RAID Capacity Calculator
Pick a level, set your drives, and see what the array will really report — usable capacity, efficiency, and what happens when a drive fails. Mixed drive sizes are pooled to the smallest member, which is what real controllers do and what most calculators quietly ignore.
Drive capacities — change any slot to model mixed sizes:
Raw capacity
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Usable capacity
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Efficiency
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Survives
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How the calculation works
For every level the array's usable capacity is computed from the smallest drive, because each member participates in every stripe or mirror: RAID 0 pools n × smallest; RAID 1 yields one drive's size; RAID 5 yields (n−1) × smallest; RAID 6 yields (n−2) × smallest; RAID 10 yields (n÷2) × smallest and requires an even drive count. Efficiency is usable divided by the raw sum of all drives.
Two limits are enforced rather than assumed: minimum drive counts (RAID 5 needs 3, RAID 6 needs 4, RAID 10 needs 4 and even), and honest fault tolerance — RAID 6 survives two failures anywhere; RAID 10 survives one failure per mirror pair, so two failures in the same pair still lose the array.
Rebuild time is the hidden cost the table cannot show: the larger the drives, the longer a degraded array runs exposed while the replacement rebuilds. On 20 TB drives that window is measured in days — one more reason RAID 6 earns its parity tax on big arrays.
Is RAID a backup?
No. RAID protects against a drive dropping out of the array. It does not protect against accidental deletion, ransomware encryption, file-system corruption, theft, fire, or a failed rebuild — all of those propagate to every drive in the array. Keep at least one independent backup copy on separate hardware, ideally off-site.
Can I mix drive sizes in one array?
Technically yes with most controllers, but usable capacity pools to the smallest drive, so the difference is wasted. The calculator applies that rule rather than adding labels together. Match drive sizes where you can.
Why does the calculator reject two drives for RAID 5?
RAID 5 needs at least three drives by definition: two hold data and the parity that survives one failure has to live somewhere. Two drives is RAID 1 territory.