How much smaller will my MP3 get?
An MP3's size is almost entirely its bitrate multiplied by its length, so you can work it out before you start. One minute of audio takes about this much space at each setting:
- 160 kbps — about 1.2 MB per minute
- 128 kbps — about 0.96 MB per minute
- 96 kbps — about 0.72 MB per minute
- 64 kbps — about 0.48 MB per minute
- 48 kbps — about 0.36 MB per minute
A worked example
A four-minute song downloaded at 320 kbps is around 9.6 MB. Compressed to 128 kbps it drops to roughly 3.8 MB, and to 64 kbps about 1.9 MB. Choosing Mono halves the bitrate again, which is why it's the biggest single saving for voice recordings.
Compression only helps when the target is lower than the original. If your file is already 128 kbps, re-encoding it at 160 kbps makes it larger without making it sound any better.
Which bitrate should I choose?
Match the bitrate to what's in the file rather than picking the smallest number:
- Music you'll listen to: 128 kbps stereo. Most people can't tell it from the original on phone speakers or earbuds.
- Podcasts and interviews: 64 kbps, Mono on.
- Voice notes, lectures and audiobooks: 48–64 kbps, Mono on.
- Background music for a website or app: 96 kbps stereo.
Does compressing an MP3 lose quality?
Yes, a little. MP3 is a lossy format, so each re-encode throws away detail the encoder judges you're least likely to hear. At 128 kbps and above the difference is hard to notice; below 64 kbps music starts to sound thin and swishy, though speech stays perfectly clear.
Because quality drops a bit every time, keep your original and compress from it if you need a different size later — don't compress an already-compressed copy again.
Getting an MP3 under an email size limit
Gmail and Outlook cap attachments at 25 MB and 20 MB respectively. To fit, divide the limit by the length in minutes and pick the next bitrate down: a 30-minute recording at 64 kbps Mono is about 7 MB, comfortably under either limit. If you only need part of the recording, cut it first with the Audio Trimmer — a shorter file is smaller at any bitrate.