The absolute lowest MP3 bit rate you should consider is 128kbps. This was often referred to as being CD quality, but it's far from being so. This bit rate will allow you to get much more music on to your MP3 player but you'll sacrifice a great deal of audio quality as a result. If you use the headphones that came with your player and don't listen to that much music, 128kbps will probably suffice for you.

The happy medium between small file sizes and true CD quality audio is a 256kbps bit rate. This offers vastly superior sound quality over 128kbps, without eating up too much disk space. The difference in quality between 128kbps and 256kbps is vast and immediately obvious -- your favourite tunes encoded at 256kbps will sound stunning in comparison to the same tracks encoded at 128kbps. Consider this bit rate if you own a player with 4GB of memory or above, or if you only carry a few albums with you at any one time.

At the top end of the scale we get into what is known as 'lossless' audio. 128kbps, 256kbps and similar bit rates are known collectively as 'lossy' formats. This is because in order to reduce the size of a music file, the MP3 encoder literally throws away data from the original recording. Lossless encoding throws away nothing -- your music file is identical in quality to that of the original CD. Although lossless audio is still compressed, it's done so using a complex algorithm that retains every single bit of data while still managing to reduce file sizes. However, whereas an average song encoded at 128kbps will be around 3.5MB, the same song in lossless format will be around 30MB. Lossless is the crème de la crème of audio compression but not all MP3 players support the formats. Check out our MP3 & Digital Music Reviews section to find a player that supports the wonderful lossless compression formats, such as FLAC or Apple Lossless.

One final consideration should be to encode MP3s using a 'variable bit rate' instead of a 'constant bit rate'. Using VBR allows the MP3 encoder to use lower bit rates during quieter or less complex parts of a song, and significantly higher bit rates during more complex sections. This allows you to have a file of greater quality but without pushing file sizes up too much.

Good luck!