Free tool
RAID Capacity Calculator
Work out the usable capacity, fault tolerance and rebuild risk of a RAID array for every common level, from RAID 0 through RAID 6 and RAID 10.
RAID is uptime, not backup
RAID keeps a server running when a disk dies. It does nothing about a deleted file, a ransomware attack, a failed controller, a fire or somebody formatting the wrong volume, and every one of those is more likely than the thing RAID protects against. An array with no separate backup is one mistake away from total loss.
The rebuild is the dangerous moment. When a disk fails in RAID 5 the array reads every sector of every remaining disk to rebuild, which is exactly the workload most likely to expose a second failure, on drives of the same age from the same batch. With large modern disks that rebuild can run for a day or more, and the array has no protection at all for the whole of it. This is why RAID 6 or RAID 10 is the sensible choice above about six terabyte disks.
Note the formatted capacity figure as well. Drives are sold in decimal terabytes and reported by the operating system in binary, which quietly removes around nine percent before you have stored anything.