Pick your RAID level, drive count, and drive size. See usable capacity, parity overhead, fault tolerance, and a realistic rebuild time.
Drive manufacturers sell capacity in decimal terabytes (1 TB = 1,000,000,000,000 bytes), but operating systems generally report in binary tebibytes (1 TiB = 1,099,511,627,776 bytes). That's roughly a 9.3% loss before any RAID parity. Our "usable after format" number reflects that conversion plus filesystem overhead (we use ~93% of the post-RAID raw figure as a conservative estimate for ZFS, ext4, and btrfs).
Rebuild time is estimated at a sustained 150 MB/s, which is a realistic average for modern HDDs reading sequentially during a rebuild. Real rebuild times vary wildly depending on array activity, drive health, and pool fragmentation — for ZFS rebuilds (resilvers) on a mostly-empty pool, you may see far better; on a 90%-full pool with active workloads, much worse.
Synology Hybrid RAID (SHR) is the only level here that handles mixed drive sizes efficiently. With same-size drives, SHR-1 yields the same capacity as RAID 5 and SHR-2 matches RAID 6. The advantage shows up when you have, for example, two 8TB and two 16TB drives — SHR will use more of that capacity than traditional RAID, which is forced to treat all drives as the smallest size.