LUFS metering discrepancies

You'd think meters would be such a simple thing! I have an item whose LUFS-I I'm measuring, since I want to normalize all such clips to the same LUFS-I value as a standard starting point for mixing (this is part of a film). JS Loudness Meter says the LUFS-I value is -18.7. So do Youlean and iZotope Insight. And furthermore, so does Reaper's master meter, set to give LUFS-I as its readout. I get these values by playing through the clip with it soloed. HOWEVER... when I use Extensions

A LUFS difference of 3 or 6 LU is almost always caused by channel count or pan law differences. For example a mono media that measures -18 LU will play back at -15 LU on a stereo track and -12 LU if playing back on 4 channels. So it's likely that both numbers you are seeing are correct, but measuring the loudness at different places. The simplest and most consistent way to do what you want would be to run the built-in action "Normalize items (peak/RMS/LUFS)".

Is there any consensus on a LUFS meter that everyone agrees with the results across all channel formats? I still get the impression that surround formats read higher with more channels. A 5.1 mix hitting at the same levels reads louder that stereo and a 7.1.4 mix hitting the same reads louder still. Some of this is apples vs oranges. A surround mix isn't necessarily supposed to match a stereo mix with all the levels. The idea is literally defining mix elements that are obscured in stereo and it naturally leads to more sound content overall in the file. Now, if the idea IS to match a stereo mix with equal buried mix elements and simply more separation across channels, there it is and the levels should match. Movie mixes are niche style mixes and do this backwards... the surround mix, if any, is made to fold down to an intentional stereo mix and the surround is what it is. In music, the surround mix is a main event and full audio all around. So there's some stuff to argue about! I called the style of movie mixing "niche" with the center dialog and everything else is sounds that go "zing" around the room vs music mixes with full audio in all channels. Of course surround sound for music listening is considered niche and movie watching is the mainstream. LUFS metering though. I see some desperate workarounds going on like Dolby recommending to meter 7.1.4 mixes folded down to 5.1 for the meter reading instead of 7.1.4. Kind of calls it out doesn't it!

The LUFS specification is clear about how to treat channel weightings up to 5.1 surround. Beyond that is not specified. REAPER treats any channel after channel 6 with the same weighting as the surround channels in 5.1, which seems reasonable.

All clear! Thanks.