LUFS (Loudness Units relative to Full Scale) is the unit for how loud audio sounds to a listener on average, measured with a standard method (ITU-R BS.1770) that weights frequencies the way the ear hears them.
A track at -9 LUFS integrated is turned down 5 dB on Spotify's default setting (-14 LUFS). An episode at -19 LUFS is 3 LU under Apple Podcasts' -16 LUFS.
Measure your own file's LUFS — in your browser, nothing uploaded.
Also used in: Track Ready Checker, Episode Ready Checker, Audio Polish
Peak meters only tell you how close a file gets to distortion; two files with the same peak can sound very different in loudness. LUFS measures what listeners actually hear, which is why streaming services use it to set every track to the same level.
Spotify plays music at -14 LUFS on its default setting, and Apple Podcasts asks for about -16 LUFS. Knowing your file's integrated LUFS (the average over the whole track or episode) tells you whether a service will turn it down, turn it up, or leave it alone, and whether pushing a master louder will gain you anything at all.
A singer masters a single at -8 LUFS integrated because it sounded powerful in the studio. On Spotify's default setting it is turned down by about 6 dB to -14 LUFS, so it plays no louder than a gentler master, and the heavy limiting used to get it that loud now just sounds flat next to tracks with more dynamics.
A podcaster's episode measures -23 LUFS; Apple Podcasts asks for -16, so it will sound quiet next to other shows until it is brought up.
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