Calculator Tools
RAID Calculator
Calculate usable RAID capacity, redundancy, efficiency, and read/write speed for RAID 0, 1, 5, 6, 10, and JBOD. Set drive count and size to plan an array.
RAID storage calculator
Array setup
RAID level
Striping with single distributed parity. One drive of capacity is spent on parity, and the array survives any one drive failing. Needs at least three drives.
Drive makers label capacity in decimal TB and GB. The calculator assumes identical drives, since a RAID array is limited by its smallest drive.
Result
Usable capacity
12 TB
10.91 TiB as reported by Windows and most NAS dashboards
Raw capacity
16 TB
Total of every drive
Lost to redundancy
4 TB
Spent on mirrors or parity
Storage efficiency
75%
Usable out of raw
Fault tolerance
1 drive
Survives one drive failure. A second failure during rebuild loses data.
Approximate performance
Read speed
~3x
Reads stripe across the data drives.
Write speed
~1x
Each write updates parity, so small writes carry a read-modify-write penalty.
Multipliers are relative to one drive and approximate. Real throughput depends on the controller, cache, and access pattern.
RAID level comparison
How each level trades usable space against protection and speed. Values assume identical drives.
| Level | Min drives | Usable | Drives can fail | Best for |
|---|---|---|---|---|
| 2 | 100% | 0 | Scratch space and speed where data loss is acceptable. | |
| 2 | 1 drive | All but one | Two-drive boot or backup volumes that must stay readable. | |
| 3 | N - 1 drives | 1 | General storage that balances capacity and protection. | |
| 4 | N - 2 drives | 2 | Large arrays where a slow rebuild risks a second failure. | |
| 4 | 50% | 1 per mirror | Databases and busy workloads needing speed plus redundancy. | |
| 2 | 100% | 0 | Pooling odd-sized drives into one large volume. |
Why usable capacity is lower than the box says
Two things shrink the headline number. First, redundancy: mirroring and parity reserve part of every array so it can survive a failed drive, which is the trade this calculator measures. Second, decimal versus binary units: drive makers count a terabyte as 1,000,000,000,000 bytes, while Windows and many NAS dashboards divide by 1024 at each step and call the result a TB, so a 4 TB drive shows up as about 3.64 TiB. The result above lists both so the figure matches whatever your system reports.
Mixed drive sizes
Standard RAID treats every drive as if it were the size of the smallest one, so two 4 TB drives and two 8 TB drives behave like four 4 TB drives and the extra space on the larger pair is wasted. That is why this calculator asks for a single per-drive size. If your drives differ, enter the smallest size to get the capacity a hardware or software RAID controller will actually present.
How to use
- Choose a RAID level, such as RAID 5 or RAID 10.
- Enter the number of drives, or tap a preset like 4 or 8.
- Type the size of one drive and pick the unit (TB, GB, TiB, or GiB).
- Read the usable capacity, efficiency, and fault tolerance in the result panel.
- Copy the summary, or open the comparison table to weigh other RAID levels.
About this tool
RAID Calculator works out what a disk array actually gives you once redundancy is taken into account. Pick a RAID level, enter how many drives you have and the size of each one, and the tool reports the usable capacity, the raw capacity (every drive added together), the space lost to mirrors or parity, the storage efficiency as a percentage, and the fault tolerance, which is how many drives can fail before data is lost. Each common level is covered: RAID 0 stripes for speed with no protection so all capacity is usable; RAID 1 mirrors every drive so usable space equals one drive but the array survives until a single drive remains; RAID 5 spends one drive on distributed parity and survives one failure; RAID 6 spends two drives on parity and survives two failures, which matters on large arrays where a rebuild takes days; RAID 10 stripes mirrored pairs for half the capacity with fast reads and protection against at least one failure; and JBOD spans drives into one volume with no redundancy. The result is shown in both decimal terabytes, the way drive makers label capacity, and binary tebibytes, the way Windows and many NAS dashboards report it, so the figure matches whatever your system shows. Approximate read and write speed multipliers relative to a single drive are included for each level, along with a comparison table and notes on why usable space is always lower than the box says and how mixed drive sizes are handled. Every value is computed in your browser with plain arithmetic; the numbers you enter are never uploaded.
Free to use. Works in your browser. No signup, no login.
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