We currently use ReplayGain, which was the most recognized standard for calculating loudness when Spotify first started.
In the future, we plan to use a new standard for calculating loudness, called ITU 1770 (from the International Telecommunication Union). This defines the integrated LUFS (Loudness Units Full Scale) measure, and it’s what we recommend you use to measure the loudness of your tracks.
ReplayGain doesn’t specify a measurement unit for loudness, so we’re unable to give an exact measure in LUFS used by ITTU 1770. However, we adjust tracks to 3 dB higher than ReplayGain algorithm specifies, which is roughly equivalent to -14 dB LUFS, according to the ITU 1770 standard."