I get half of the raw capacity, so 7.5TB. Well a bit less due to metadata, 7.3TB as reported by df (6.9TiB).

For btrfs specifically there is an online calculator [1] that shows you the effective capacity for any arbitrary configuration. I use it whenever I add a drive to check whether it’s actually useful.

1: https://carfax.org.uk/btrfs-usage/?c=2&slo=1&shi=1&p=0&dg=1&...

Here are raw outputs:

$ sudo btrfs fi df /media/storage� Data, RAID1: total=6.34TiB, used=6.22TiB System, RAID1: total=32.00MiB, used=992.00KiB Metadata, RAID1: total=11.00GiB, used=8.71GiB GlobalReserve, single: total=512.00MiB, used=0.00B $ sudo btrfs fi us /media/storage� Overall: Device size: 13.64TiB Device allocated: 12.70TiB Device unallocated: 966.54GiB Device missing: 0.00B Device slack: 0.00B Used: 12.47TiB Free (estimated): 600.31GiB (min: 600.31GiB) Free (statfs, df): 547.55GiB Data ratio: 2.00 Metadata ratio: 2.00 Global reserve: 512.00MiB (used: 0.00B) Multiple profiles: no

Data,RAID1: Size:6.34TiB, Used:6.22TiB (98.20%) /dev/sdc 2.53TiB /dev/sdk 2.54TiB /dev/sdf 837.00GiB /dev/sdb 3.40TiB /dev/sdi 3.39TiB

Metadata,RAID1: Size:11.00GiB, Used:8.71GiB (79.22%) /dev/sdc 3.00GiB /dev/sdk 4.00GiB /dev/sdf 2.00GiB /dev/sdb 6.00GiB /dev/sdi 7.00GiB

System,RAID1: Size:32.00MiB, Used:992.00KiB (3.03%) /dev/sdc 32.00MiB /dev/sdk 32.00MiB

Unallocated: /dev/sdc 198.49GiB /dev/sdk 187.49GiB /dev/sdf 92.51GiB /dev/sdb 243.02GiB /dev/sdi 245.02GiB