SAN Capacity Calculator
Calculate usable SAN capacity after RAID overhead, hot spare reservation, and metadata overhead.
Inputs
Total raw disk capacity before RAID, spares, or metadata overhead
Capacity lost to parity/mirroring (e.g. ~25% for RAID 6 in an 8-disk group, 50% for RAID 10)
Reserved for array metadata, snapshots reserve, and filesystem structures
Usable Capacity
68.87TB
Usable Efficiency
68.9%
Total Overhead (RAID + spare + metadata)
31.13TB
Capacity After RAID Overhead
75.00TB
Step by step
Values used
Raw Capacity = 100 TB; RAID Overhead = 25; Hot Spare Reservation = 4 TB; Metadata / Filesystem Overhead = 3
Usable capacity
usable = (raw × (1 − raid_overhead%) − hot_spare) × (1 − metadata_overhead%)
Usable Capacity
= 68.87 TB
Usable Efficiency
= 68.9
Total Overhead (RAID + spare + metadata)
= 31.13 TB
Capacity After RAID Overhead
= 75.00 TB
How it works
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.
Formula
Usable capacity
usable = (raw × (1 − raid_overhead%) − hot_spare) × (1 − metadata_overhead%)
- R
- Raw capacity
- o_{raid}
- RAID overhead fraction
- S_{hot}
- Hot spare reservation
- o_{meta}
- Metadata overhead fraction
Frequently Asked Questions
Why does RAID overhead vary so much between RAID levels?
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%.
Do I need more than one hot spare?
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.
Why is metadata overhead applied after the hot spare instead of on raw capacity?
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.