RAID Calculator
Compare usable drive capacity, redundancy overhead and drive-failure tolerance for five common RAID layouts.
Usable capacity before formatting
RAID is not a backup. Equal drive capacities are assumed; with mixed drives, use the smallest supported capacity. Formatting, filesystem reserves, controller metadata, spares and rebuild risk are excluded.
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How this calculator works
RAID 0 stripes across all drives with no redundancy. RAID 1 here mirrors the entire set, leaving one drive of usable capacity. RAID 5 reserves one drive-equivalent for distributed parity; RAID 6 reserves two. RAID 10 stripes mirrored pairs and requires an even drive count. Its failure tolerance depends on which drives fail: losing both members of any mirror pair can lose the array even if other pairs remain intact.
Formula
For N equal drives of capacity C: RAID 0 = NC; RAID 1 = C; RAID 5 = (N−1)C; RAID 6 = (N−2)C; RAID 10 = NC/2. Redundancy overhead = raw − usable capacity.
Assumptions & limitations
RAID is not a backup. Equal drive capacities are assumed; with mixed drives, use the smallest supported capacity. Formatting, filesystem reserves, controller metadata, spares and rebuild risk are excluded.
Worked example
Four 4 TB drives in RAID 5 provide 16 TB raw and 12 TB usable before formatting. The 4 TB difference is parity overhead; one drive may fail without losing the array under the modelled layout.
Our approach to calculations ↗Common questions
Can RAID 10 survive two failed drives?
Sometimes. Failures in different mirror pairs may be tolerated; two failures in the same pair can lose the array. Only one arbitrary drive failure is guaranteed.
Why is RAID not a backup?
Redundancy does not protect against deletion, corruption, theft or all common-mode failures. Keep separately recoverable backups.
What if the drives have different capacities?
This model uses equal effective capacity. Enter the smallest usable member capacity and check your controller or filesystem for its actual layout rules.
Does RAID 1 with more than two drives mean multiple mirror pairs?
Not in this tool. It means a single mirrored set. Use RAID 10 for striping across two-drive mirror pairs.