Estimate data used streaming music.
Working music data out per track rather than per hour matches how people actually listen and makes the quality setting's effect obvious. A three and a half minute track is about 8 MB at 320 kbps, 4 MB at 160 kbps and roughly 37 MB in lossless. The tracks-per-gigabyte figure turns that into a number worth remembering: around 128 tracks per gigabyte at high quality.
Data per track from bitrate
MB per track = kbps x track minutes x 60 / (8 x 1024); monthly GB = MB per track x tracks per day x active days / 1024
Spotify offers 24, 96, 160 and 320 kbps; Apple Music streams 256 kbps AAC as standard; and lossless tiers run 1411 kbps for CD quality and higher for hi-res. Each step up roughly doubles the data.
Exactly half. For someone streaming 45 tracks a day that is a reduction from about 10.6 GB a month to 5.3 GB, which is often the difference between fitting a plan and not.
Yes, substantially, if you replay them. Downloading over wifi and playing offline removes the traffic from your mobile plan entirely, and it also avoids re-downloading the same track every time it comes up in a rotation.