How this instrument works
RAID (Redundant Array of Independent Disks) combines multiple physical drives into a single logical storage volume, trading some raw capacity for performance, redundancy, or both. Different RAID levels make that trade-off differently, so the same set of drives yields a very different amount of usable storage depending on which level you choose — the whole point of this calculator is making that difference concrete before committing to hardware.
RAID 0 (striping) uses every byte of every drive with no redundancy at all — usable storage equals the full sum of all drives (N x capacity), the maximum possible, but a single drive failure destroys the entire array. RAID 1 (mirroring) writes identical data to every drive in the set, so the usable total is capped at just one drive's size no matter how many drives are added — a heavy overhead, but you can lose all but one drive and keep your data.
RAID 5 uses distributed parity spread across all drives, reserving the equivalent of exactly one drive's worth of space for parity data regardless of array size: usable storage is (N-1) x capacity, tolerating a single drive failure. RAID 6 extends the same idea with double parity, reserving two drives' worth of space — (N-2) x capacity — to tolerate two simultaneous drive failures, at the cost of more lost storage and slower write performance than RAID 5.
RAID 10 combines mirroring and striping: drives are paired into mirrors, and those mirrored pairs are then striped together, giving usable storage of (N/2) x capacity — half the raw total, same as RAID 1's proportional cost, but spread across a striped array that also gets RAID 0's performance benefits. RAID 10 is a common choice for demanding database and virtualization workloads that need both redundancy and speed.
- Choose RAID level — RAID 0, 1, 5, 6 or 10 — from the dropdown.
- Enter Number of drives — at least 2, with practical minimums varying by level (RAID 5 needs at least 3, RAID 6 at least 4, RAID 10 an even number).
- Enter Capacity per drive (TB) — assume identical drives, as RAID arrays generally require matching (or at least equally-sized-as-configured) drives.
- Read Usable capacity (TB) for the resulting storage available to actually use, after whatever redundancy overhead the chosen level requires.
- Compare levels with the same drive count and capacity to see the capacity-vs-redundancy trade-off directly — RAID 0 will always show the highest number, RAID 1 the lowest.
Worked example — RAID 5 with four 4TB drives
Choose RAID level = RAID 5, Number of drives = 4, Capacity per drive (TB) = 4. Usable capacity (TB) reads 12.0. Working through the formula: (4-1) x 4 = 3 x 4 = 12TB.
This is the textbook RAID 5 example: four drives, one drive's worth of space (4TB) reserved for distributed parity, leaving 12TB usable out of 16TB raw — a 25% overhead that buys tolerance for any single drive in the array failing without data loss. It's a common small-server and NAS configuration precisely because that overhead drops as a percentage with more drives, while the redundancy benefit (surviving one drive failure) stays exactly the same.
Questions
Which RAID level gives the most usable storage?
RAID 0 always gives the most usable storage for a given set of drives, since it uses every drive's full size with no redundancy overhead at all — usable storage equals N x capacity, the theoretical maximum. The tradeoff is that RAID 0 offers zero fault tolerance: a single drive failure in the array causes total data loss across the whole volume, which is why RAID 0 is generally reserved for scratch or cache storage rather than anything you can't afford to lose.
Why does RAID 5 lose exactly one drive's worth of capacity, no matter how many drives are in the array?
RAID 5 distributes parity data (information that lets the array reconstruct any single missing drive) evenly across all drives rather than dedicating one whole physical drive to it, but the total amount of parity data stored still adds up to exactly one drive's worth of capacity regardless of how many drives are in the array. That's why the formula is (N-1) x capacity: as N grows, that one-drive parity cost becomes a smaller percentage of the total, which is part of why RAID 5 scales more efficiently with larger arrays than RAID 1 or RAID 10 do.
What's the practical difference between RAID 5 and RAID 6?
RAID 5 tolerates exactly one drive failure using single parity, reserving one drive's worth of space: (N-1) x capacity. RAID 6 tolerates two simultaneous drive failures using double distributed parity, reserving two drives' worth of space: (N-2) x capacity — less usable storage for the same drive count, but meaningfully safer, especially for large arrays where a second drive failing during a long rebuild after the first failure is a real risk. Many storage administrators prefer RAID 6 over RAID 5 specifically because rebuild times on today's large drives can stretch long enough for that second-failure risk to matter.
Why does RAID 1 usable capacity stay the same no matter how many drives you add?
RAID 1 mirrors identical data across every drive in the set — adding a third or fourth mirror drive just adds more redundant copies of the same data, not more usable space. Usable storage is always exactly one drive's worth of space, regardless of array size, which is why RAID 1 is inefficient for large arrays but is still chosen for small, critical two-drive setups (like a boot volume) where simplicity and straightforward recovery matter more than storage efficiency.
Why is RAID 10 usable capacity half of the raw total, same as RAID 1?
RAID 10 is built from mirrored pairs (each pair works exactly like RAID 1, one drive's worth of space lost to redundancy) that are then striped together (RAID 0's approach for combining the mirrored pairs). Because the mirroring layer always costs exactly half the space, RAID 10's usable-storage formula, (N/2) x capacity, produces the same 50% overhead as plain RAID 1 — but spread across a striped array that adds RAID 0-like read/write performance on top, which is the actual reason it's chosen over plain RAID 1 for demanding workloads despite an identical overhead.