Calculate usable storage area network capacity after RAID overhead and reserved space.
Raw disk capacity in a SAN is reduced in three stages before it becomes usable: RAID overhead (parity or mirror copies needed for redundancy), one or more hot spares set aside to automatically rebuild after a drive failure, and metadata/filesystem overhead reserved by the array controller and file system structures. Each stage takes a bite out of what's left from the previous stage, so the order of subtraction matters — this calculator applies RAID overhead first, then reserves the hot spare from the RAID-protected capacity, then applies metadata overhead to what remains.
Usable capacity
usable = (raw × (1 − raid_overhead%) − hot_spare) × (1 − metadata_overhead%)
RAID 10 mirrors every drive, so it always costs 50% of raw capacity regardless of drive count. RAID 5 costs one drive's worth of parity per group (overhead shrinks as group size grows), and RAID 6 costs two drives' worth of dual parity for better fault tolerance — so an 8-disk RAID 6 group has roughly 25% overhead while the same group in RAID 5 has about 12.5%.
Larger disk pools and higher-capacity drives (which take longer to rebuild) increase the odds of a second failure during a rebuild window, so many SAN admins reserve one spare per 20-30 drives or per shelf rather than a single global spare.
Metadata and filesystem structures are allocated on the capacity actually presented to hosts after redundancy and spares are accounted for, not on the raw disk capacity — so applying it last reflects how storage arrays actually carve up space.