Storage Expansion Calculator
Calculate additional disks or capacity needed to reach a target storage size.
About Storage Expansion Calculator
Expanding storage to a target capacity requires accounting for the RAID overhead of the new disks being added, not just their raw size — RAID 5 sacrifices one disk's worth of capacity to parity regardless of array size, RAID 6 sacrifices two, and mirrored levels (RAID 1/10) sacrifice half. This calculator finds the minimum number of same-size new disks, at the chosen RAID level, needed to close the gap between current and target usable capacity.
Formula
New capacity
new_capacity = current_capacity + disks_added × disk_size × raid_efficiency
- n
- disks added
- S
- size per disk
- \eta
- RAID efficiency for that level/count
Worked example
With the default values below, Storage Expansion Calculator returns New Disks Needed of 4. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Current Usable Capacity (GB)minimum 0
- 4,000
- Target Usable Capacity (GB)minimum 0
- 10,000
- New Disk Size Each (GB)minimum 1
- 2,000
- RAID Level for New Disksone of None / JBOD (100% efficiency), RAID 0 (100% efficiency), RAID 1 (50% efficiency, 2 disks), RAID 5 ((n-1)/n efficiency), RAID 6 ((n-2)/n efficiency), RAID 10 (50% efficiency)
- RAID 5 ((n-1)/n efficiency)
Results
- New Disks Needed
- 4
- New Total Usable Capacity (GB)
- 10,000.00
- Added Usable Capacity (GB)
- 6,000.00
- Raw Capacity of New Disks (GB)
- 8,000.00
- RAID Efficiency Used
- 75.0%
Step by step
- 1Values used
- 2New capacity
- 3New Disks Needed= 4
- 4New Total Usable Capacity (GB)= 10,000.00
- 5Added Usable Capacity (GB)= 6,000.00
- 6Raw Capacity of New Disks (GB)= 8,000.00
- 7RAID Efficiency Used= 75.0
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
Why does RAID 5 efficiency depend on the number of disks?
RAID 5 always dedicates exactly one disk's worth of capacity to distributed parity, so efficiency is (n-1)/n — with 4 disks that's 75% usable, but with 8 disks it rises to 87.5%, since the fixed one-disk parity cost is spread across more capacity.
Is it better to add disks to an existing array or build a new one?
Growing an existing RAID array (if your RAID controller/mdadm setup supports online reshaping) avoids downtime and data migration, but adding a separate new array is often simpler, avoids a lengthy and I/O-intensive reshape/resync operation, and lets you choose a different RAID level for the new capacity.
Does this account for hot spares?
No — this calculates usable data capacity only. If you want dedicated hot-spare disks for automatic rebuild, add their count on top of the disksNeeded result; they don't contribute to usable capacity in the array itself.