Bit Rate ↔ File Size for Audio Codecs (MP3/AAC/FLAC estimate) mechanism

Model scope

The base bitrate-duration arithmetic is standard, but the specification needs explicit input and output units, validation rules, and a separate PCM-size formula for the FLAC ratio estimate. The honest-label warning is appropriate but should state that the FLAC range is empirical and content-dependent.

• size = bitrate × duration ÷ 8 is correct only when bitrate is in bits per second, duration is in seconds, and the result is bytes.
• The FLAC estimate is incomplete: it needs an explicit PCM-size formula and a selected compression ratio or an output range. The stated 50–60% ratio is empirical and content-dependent, not a universal FLAC formula.
• If FLAC is selected without either a measured bitrate or PCM parameters such as sample rate, bit depth, and channel count, the file size cannot be computed from the listed inputs alone.

Units and conventions

• Bitrate is assumed to be measured in bits per second; values displayed as kbps must be converted using 1 kbps = 1,000 bits per second.
• Duration is assumed to be in seconds.
• The base formula outputs bytes because 8 bits = 1 byte.
• Displayed decimal file sizes use 1 MB = 1,000,000 bytes; binary sizes use 1 MiB = 1,048,576 bytes.
• The FLAC 50–60% estimate is assumed to be relative to uncompressed PCM size.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "codec": "MP3",
    "bitrate_bps": 128000,
    "duration_seconds": 180
  },
  "expected_outputs": {
    "estimated_size_bytes": 2880000,
    "estimated_size_MB_decimal": 2.88,
    "estimated_size_MiB": 2.74658203125
  },
  "working": "size = bitrate × duration ÷ 8\n= 128,000 bits/s × 180 s ÷ 8\n= 23,040,000 bits ÷ 8\n= 2,880,000 bytes\n= 2.88 MB decimal\n= 2,880,000 ÷ 1,048,576\n= 2.74658203125 MiB"
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

An extreme but numerically representable input overflows ordinary JavaScript floating-point arithmetic and produces Infinity instead of a finite size. The convention probe also exposes materially different displayed values if decimal and binary bitrate or storage units are mixed.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche5/tranche5-results.json
• BOUNDARY_HAZARD_CATALOG.json