Source: https://developers.google.com/actions/tools/audio-loudness
Source: http://k.ylo.ph/2016/04/04/loudnorm.html (Credits: k@ylo.ph | Source discovered by Mulvya)
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./ffmpeg -i /path/to/input.wav -af loudnorm=I=-16:TP=-1.5:LRA=11:print_format=summary -f null -This instructs FFmpeg to measure the audio values of your media file without creating an output file. You will get a series of values presented as follows:
Input Integrated: -27.2 LUFS Input True Peak: -14.4 dBTP Input LRA: 0.1 LU Input Threshold: -37.7 LUFS Output Integrated: -15.5 LUFS Output True Peak: -2.7 dBTP Output LRA: 0.0 LU Output Threshold: -26.2 LUFS Normalization Type: Dynamic Target Offset: -0.5 LUThe sample values above indicate important information about the incoming media. For instance, the
Input Integratedvalue shown indicates audio that is too loud. TheOutput Integratedvalue is much closer to -16.0. Both theInput True PeakandInput LRA, or loudness range, values are higher than our provided ceilings and will be reduced in the normalized version. Finally, theTarget Offsetrepresents the offset gain used in the output. - Run a second pass of the loudnorm filter, supplying the values from step 1 as “measured” values in the loudnorm options.
./ffmpeg -i /path/to/input.wav -af loudnorm=I=-16:TP=-1.5:LRA=11:measured_I=-27.2:measured_TP=-14.4:measured_LRA=0.1:measured_thresh=-37.7:offset=-0.7:linear=true:print_format=summary output.wav
bytes and bones 
Hi, first of all thanks for this guide. It has become a reference for me when I started using ffmpeg to control the loudness of my mixes from video productions to music mixes. It has been a great reference during my research.
Just a short correction though; you do not need to type in the target levels on first pass… loudnorm=print_format=summary works exactly the same. I have tried and tested that this morning.
I have seen it that way on the Google developer’s page as well and I honestly don’t know why they wrote the unnecessary options, but they lack a “suggest” feature on their site (weird, right?) so here I am. Thanks to update so Noobs like me don’t waste their time on that first line at least, lol.
Huge thx for your comment! I really appreciate your contribution, it’s very useful for me. And it feels good to share some hardly collected knowledge. :) Wish you good luck in normalizing your environment. :)))
Axel
@Alexandre Pigeot
Google seems to have copied that section from the filter author’s blog. See http://k.ylo.ph/2016/04/04/loudnorm.html
Thank you very much for the info.