SAN Capacity Calculator
Calculate usable SAN capacity after RAID overhead, thin provisioning and reserved space.
Inputs
Percent lost to RAID parity/mirrors
Ratio of provisioned to physical (1.0 = thick, >1 = overprovisioned)
Usable Physical Capacity
71.25TB
Provisionable (with thin)
106.88TB
RAID Overhead
25.00TB
Reserved Space
3.75TB
Step by step
After RAID overhead
100 × (1 - 0.25)
= 75 TB
After reserved space
75 × (1 - 0.05)
= 71.25 TB
Provisionable (thin)
71.25 × 1.5
= 106.88 TB
How it works
SAN usable capacity is the raw capacity minus RAID overhead (parity/mirror space), minus space reserved for hot spares, metadata, and snapshots. Thin provisioning allows allocating more logical capacity than physical — beneficial for sparse workloads but risky if actual consumption exceeds physical capacity without monitoring.
Formula
Usable capacity
usable = raw × (1 - raid_overhead%) × (1 - reserved%)
- R
- raw capacity
- p
- RAID overhead fraction
- r
- reserved fraction
Frequently Asked Questions
What is thin provisioning?
Thin provisioning allocates storage on-demand rather than reserving the full logical size upfront, allowing you to present more capacity to hosts than physically exists — at the risk of running out if actual writes exceed physical capacity.
How much should I reserve for hot spares?
Typically 1-2 disks worth of capacity per shelf (roughly 3-5% of total), plus additional space for snapshots and controller metadata depending on the SAN vendor's recommendations.