The one formula
For any constant bitrate file:
size in bytes = bitrate ÷ 8 × seconds
A 4 minute track at 320 kbps is 320,000 ÷ 8 × 240 = 9.6 million bytes, about 9.2 MB. At 128 kbps the same track is 3.7 MB. Bitrate and length are the only things that matter; the music does not change the size of a constant bitrate file.
Where WAV's bitrate comes from
WAV is uncompressed, so its bitrate is fixed by the recording format:
bitrate = sample rate × bit depth × channels
CD audio is 44,100 × 16 × 2 = 1,411,200 bits per second, or 1,411 kbps. That is 10.1 MB per minute, so a 4 minute track is 40.3 MB. A 24-bit 96 kHz studio recording is 4,608 kbps, or 33 MB per minute.
What the common bitrates are for
- 64 to 96 kbps: speech, podcasts and audiobooks, often in mono, where it is plenty.
- 128 kbps: the old default. Fine on phone speakers, audibly thin on cymbals and reverb through good headphones.
- 192 kbps: a sensible middle for music you will mostly hear on the move.
- 256 to 320 kbps: the practical ceiling for MP3, where most listeners stop hearing a difference from the original.
- 1,411 kbps: CD quality WAV or FLAC, bit for bit identical to the master.
Variable bitrate
VBR spends more bits on busy passages and fewer on quiet ones, which usually sounds better than CBR at the same average size. The quoted number becomes an average, so the file size formula only gives an estimate. For MP3, a VBR setting around 190 kbps average is roughly the quality of 256 kbps CBR at two thirds the size.
Lossless, lossy and generations
FLAC and ALAC compress losslessly, usually to 50 to 60% of WAV, and decode back to exactly the original samples. MP3 and AAC throw information away permanently. That matters most when you re-encode: converting MP3 to MP3, or MP3 to AAC, compounds the loss each time. Keep a lossless master if you will ever edit or re-export, and treat the lossy file as the final delivery.
Which format
- MP3: plays on everything, including old car stereos. The safe choice for handing a file to someone.
- AAC or M4A: better sound than MP3 at the same bitrate, standard on Apple devices and in video.
- Opus: the best of the three at low bitrates, and excellent for voice, but not universally supported.
- WAV: for editing, mastering and anything going into a studio or a sampler.
- FLAC: for archiving a collection without the size of WAV.
Sample rate and bit depth
Raising the sample rate above 44.1 or 48 kHz extends the frequency range past hearing; it is useful while recording and processing, not for listening. Bit depth sets the noise floor: 24-bit gives headroom to record into, and 16-bit is more than enough for the finished file.
Tools
The Audio Bitrate Calculator turns bitrate and length into a file size, or works out the bitrate that fits a size limit. Audio Converter changes format and bitrate, Trim Audio cuts the length, which is the other half of the size, and Video to MP3 pulls the track out of a video.