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How Big Will the Converted Audio File Be?

Lossy audio size is arithmetic: bitrate times duration. Lossless size depends on how compressible your recording is. Here are measured sizes for MP3, M4A, FLAC, Opus, WAV and AIFF from the same three-second source.

How Big Will the Converted Audio File Be?

The short answer

For lossy formats the size is pure arithmetic: bitrate x duration / 8. A 192 kbps MP3 is about 1.4 MB per minute; 320 kbps is about 2.4 MB per minute; 128 kbps is about 0.9 MB per minute. That is why people who need a small file reach for a lower bitrate rather than a different codec.

Lossless formats work differently. WAV and AIFF are fixed by sample rate and bit depth, not by content: 44.1 kHz, 16-bit, stereo is about 10.6 MB per minute no matter what you recorded. FLAC sits in between — it is lossless too, but how much it saves depends entirely on how compressible the recording is.

Measured sizes from one source

Source: three seconds of 44.1 kHz stereo audio, 529,278 bytes as 16-bit PCM WAV.

MP3 320 kbps: 122,297 bytes, measuring about 326 kbps. MP3 192 kbps: 73,395 bytes, about 195 kbps. MP3 128 kbps: 48,945 bytes, about 130 kbps.
M4A (AAC) 192 kbps: 73,757 bytes. M4A (AAC) 128 kbps: 49,716 bytes.
Opus 128 kbps: 49,015 bytes. FLAC: 45,245 bytes on this particular source, which is misleading — see below.
WAV: 529,278 bytes. AIFF: 529,254 bytes. The two uncompressed formats differ by 24 bytes.
Note that the measured bitrates run slightly above the label: 192 kbps MP3 reads as 195, and 128 kbps AAC reads as 132. The difference is container and per-frame overhead, and it is always there.

How much FLAC saves depends on the recording

We converted the same WAV to FLAC three times from three different sources. A pure sine wave compressed to 8.5% of the original — a waveform that barely changes can be described extremely compactly. A three-note chord came out at 37.9%. Pink noise, which is close to the hardest case for a lossless compressor, came out at 40.9%.

The 8.5% figure is an outlier, not a promise. Real music and speech recordings typically land between 50% and 70%, because they contain a lot of detail that cannot be predicted away. If you are choosing FLAC to save space, the honest expectation is roughly half the size of WAV, not a tenth.

Choosing a target format

Sending to another person, or playing in a car: MP3 at 192 to 256 kbps. Compatibility is the entire point of this choice.
Voice recordings and podcasts: M4A or Opus at the same bitrate rather than MP3, because both handle speech better at a given size.
Feeding an editor: WAV or AIFF, whichever your software prefers. They behave identically on a timeline.
Archiving something you may edit later: FLAC. It keeps the samples exactly while costing noticeably less than WAV.
Embedding in a web player: Opus or OGG, which hold up better than MP3 when the bitrate is pushed down.