RAID 1 Capacity Calculator
Calculate usable capacity of a RAID 1 mirrored array from disk count and size.
About RAID 1 Capacity Calculator
RAID 1 mirrors identical copies of data across all disks in the array, so usable capacity equals a single disk's size regardless of how many mirrors exist. The trade-off is maximum redundancy — all disks minus one can fail without data loss — at the cost of the lowest storage efficiency of any standard RAID level.
Formulas
RAID 1 usable capacity
usable = disk_size (one disk's worth regardless of mirror count)
- S
- capacity of each disk
Storage efficiency
efficiency = 1/n × 100%
- n
- number of mirrored disks
Worked example
With the default values below, RAID 1 Capacity Calculator returns Usable Capacity of 2,000.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of Disks (mirrors)minimum 2
- 2
- Size per Diskminimum 0.001 GB
- 2,000 GB
Results
- Usable Capacity
- 2,000.00 GB
- Raw Capacity
- 4,000.00 GB
- Mirror Overhead
- 2,000.00 GB
- Storage Efficiency
- 50.00%
- Fault Tolerance
- Tolerates 1 disk failure(s)
Step by step
- 1Disk count (n)= 2
- 2Disk size= 2000 GB
- 3RAID 1 usable capacity= 2000 GB
- 4Fault tolerance= 1 disk failure(s)
- 5Storage efficiency= 50%
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Can RAID 1 have more than 2 disks?
Yes. A 3-way or even 4-way mirror is possible — usable capacity stays at one disk's size, but fault tolerance increases to n-1 simultaneous failures.
Does RAID 1 improve read performance?
Read throughput can improve because the controller can read from multiple mirrors in parallel. Write performance does not improve because every write must be committed to all mirrors.