What this calculator does
A RAID calculator works out how much usable storage an array actually delivers once drives are combined for speed, redundancy, or both. RAID (Redundant Array of Independent Disks) spreads data across several physical drives, but the usable capacity is almost always less than the sum of the drives, because some of that raw space is spent protecting against a drive failure rather than storing files.
The trade-off is fixed by the RAID level chosen, not by anything the drives themselves decide. RAID 0 stripes data with no redundancy at all, so a single drive failure loses everything on the array; RAID 1, 5, 6 and 10 all sacrifice some capacity in exchange for the array surviving one or more drive failures. Enter the number of drives, the capacity of each, and the RAID level to see the usable capacity and how many drives can fail before data is lost.
The formula
Each RAID level has a fixed, standard formula for usable capacity. RAID 0 stripes with no parity, so usable capacity is the full sum of every drive. RAID 1 mirrors data across drives, so usable capacity equals the size of a single drive no matter how many drives are mirrored. RAID 5 reserves the equivalent of one drive for parity, so usable capacity is (n − 1) drives worth of space. RAID 6 reserves two drives worth of parity for (n − 2) drives worth of usable space, tolerating two simultaneous failures. RAID 10 mirrors pairs of drives and then stripes across those pairs, halving raw capacity to (n ÷ 2) drives worth of space.
| Term | Meaning |
|---|---|
| n | Number of physical drives in the array. |
| Raw capacity | The sum of every drive's capacity, before any redundancy overhead. |
| Usable capacity | The space actually available for storing files once redundancy overhead is set aside. |
| Fault tolerance | How many drives can fail simultaneously without the array losing data. |
The inputs explained
| Field | What to enter |
|---|---|
| Number of drives | The number of physical drives that make up the array. Some RAID levels have a minimum: 2 for RAID 0 and 1, 3 for RAID 5, 4 for RAID 6, and an even number of at least 4 for RAID 10. |
| Capacity per drive (TB) | The capacity of each individual drive. All drives in a real array should be the same size, since RAID levels other than RAID 0 treat every drive as if it matched the smallest one. |
| RAID level | The RAID level to calculate. Each has a different balance of usable capacity against how many drives can fail. |
When to use it
Sizing a NAS or server build
Before buying drives, working out the usable capacity at a given RAID level shows how many and how large the drives need to be to hit a target storage size, rather than discovering the shortfall after the array is built.
Comparing RAID levels for the same drives
The same set of drives delivers very different usable capacity and fault tolerance depending on the RAID level chosen, so comparing RAID 5 against RAID 6 or RAID 10 for an identical drive count makes the trade-off explicit.
Checking a vendor's advertised capacity
Storage vendors sometimes quote raw capacity rather than usable capacity. Recalculating from the drive count, size and RAID level independently confirms what will actually be available once the array is formatted.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How usable capacity compares across RAID levels for 6 × 4 TB drives
The same six drives, reformatted under each RAID level.
| RAID level | Usable capacity | Raw (total) capacity |
|---|---|---|
| RAID 0 | 24.00 TB | 24.00 TB |
| RAID 1 | 4.00 TB | 24.00 TB |
| RAID 5 | 20.00 TB | 24.00 TB |
| RAID 6 | 16.00 TB | 24.00 TB |
| RAID 10 | 12.00 TB | 24.00 TB |
How usable capacity in RAID 5 grows as more drives are added
A RAID 5 array with a fixed drive size, as the drive count increases.
| Number of drives | Usable capacity | Storage efficiency |
|---|---|---|
| 3 | 4.00 TB | 66.7% |
| 4 | 6.00 TB | 75.0% |
| 5 | 8.00 TB | 80.0% |
| 6 | 10.00 TB | 83.3% |
| 8 | 14.00 TB | 87.5% |
| 10 | 18.00 TB | 90.0% |
Questions
Which RAID level should I use?
It depends on the priority. RAID 0 maximises capacity and speed with no protection at all, RAID 1 and RAID 10 favour read speed and rebuild simplicity at the cost of losing half the raw capacity, and RAID 5 or RAID 6 favour capacity efficiency for arrays where a second parity drive (RAID 6) is worth the extra protection against a failure during a rebuild.
Why is usable capacity less than the sum of the drives?
Every RAID level except RAID 0 reserves some of the raw capacity to store redundant data (a mirror copy or parity information) that can rebuild the array after a drive fails. That reserved capacity is the cost of the array surviving a failure instead of losing everything.
Is RAID a backup?
No. RAID protects against a hardware failure inside the array, but it does not protect against accidental deletion, ransomware, fire, theft or a controller fault that corrupts the whole array at once. A separate backup, ideally stored somewhere else entirely, is still needed.
Why does RAID 10 need an even number of drives?
RAID 10 works by mirroring drives in pairs and then striping data across those pairs, so the drives have to divide evenly into pairs. An odd number, or fewer than four drives, cannot form the mirrored pairs the level depends on.
For a plain unit conversion between storage sizes, see the file size calculators, and for elapsed-time maths on a rebuild or backup window, the minutes to hours calculator converts between the two.