Decibels in plain words
Loudness is changed in decibels (dB). +6 dB doubles the amplitude of the sound wave and sounds clearly louder; +10 dB is perceived as roughly twice as loud. -6 dB halves the amplitude. The percentage next to the slider shows the same change as a factor, for example +6 dB is about 200%.
Raising the volume also raises the peaks. When a peak would go past the top of the scale, a hard cut would make it crackle. Instead, the tool lets peaks above -1 dB approach the maximum along a soft curve, so loud parts stay clean, although very large boosts on already loud files make them sound compressed.
Normalizing: same loudness for every file
Normalize measures each file first and then sets the gain needed to reach the target. Peak at -1 dB makes the file as loud as possible without clipping; the result depends on the single loudest moment. The LUFS targets measure the loudness people actually hear, averaged over the whole file the way broadcasters do it (ITU-R BS.1770 with gating). Our measurement is a close approximation, not a certified meter.
-14 LUFS is close to what YouTube and Spotify play at, -16 LUFS is common for podcasts and speech, and -23 LUFS is the European broadcast standard (EBU R128). Normalizing a whole batch of files to the same LUFS value evens out a playlist or a series of recordings. The result line shows the measured value and the change, for example from -20.3 to -14 LUFS.
Videos and formats
For videos (MP4, MOV, WebM, MKV) only the sound is changed: the picture is copied without re-encoding. Every file keeps its format; FLAC files are saved as WAV, which is also lossless, because browsers cannot write FLAC. MP3, M4A and OGG are encoded again at a high bit rate, so the quality stays close to the original. Firefox cannot create AAC sound, which M4A, MP4 and MOV need; use Chrome, Edge or Safari for those.