How to check podcast loudness (LUFS and true peak) on a Mac

Free: measure with ffmpeg in Terminal

ffmpeg is a free, open-source audio and video tool. It isn't built in to macOS; the usual way to install it is Homebrew (brew.sh). Its ebur128 filter measures loudness the EBU R 128 way, built on ITU-R BS.1770.

1. Install Homebrew from brew.sh if you don't have it yet, then install ffmpeg with brew install ffmpeg.
2. Go to the folder with your audio: type cd (with a space), drag the folder onto the Terminal window and press Return.
3. Measure one file with the second line below (change episode.wav to your file's name). At the end ffmpeg prints a Summary: I is the integrated loudness in LUFS, LRA the loudness range, and Peak under True peak is the true peak.
4. To check every WAV file in the folder, paste the last line (change *.wav to *.mp3 or *.m4a for other files). It prints each file name with its three readings.
5. Compare the numbers with your spec. For Apple Podcasts: I between −17 and −15, and a true peak of −1 or lower.

brew install ffmpeg
ffmpeg -hide_banner -nostats -i episode.wav -af ebur128=peak=true -f null -
for f in *.wav; do echo "$f"; ffmpeg -hide_banner -nostats -i "$f" -af ebur128=peak=true -f null - 2>&1 | grep -E '^ +(I|LRA|Peak):'; done

ffmpeg's loudnorm filter can also change the loudness, but unless you give it the values measured in a first pass it uses dynamic processing, which changes how the audio sounds. A plain gain change needs the two-pass method.

Free: Audacity's Loudness Normalization

Audacity is a free, open-source audio editor (audacityteam.org).

1. Open the file in Audacity and select all of it (Select › All).
2. Choose Effect › Volume and Compression › Loudness Normalization, choose perceived loudness, type the target (for example −16 LUFS) and apply it.
3. Export the result as a new file (File › Export Audio), so the original stays as it is.

This changes the audio rather than telling you whether it passes, so measure the new file, true peak included, before you deliver it.