btrfs: how to calculate btrfs compression space savings?

In Debian/Ubuntu:

apt install btrfs-compsize
compsize /mnt/btrfs-partition

In Fedora:

dnf install compsize
compsize /mnt/btrfs-partition

output is like this:

Processed 123574 files, 1399139 regular extents (1399139 refs), 69614 inline.
Type       Perc     Disk Usage   Uncompressed Referenced  
TOTAL       73%      211G         289G         289G       
none       100%      174G         174G         174G       
lzo         32%       37G         115G         115G

Per the btrfs wiki, there's no built-in support for this, but the compsize tool can measure compression on btrfs.

The df output will be similar to btrfs fi df in that it will report how much disk space is used, not the uncompressed size. The wiki says you can "kind of guess" by comparing df output before and after compressing a file. Another option is using some program that will total up the actual data such as rsync --stats on a dry run, which will report how much data is read.