SAN Capacity Calculator
Calculate usable SAN capacity after RAID overhead, thin provisioning and reserved space.
About SAN Capacity Calculator
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
Worked example
With the default values below, SAN Capacity Calculator returns Usable Physical Capacity of 71.25. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Raw SAN Capacityminimum 0.1 TB
- 100 TB
- RAID Overheadaccepts 0–80 %
- 25 %
- Reserved Space (hot spare, metadata)accepts 0–50 %
- 5 %
- Thin Provisioning Ratioaccepts 1–10
- 1.5
Results
- Usable Physical Capacity
- 71.25 TB
- Provisionable (with thin)
- 106.88 TB
- RAID Overhead
- 25.00 TB
- Reserved Space
- 3.75 TB
Step by step
- 1After RAID overhead= 75 TB
- 2After reserved space= 71.25 TB
- 3Provisionable (thin)= 106.88 TB
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.