How to use this calculator
- Enter or choose rAID level.
- Enter or choose number of drives.
- Enter or choose hot spares.
- Enter or choose drive size.
- Enter or choose size unit.
- The usable capacity updates as you type. Use Copy result or Share to keep it.
Method
Usable = Data drives × Drive size, where data drives = n (RAID 0), 1 (RAID 1), n − 1 (RAID 5), n − 2 (RAID 6), n ÷ 2 (RAID 10), n = drives − hot spares; TiB = bytes ÷ 2⁴⁰
- n
- active drives in the array
- Drive size
- decimal: 1 TB = 10¹² bytes
Worked example
- RAID 6 uses two drives’ worth of parity: 6 − 2 = 4 data drives.
- 4 × 4 TB = 16 TB = 16 × 10¹² bytes.
- 16 × 10¹² ÷ 2⁴⁰ = 14.552 TiB; 16 ÷ 24 TB raw = 66.67%.
Six 4 TB drives in RAID 6 give 16 TB (14.55 TiB) usable and survive any two drive failures.
What the result means
Leave free space for file-system metadata and snapshots, and remember RAID is not a backup: deletion, corruption and ransomware are copied to every drive.
Assumptions and limits
- All drives are the same size; with mixed sizes most RAID levels use only the smallest size on every drive.
- File-system overhead, ZFS/Btrfs specifics and vendor RAID variants (RAID-Z, SHR) are not modelled.
Questions
Why does my 4 TB drive show 3.64 TB?
The drive has 4 × 10¹² bytes. Windows divides by 2⁴⁰ and shows 3.64, but labels it “TB” rather than TiB.
RAID 6 or RAID 10?
RAID 6 survives any two failures and gives more space with many drives; RAID 10 gives better write performance and faster rebuilds but can be lost if both drives in one mirror fail.