What does it cost to convert WAV to MP3?

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

How large can the WAV 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 / WAV to MP3

Convert WAV to MP3

To convert WAV to MP3, send one POST to /v1/audio-convert with the WAV 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 WAV 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/waveform-beeps.wav",
  "to": "mp3"
}

Response

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/waveform-beeps.wav",
  "bytes": 32044,
  "source_container": "wav",
  "format": "mp3",
  "bitrate_kbps": null,
  "sample_rate": null,
  "audio_bytes": 2180,
  "audio_base64": "SUQzBAAAAAAACgAAAAAAAAAAAAD/4xjEAApAAx75QRACiHZUlw94AlAQBAEwfB8H..."
}

Source file: av/waveform-beeps.wav, 31.3 KB (32,044 bytes). Two-second mono 16-bit PCM WAV at 8 kHz: three 0.4 s 500 Hz triangle-wave beeps at rising level separated by silence, generated from integer arithmetic. The recorded run returned 2.1 KB (2,180 bytes) of MP3, 7% of the source size, with the source read as the wav 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 RIFF header counts 32,036 bytes after itself, and the fmt chunk, in its own field order, declares audio format 1 (integer PCM), 1 channel, 8,000 Hz, 16,000 bytes per second, a block align of 2 bytes and 16 bits per sample. The data chunk right after it holds 32,000 bytes of samples, 2 s at that rate. 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

WAV as a source

WAV is a RIFF file whose form type at byte eight reads WAVE, and it usually carries uncompressed PCM samples. Grist requires both markers, which keeps WebP images and AVI video, the other RIFF users, out of the audio path. Because a WAV holds audio alone, video-thumbnail and subtitle-extract refuse it as a container without a picture or caption stream, and only audio-convert accepts it. Before any sample is encoded, the length of the first audio stream is measured, first from the container header and, when that is missing or doubtful, by scanning packet timestamps; a file whose duration cannot be established is refused instead of being encoded blind. Since PCM is already decoded audio, the converter reads the samples directly rather than undoing another codec before the destination encoder runs.

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 WAV to MP3?: $0.003 per call, paid in USDC on Base via x402.
How large can the WAV 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.