Storage Array Usable Space Calculator
Follows raw disk capacity down to the space you can actually fill.
Space you can plan to use
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Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
In binary units (powers of 1,024) —
Total raw capacity —
After spares and RAID —
After file system overhead —
Share of raw capacity — %
Reading this —
The formula
usable = RAID capacity of (disks − spares) × (1 − overhead) × (1 − reserve)
Spares come off first, then the RAID level, then the overhead and the reserve.
What to watch out for
Compression and deduplication can win some back, but how much depends on the data, so they are left out here.
Frequently Asked Questions
How much of 12 × 16 TB can I use in RAID 6?
With one hot spare, 5% overhead and 20% reserve: 109.44 TB, which is 99.54 TiB.