You can try yourself: https://carfax.org.uk/btrfs-usage/

But generally, RAID1 gets you 50% of storage space, whereas RAID5 gets you 66% (and any odd combination of disks).

RAID5 should actually get you n-1 disks of space (if they're equally large), while raid1 only gets you n/2

Raid1 actually gets you less than that. If you mirror 5 drives that only gives you 1 drive worth of data (with 4 redundancies).

But… you have to weigh it against losing all your data. With raid5 if any 2 of your disks die at the same time then you've lost your volume and most likely 100% of your data (no matter how many disks you have).

With raid1 you have to lose _all_ your disks before that happens. Typically that's also just 2 but you can mirror 3 or more drives if you need some data to be _really_ resistant to disk failures and you don't care about "wasting" n-1 times the space.

So you end up trading off efficiency of storage space and resiliency to data loss.

Myself, disks got cheap enough that I always just buy 2 disks and mirror them. I find it easier to reason about overall, especially in the face of a degraded array.

We're talking about btrfs raid1 here which always gives you half of total capacity of all disks (except for extreme cases). With 5 drives it's 2.5 drive worth of data. If 2 drives die means 1 drive worth of data was lost (more or less, depends on balance).