIFF, AVI, FLAC, MKV, MP3, MP4, Ogg, WAV
Output formats	AAC, FLAC, MP3, Opus, WAV

02Example

A real MKV to MP3 run of audio-convert.

Request

POST /v1/audio-convert
content-type: application/json
payment-signature: <base64 x402 payload>

{
  "url": "https://grist.tools/samples/av/pair-clip.mkv",
  "to": "mp3"
}

Response

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/pair-clip.mkv",
  "bytes": 10531,
  "source_container": "matroska",
  "format": "mp3",
  "bitrate_kbps": null,
  "sample_rate": null,
  "audio_bytes": 4196,
  "audio_base64": "SUQzBAAAAAAACgAAAAAAAAAAAAD/4xjEAAxoAub5QBAAh1QgCdy3AiAECgDB/D8Q..."
}

Source file: av/pair-clip.mkv, 10.3 KB (10,531 bytes). 4 s Matroska file made by ffmpeg: a 64x36 PNG video stream at 1 fps (a slate field with a small amber square moving right), a mono 8 kHz FLAC audio track of two 500 Hz triangle beeps, and an English SubRip subtitle stream with two cues. The recorded run returned 4.1 KB (4,196 bytes) of MP3, 40% of the source size, with the source read as the matroska container. Decoded, the MP3 in audio_base64 opens with 49 44 33 04 00 00 00 00 00 0A 00 00. Recorded with ffmpeg 8.1.2.

In the sample itself, the EBML header names DocType matroska, version 4, readable by a version 2 parser; Segment Info sets a timestamp scale of 1,000,000 ns and a duration of 4,000 ticks, 4 s, with Lavf as the muxing application; the Tracks element lists codec IDs V_MS/VFW/FOURCC (video, language und), A_FLAC (audio, language und) and S_TEXT/UTF8 (subtitle, language eng). In the returned MP3: it opens with an ID3v2.4 tag with 10 bytes of body, then an MPEG 2.5 Layer III frame header at byte 20: 8 kbps, 8,000 Hz, mono.

03Format notes

MKV as a source

Matroska files begin with the EBML magic 1A 45 DF A3, and WebM uses the same signature, so the sniffer treats both as a single container and reports it as matroska in the container field of each response. Matroska is built from tracks that ffmpeg can address independently. When audio is the target, the first audio track is decoded and any picture is ignored. For a still frame, the requested timestamp is sought at the demuxer level before one picture is decoded, with sound excluded. A subtitle track is chosen by its index counted among subtitle tracks only, starting from zero, and an index that matches nothing is refused rather than answered with an empty file. SubRip or WebVTT text written by ffmpeg comes back together with a cue count taken from the timing arrows in that text.

MP3 as a target

MP3 is the default value of audio-convert's to field, encoded with the pinned libmp3lame codec. The mp3 muxer needs no seeking, so output streams straight to a pipe instead of a temporary file. bitrate_kbps applies to this lossy destination; when it is omitted, no audio bitrate argument is passed, the encoder's own default applies, and the response reports the bitrate as null. An optional sample_rate selects from the closed set, otherwise ffmpeg normally follows the source. Only the first audio stream is encoded, and video, chapters and metadata are left out, with bit-exact flags keeping the bytes reproducible for the pinned ffmpeg. Even with source metadata removed, each recorded MP3 output still begins with an ID3v2 tag written by the muxer. Inputs whose duration exceeds the service limit are refused before encoding.

04Errors

Code	HTTP
`invalid_input`	`400`
`blocked_target`	`403`
`unreachable_target`	`424`
`upstream_timeout`	`424`
`unsupported_content_type`	`415`
`too_large`	`413`
`unprocessable`	`422`
`internal`	`500`

When each is raised, and what it means for payment: /docs/audio-convert.

05Questions

What does it cost to convert MKV to MP3?: $0.003 per call, paid in USDC on Base via x402.
How large can the MKV file be?: Up to 100 MB (104,857,600 bytes), and at most 60 min (3,600 s) of media. Past a limit the call answers too_large (413).
What happens if the conversion fails?: The tool answers with a typed JSON error from the errors table. Payment is settled only after the tool has produced its result; a call that fails inside the tool is never settled.