Theory

These days audio loudness is measured in LUFS (Loudness Units relative to Full Scale). This is a relative measurement, where, as I understand, 0 LUFS is the loudest audio you’re allowed to broadcast according to the standard by European Broadcasting Union. And -10 LUFS is 10 decibel quieter than 0 LUFS. Although LUFS is a bit more involved than decibel and measures “weighted perceived loudness”.

Youtube guarantees that videos are not louder than -13 LUFS, i.e. if you upload content louder than that, it will be normalised to -13 LUFS. In terms of the loudness of actual youtube videos, I looked at couple channels and found that some of them try to be as loud as possible at -13 or -15 (maybe because that’s the loudness of youtube ads), while other channels are not that loud, around -30 (in particular, I looked at conference talks).

I didn’t come to a conclusion what is the “right” loudness level for youtube but in general the recommended broadcasting level is -23 LUFS (see EBU R128 Introduction video for details).

Practice

You can measure LUFS of a video using ffmpeg. For example,

ffmpeg -nostats -i 'video.mp4' -filter_complex ebur128 -f null -

will print few lines to console and in the end a summary with something like:

Integrated loudness:
I:         -15.9 LUFS

where “integrated” means it was calculated for the whole video, not just a part of it.

You can also download videos from youtube using youtube-dl and then run them through ffmpeg to determine their loudness.

To increase volume of a video you can do

ffmpeg -i inputfile -vcodec copy -af "volume=10dB" outputfile