The MP3 target is fixed at 192 kbps
PrivateMP3 does not present 128, 256 or 320 kbps buttons. Every MP3 output is requested from the encoder at 192,000 bits per second. The number describes the target data rate of the new MP3 stream; it does not describe the bitrate, sample rate or quality of the source track.
Cuts affect how many seconds are encoded, not the target bitrate. Changing from MP3 to M4A or WAV changes the output method, while selecting a different audio track changes the source being processed.
Calculate the data portion from duration
The estimate beside Convert uses target bitrate divided by eight, multiplied by the kept duration. Using decimal megabytes, the calculation is:
The audio file size calculator does this arithmetic for any duration, and works backwards from a size limit to the longest recording that fits.
size in MB = bitrate in kbps × seconds ÷ 8 ÷ 1000
| Reference bitrate | Per minute | Per hour |
|---|---|---|
| 128 kbps | 0.96 MB | 57.6 MB |
| 192 kbps used here | 1.44 MB | 86.4 MB |
| 256 kbps | 1.92 MB | 115.2 MB |
| 320 kbps | 2.40 MB | 144.0 MB |
The other rows are reference calculations, not controls available on PrivateMP3. Headers and encoder details can make the saved file differ slightly from the arithmetic, which is why the interface labels its number About rather than promising an exact byte count.
A long screen recording is where these per-hour figures matter most; the screen recording guide covers finding the file and its tracks, and the converter writes its audio at the same 192 kbps.
At the same total bitrate, mono does not automatically halve these figures. A mono file becomes smaller only if it is encoded at a lower total bitrate. PrivateMP3 keeps the selected track's channel count and offers no mono switch.
Measured: how close the arithmetic is
These are MP3 files the converter saved in desktop Chrome between 13 and 17 October 2026, from generated test recordings, set against the formula above. They show the same thing every time: the saved file is the arithmetic plus a small fixed amount, not a percentage.
| Source track | Output length | Arithmetic at 192 kbps | Saved MP3 | Difference |
|---|---|---|---|---|
| AAC stereo 128 kbps | 10:00 | 14,400,000 bytes | 14,400,744 bytes | +744 |
| Same, two breaks removed | 7:30 | 10,800,000 bytes | 10,800,744 bytes | +744 |
| Same, part 2 of a 5 MB split | 3:28 | 4,992,000 bytes | 4,992,936 bytes | +936 |
| Same, part 3 of that split | 3:04 | 4,416,000 bytes | 4,416,936 bytes | +936 |
| AAC stereo 96 kbps (sample recording) | 0:20 | 480,000 bytes | 481,704 bytes | +1,704 |
| Opus mono (sample recording) | 0:20 | 480,000 bytes | 481,128 bytes | +1,128 |
Two things follow. For a size limit, the formula is safe to plan with: a part sized from it came out under 5,000,000 bytes with about a kilobyte to spare, which is how the size-limit advice in File check sizes its parts. And the source does not move the MP3 size: a 128 kbps or 96 kbps stereo track and a mono track all produce 24,000 bytes per second plus that small remainder, because the encoder target is fixed.
An unchanged M4A copy is different, because it keeps the source rate. For the same 10-minute 128 kbps track the copy saved as 9,739,957 bytes. The source rate gives 9,600,000 bytes; the calculator adds about 188 bytes per second for the MP4 packet index, 112,800 bytes here, which brings the estimate to 9,712,800, 0.3 percent under the saved file. Removing part of that recording turns the copy into a 192 kbps encode, which is why cutting it can make the file larger; the guide to smaller lecture audio has that measurement.
Bitrate cannot repair the source
A higher output bitrate cannot restore clipped peaks, remove room noise or reconstruct detail discarded by an earlier lossy encode. If a video contains AAC, converting that track to MP3 first decodes the AAC and then performs another lossy encoding. If the input is already MP3, cutting and exporting MP3 creates another MP3 generation.
There is no universal bitrate at which speech or music becomes indistinguishable. The result depends on the source, encoder, audio material, listening equipment and listener. Claims that 128 kbps is always transparent, or that nobody can hear a difference above 192 kbps, are not reliable operating rules.
Choose the processing route before comparing numbers
- Use MP3 when the receiving application requires MP3 or its documented support is the safest choice. PrivateMP3 will write 192 kbps.
- Use M4A packet copy when the selected source is AAC, AC-3 or E-AC-3, the complete timeline is unchanged and the receiving application accepts M4A. The original packets are then kept instead of making a new MP3.
- Use WAV when a downstream editor or analysis program wants 16-bit PCM. WAV avoids another lossy encode but is much larger and cannot improve an already lossy source.
The practical decision is therefore not “which unsupported bitrate button should I choose?” It is whether the next application needs MP3, can accept the existing AAC in M4A, or needs PCM for editing.