Picking a bitrate
Bitrate is how many bytes each second of audio is allowed to use. Higher means better sound and a larger file. The right choice depends almost entirely on whether you are compressing speech or music.
| Bitrate | Roughly per hour |
|---|---|
| 64 kbps mono | 29 MB |
| 96 kbps | 43 MB |
| 128 kbps | 58 MB |
| 192 kbps | 86 MB |
| 256 kbps | 115 MB |
For an interview, lecture, voice memo or podcast, 64 to 96 kbps in mono is genuinely enough and most listeners will not hear a difference. For music, start at 192.
Where the big savings are
If your file is a WAV, you are carrying uncompressed audio — roughly 10MB per minute in stereo. Converting to MP3 typically cuts that by 90 percent or more, which is why a voice recording that will not attach to an email suddenly does.
Converting mono is the other easy win. A single microphone recording has no meaningful stereo information, so storing two channels doubles the size for nothing.
Questions
Can I compress an MP3 that is already an MP3?
Yes, by re-encoding it at a lower bitrate. Be aware this is a lossy step on top of a lossy file, so quality degrades slightly more than the bitrate alone suggests. Going from 320 to 128 kbps is usually fine; going from 128 to 64 is noticeable.
Is my audio uploaded?
No. Decoding and encoding both happen in your browser, on your device.
Why does a long file take a while?
Encoding is real computation and your device is doing all of it. Expect roughly ten to thirty seconds per hour of audio on a modern laptop, longer on a phone.