What is LUFS?
Also called: Loudness Units relative to Full Scale
LUFS (Loudness Units relative to Full Scale) is a unit for measuring how loud audio sounds to people, not just how high its peaks reach. Streaming services and broadcasters use it to compare and balance the loudness of different songs.
A LUFS meter follows the ITU-R BS.1770 standard: it filters the audio to weight the frequencies our ears are most sensitive to, then averages the energy over time. One LU (loudness unit) equals one decibel, so a track at -9 LUFS is 5 dB louder than a track at -14 LUFS.
Meters usually show three readings. Integrated loudness is the average over the whole song and is the number platforms use. Short-term loudness uses a 3 second window and momentary loudness a 400 millisecond window, which help you see how loud individual sections are.
There is no single correct LUFS value for a master. Spotify and YouTube play music at about -14 LUFS by default and Apple Music at about -16 LUFS, so a master that is much louder than that is simply turned down. Pick a loudness that suits the genre and keeps the punch you want.
Example
A master measuring -8 LUFS integrated is turned down by about 6 dB on a service that plays at -14 LUFS, so the extra loudness you added with heavy limiting is not heard by listeners there, but the lost dynamics are.
Put it to work
Measure the integrated, short-term and momentary loudness and the true peak of your track, free in your browser.
LUFS Loudness Meter