By LabBench · Calculation review: October 19, 2026. These examples are arithmetic, not measurements of a particular NAS. They cover conventional arrays of equal-size drives, without hot spares.
Start with four 8 TB drives
The labels describe 32 TB of raw capacity: 4 × 8 TB. In RAID 5, the equivalent of one drive is used for parity, leaving (4 − 1) × 8 = 24 TB usable before system and filesystem overhead. That is the first reduction. It buys a specific form of drive-failure tolerance; it is not free space lost to unit conversion.
Open the four-drive RAID 5 example →
24 TB is about 21.83 TiB
A decimal TB contains 10¹² bytes. A binary TiB contains 2⁴⁰ bytes. Divide 24,000,000,000,000 by 1,099,511,627,776 to get approximately 21.83 TiB. These two figures describe the same number of bytes. Conversion does not consume storage. Some operating systems display a binary quantity with a label that looks like a decimal unit, which makes comparisons harder.
A smaller number after formatting can reflect both unit presentation and actual reservations. Check the units first, then the system allocation. Do not count the same reduction twice.
Compare the array layout before comparing the display
| Layout | Usable TB | Approx. TiB | Protection limit |
|---|---|---|---|
| RAID 0 | 32 | 29.10 | No drive-failure protection |
| RAID 1, four-way mirror | 8 | 7.28 | Any three drives may fail |
| RAID 5 | 24 | 21.83 | One drive failure |
| RAID 6 | 16 | 14.55 | Two drive failures |
| RAID 10 | 16 | 14.55 | One failure per mirror pair |
These are drive-failure tolerances, not guarantees against data loss. RAID 10 can survive two failures if they occur in different pairs, but lose the array if both drives of one pair fail. A four-way RAID 1 mirror is a specific layout; verify that your NAS actually offers it.
Reserve room for growth and ordinary operation
Suppose your current dataset is 12 TB and you plan for 20% growth: 12 × 1.20 = 14.4 TB. If you also choose to keep 20% of usable capacity free, the planning capacity is 14.4 ÷ 0.80 = 18 TB, before any additional vendor or filesystem reservations. A 24 TB RAID 5 array passes that simple capacity budget; a 16 TB RAID 6 array does not.
The 20% figures are editable planning assumptions, not universal requirements. Snapshots, retention, metadata, reserved blocks and rebuild behavior depend on the software and workload. Apply your platform's guidance and include backup storage separately.
Three checks before ordering drives
- Confirm the required RAID layout, supported drive sizes and available bays with the enclosure vendor.
- Budget in one unit throughout. Include system reservations, growth and retained snapshots.
- Plan a separate recoverable backup. Redundancy does not undo deletion or protect the only copy from ransomware, theft or fire.
For mixed-size drives, the simple conventional model generally uses the smallest member's capacity. SHR and other specialized layouts may reclaim space differently and are outside this calculator's model.
Sources: Synology capacity formulas and reservations; NIST decimal and binary prefix reference. Vendor behavior must be checked for your actual system.
Calculate your own RAID capacity → · Next: estimate the backup window →