MP3 bitrate explained: which quality should you save at?
Bitrate is the amount of data used for each second of audio. More data can mean better sound, but only up to the quality the recording already had.
What kbps means
An MP3's bitrate is measured in kilobits per second (kbps). A 128 kbps file stores 128,000 bits for every second of sound. The encoder decides which details of the audio you're least likely to notice and discards them to fit that budget. The higher the budget, the less it has to throw away.
Because the bitrate is a rate, file size follows directly from it. One minute at 128 kbps is 128 × 60 ÷ 8 = 960 kilobytes, a little under 1 MB.
| Bitrate | Size per minute | Typical use |
|---|---|---|
| 96 kbps | ≈ 0.7 MB | Speech, voice notes, podcasts in mono |
| 128 kbps | ≈ 0.96 MB | Podcasts, audiobooks, casual music listening |
| 192 kbps | ≈ 1.4 MB | Music where you want a safe margin |
| 256 kbps | ≈ 1.9 MB | Music for good headphones or speakers |
| 320 kbps | ≈ 2.4 MB | The highest MP3 bitrate; archiving music in MP3 |
| WAV (16-bit stereo) | ≈ 10 MB | Editing, video projects, no compression at all |
Constant versus variable bitrate
A constant bitrate (CBR) file uses the same budget for every second, so silence costs as much as a cymbal crash. A variable bitrate (VBR) file spends more data on complex passages and less on simple ones, and its quality is usually described by its average bitrate.
Mp3Blocks recognises both. For CBR files it reads the exact value from the first frame header. For VBR files it finds the encoder's information tag, calculates the average bitrate from the file size and duration, and suggests the next standard value above it. The export itself is always constant bitrate, which every player handles.
Why going higher than the original doesn't help
Once an MP3 has been encoded at 128 kbps, the detail it discarded is gone. Saving that audio at 320 kbps preserves the 128 kbps sound very faithfully, artefacts included, in a file 2.5 times bigger. It cannot bring back what the first encoder removed.
Going lower is a different matter. Re-encoding at a lower bitrate removes more detail on top of what was already lost, so the loss adds up. That's why Mp3Blocks preselects the standard value that matches or slightly exceeds the original, and tells you in plain words what happens if you change it.
Rule of thumb: keep the bitrate Mp3Blocks recommends unless you have a reason to change it. Go lower when file size matters more than sound (sending voice recordings by message). Choose WAV when the clip is going into further editing.
Speech needs less than music
A single voice has a much narrower range of sound than a full band, so it survives lower bitrates well. A lecture or interview at 96 kbps is perfectly clear; the same bitrate on an orchestral recording will sound noticeably dull. If you are splitting a podcast into parts for listeners on mobile data, 96 or 128 kbps is a reasonable choice even if the original was 192.
Can you hear the difference between 256 and 320 kbps?
For most listeners on most equipment, a well-encoded MP3 at 256 kbps or higher is hard to tell apart from the uncompressed source. Differences, when they exist, show up in demanding material such as cymbals, applause and dense reverb. If you're unsure, export the same short block at both settings and compare them blind on the headphones you normally use.
When to choose WAV
WAV stores the decoded audio without compression. It's the right choice when your clip is an intermediate step: going into a video editor, a music production program, or another round of editing. Every MP3 encode costs a little quality, while WAV costs none, so you only compress once, at the very end.