RAID 1 Calculator
RAID 1 writes identical data to every drive in the mirror. Usable capacity is one drive, and the array keeps running as long as one copy survives.
RAID 1 at a glance
- 2 × 1.92TB in RAID 1 = 1.92TB usable (1.75 TiB), 50% of raw capacity; survives 1 drive failure.
Identical drives; decimal TB as drives are sold, binary TiB as most operating systems show it; before filesystem overhead. Hot spares are extra drives and add no capacity.
RAID 1 usable capacity by drive count
| Drives | 1.92TB | 3.84TB | 8TB | 12TB | 16TB | 24TB |
|---|---|---|---|---|---|---|
| 2 | 1.92TB | 3.84TB | 8TB | 12TB | 16TB | 24TB |
| 3 | 1.92TB | 3.84TB | 8TB | 12TB | 16TB | 24TB |
Usable capacity in decimal TB, before filesystem overhead. — means the drive count is not valid for RAID 1.
RAID 1 formula
Usable capacity = S (one drive). Minimum 2 drives. Fault tolerance: n − 1 drives. Example: 2 × 1.92TB = 1.92TB (1.75 TiB).
Two drives or more
Most hardware RAID controllers build RAID 1 from exactly 2 drives and call a larger mirrored set RAID 10. Software RAID (Linux mdadm, for example) can keep three or more copies; each extra drive adds a copy, not capacity.
Typical use
Boot and operating-system volumes, where capacity needs are small and a single drive failure must not stop the server.
1.92TB drives for this array
In-stock 1.92TB drives from our catalogue, with the price for the 2 drives in the example above. Hot spares are extra.
- 005051748 - EMC 1.92TB SAS 12Gb/s 2.5-inch Solid State Drive $154.95 each · 2 drives $309.90
- HPE Q8F87B 1.92TB SATA 6Gb/s 2.5-inch Solid State Drive $349.47 each · 2 drives $698.94
- MTFDDAK1T9TDC-1AT1ZABYY Micron 5200ECO Series 1.92TB 2.5 inch SATA3 TCG Disabled $447.37 each · 2 drives $894.74
- MZ1LB1T9HALS-000FB - Samsung PM983 Series 1.92TB Triple-Level Cell PCI Express 3.0 x4 $461.84 each · 2 drives $923.68
- WPP9G - Dell 1.92Tb Pcie 4.0 2.5 inch Solid State Drive $661.05 each · 2 drives $1,322.10
- MZ-ILT960A - Samsung PM1643 Series 1.92TB Triple-Level Cell SAS 12Gb/s 2.5-inch Solid $212.63 each · 2 drives $425.26
- THNSN81Q92CSE - Toshiba 1.92TB Multi-Level Cell SATA 6Gb/s 2.5-inch Read Intensive Solid $412.63 each · 2 drives $825.26
- MZ-QLW1T90 - Samsung PM963 Series 1.92TB Multi-Level Cell PCI-Express 3.0x4 2.5-inch $465.26 each · 2 drives $930.52
Frequently asked questions
What is the usable capacity of RAID 1 with two 4TB drives?
4TB (3.64 TiB). The second drive holds a copy of the first.
Can RAID 1 give me more than one drive of capacity?
No. To mirror more capacity, use RAID 10, which stripes across several mirrored pairs.
How many drive failures can RAID 1 survive?
All but one: a 2-drive mirror survives 1 failure, a 3-way mirror survives 2.
RAID calculators by level
- RAID calculator (all levels)
- RAID 0 calculator
- RAID 5 calculator
- RAID 6 calculator
- RAID 10 calculator
- RAID 50 calculator
- RAID 60 calculator
RAID planning guides
- How Many Drives Can Fail in Each RAID Level?
- RAID Hot Spare: What It Does and How It Affects Capacity
- TB vs TiB: Why a 24TB Drive Shows 21.8 TiB
- RAID Write Penalty by Level
- Best RAID Level for 2, 3, 4, 5, 6 or 8 Drives
- RAID 50 vs RAID 60
- RAID 6 vs RAID 60
Related
Updated 2026-10-22.