How the capacity estimate works
For N equal-size drives of S TB each, RAID 0 provides N × S; an N-way RAID 1 mirror provides S; RAID 5 provides (N − 1) × S; RAID 6 provides (N − 2) × S; RAID 10 provides N × S ÷ 2. RAID 10 requires an even drive count. This tool describes one conventional array, without hot spares.
Why TB and TiB differ
One TB is 1,000,000,000,000 bytes. One TiB is 1,099,511,627,776 bytes. Four 8 TB drives in RAID 5 yield 24 TB, or approximately 21.83 TiB, before formatting and metadata. Unit conversion does not consume storage.
Can I use different drive sizes?
This calculator assumes equal-size drives. For a conventional mixed-size array, enter the smallest drive capacity for every drive; larger drives may have unused space. SHR, RAID-Z expansion, vendor layouts, reserved blocks, snapshots and compression need separate modeling.
Is RAID a backup?
No. Redundancy can keep an array available through certain drive failures. It does not protect against deletion, ransomware, theft, fire or every rebuild failure. Keep a separate backup and verify restores.
Before buying storage
Check drive compatibility, usable bay count, workload rating, noise and backup needs. Keep free space for growth. Choose based on your workload rather than capacity alone.
Reference: Synology’s capacity examples. Vendor system and filesystem reservations are excluded here.