An MP3's bitrate is how much data it uses for each second of sound, measured in kilobits per second (kbps). More bits let the encoder keep more of the original detail, and make the file bigger. Somewhere between 128 and 192 kbps, the extra detail stops being something people can hear - which is why picking the highest number isn't always the smart choice.
#How much space each bitrate takes
The maths is simple: divide the bitrate by 133 to get megabytes per minute. A 192 kbps MP3 uses about 1.4 MB a minute, so a 4-minute song is under 6 MB.
Common MP3 bitrates compared
| Bitrate | Per minute | 4-minute song | Per hour | What it sounds like |
|---|---|---|---|---|
| 64 kbps (mono) | 0.5 MB | 1.9 MB | 29 MB | Fine for speech, thin for music |
| 128 kbps | 1.0 MB | 3.8 MB | 58 MB | Good on phone speakers; flaws can show on good headphones |
| 192 kbps | 1.4 MB | 5.8 MB | 86 MB | Sounds like the original to nearly everyone |
| 256 kbps | 1.9 MB | 7.7 MB | 115 MB | Sounds like the original |
| 320 kbps | 2.4 MB | 9.6 MB | 144 MB | MP3's maximum; rarely audibly better than 256 |
#Can you actually hear the difference?
Up to a point. MP3 works by throwing away sound your ears are unlikely to notice, and at low bitrates it has to throw away more than that. At 128 kbps, careful listeners on good headphones can often pick out flaws on demanding material: cymbals that sound swishy, applause that turns watery, sharp "s" sounds that smear. On phone speakers, in a car or on a train, most people can't.
From around 192 kbps up, blind tests - where listeners switch between the original and the MP3 without knowing which is which - find that almost nobody can reliably tell them apart. The audio community at Hydrogenaudio describes the LAME encoder's high-quality settings, averaging about 175-245 kbps, as normally transparent: indistinguishable from the source. Going from 256 to 320 kbps mostly buys you a bigger file.
#CBR vs VBR
- CBR (constant bitrate) uses the same number of bits every second, whether the music is a quiet piano note or a full orchestra.
- VBR (variable bitrate) spends more bits on complicated passages and fewer on simple ones. For the same average size it sounds as good or better, so it's what audio enthusiasts recommend.
The widely used LAME encoder labels its VBR settings V0 to V9. V0 averages about 245 kbps, V2 about 190 kbps, V4 about 165 kbps and V6 about 115 kbps. A few very old players showed the wrong length for VBR files, but anything made in the last fifteen years handles them fine.
#Converting to a higher bitrate doesn't help
Once an MP3 has been encoded at 128 kbps, the discarded detail is gone. Converting it to 320 kbps - or to a lossless format like WAV or FLAC - produces a bigger file that sounds exactly the same. To get better quality, go back to a better source: the original recording, a CD or a lossless download.
#Which bitrate to choose
- Music you'll listen to anywhere: 192 kbps VBR (LAME V2) or higher. If storage isn't a concern, 256 kbps or V0.
- A music collection you want to keep: store it lossless (FLAC or ALAC) and make MP3s from that when you need them.
- Podcasts and speech: 64-96 kbps mono is plenty for a single voice; 128 kbps stereo if there's music or several people.
- Audiobooks: 64 kbps mono keeps long books small without hurting a narrator's voice.
For comparison, Apple Music's standard streams use AAC at 256 kbps, a slightly more efficient format than MP3 (see M4A vs MP3).
#What Wave uses
Wave writes MP3s with the LAME encoder at its V2 setting - variable bitrate averaging about 190 kbps, one of the presets Hydrogenaudio lists as normally transparent. When the source audio is already an MP3, Wave copies it out untouched instead of re-encoding it. Try it with MP4 to MP3 or the MP3 converter for any file; nothing is uploaded.
#The short version
- MP3 size per minute is about the bitrate divided by 133: 1.4 MB a minute at 192 kbps.
- 128 kbps can show flaws on good headphones; from about 192 kbps, almost nobody can tell an MP3 from the original.
- VBR gives better sound for the size. LAME V2 (about 190 kbps) is a sensible default.
- Re-encoding a low-bitrate MP3 at a higher bitrate doesn't improve it.