No worries lol its confusing for me too
If we assume PS4/5 OS uses GiB (base 2 aka binary) then we get this
PS4 500 GB = 465 GiB and we see ~ 405 GB free space in UI, so OS uses 60 GiB roughly
PS4 Pro 1 TB = 930 GiB and we see ~ 870 GB free space in UI, so OS uses 60 GiB roughly.
PS5 825 GB = 768 GiB and we see 664 GB free space in UI, so OS uses 104 GiB.
Looking back,
J
jasondefaoite
was correct in pointing this out to me / correcting me. My bad.
Seems PS4/5 OS uses GiB (base 2).
Seems clear because a 2 TB external USB HDD is listed as 1.84 TB in the UI.
2 TB in decimal (base 10) = 1.86 GiB. So the PS4 UI sees it as GiB (but is 1.84 not 1.86 due to formatting.)