I'd like to create a parity storage space in Windows 10 with a 5 tb disk and two 3 tb disks. Is there a way to calculate how much usable space this will give me?

Yes, there is. The only redundant storage technique available that uses 3 drives is called RAID-5. This storage array allows one drive in your system to fail, while still retaining all of your data.

The basic math behind Raid-5 is
`(# of Drives * GB per drive) - ( GB per one Drive)`

Raid can only use the maximum disk space for the smallest drive you have. In your case, a typical 3TB drive is 2794 GB. This plugged into the equation gives:

**(3 * 2794) - (2794) = 5588 GB of usable storage.** (5.58 TB)

Your 5TB drive will essentially waste it's extra 2TB and be used as a 3TB drive.

While the above answer is correct for Raid-5, Windows Storage Spaces is a slightly different animal. For example, I have the following drives in a Parity Space formatted with ReFS (gotta use PowerShell in Windows 10 Pro if you want to use ReFS for Parity, as far as I can tell):

- 4x1 TB (3.63 TB usable)
- 3x2 TB (5.44 TB usable, 2.72 TB each)
- 2x3 TB (5.43 TB usable, 1.81 TB each)
- 1x1 TB (931 GB usable)

Total usable space before parity is 15.4GB (actual capacity of all drives). Total usable space after Parity is 10.2GB. Therefore, it appears in Storage Spaces, you can add up all usable space for the pool and divide by 1.5 to get an estimate for usable storage space on a Parity Pool.

Therefore, in your case you would have the following:

(2.72 * 2 + 4.54) / 1.5 = 6.65 TB of usable space.