Sorry for the space error:
Below a more correct example
$ btrfs filesystem disk-free /
Summary:
Total: 135.00GB
Allocated: 10.51GB
Unallocated: 124.49GB
Free_(Estimated) 86.56GB
Average_disk_efficiency: 62 %
Details:
Chunk-type Mode Allocated Used Free
---------- ---- --------- -------- ---------
Data Single 4.01GB 2.16GB 1.87GB
System DUP 16.00MB 4.00KB 7.99MB
System Single 4.00MB 0.00 4.00MB
Metadata DUP 6.00GB 429.16MB 2.57GB
Metadata Single 8.00MB 0.00 8.00MB