Drive bays, protection cost, and operational headroomLive current inputs
Computer & IT
RAID Capacity Calculator
Plan RAID storage from physical drives, hot spares, parity or mirroring, filesystem overhead, free-space reserve, rebuild throughput, and failure exposure.
Active array drives —
Raw installed capacity —
RAID usable before overhead —
Operational data capacity —
Simultaneous drive tolerance —
Single-drive rebuild window —
Annual any-drive exposure —
Write penalty reference —
Array anatomy
Array anatomy
Drive bays, protection cost, and operational headroom
Drive bays identify data, protection, and hot-spare roles. The capacity rail reconciles physical terabytes with protected and deliberately unused space.
Current scenario Capacity or comparison
Result composition and constraint comparisonUpdates with every input
Planning checkpoints
Plan the array review and recovery checkpoints
Turn the capacity result into an operating plan before the first production dataset is placed on the array.
Current modelLive
Review steps6
01Checkpoint
Confirm qualified drive, controller, and firmware combinations.
02Checkpoint
Benchmark healthy and degraded throughput.
03Checkpoint
Simulate a permitted member failure and spare activation.
04Checkpoint
Record measured rebuild time and alert delivery.
05Checkpoint
Complete an independent backup restore test.
06Checkpoint
Set the occupied-capacity warning below the reserve boundary.
How to use the storage resilience planner
- Enter equal-size members using manufacturer decimal TB.
- Remove dedicated spares before evaluating the active layout.
- Use rebuild throughput measured under representative foreground load.
- Keep reserve for snapshots, rebalance work, and performance.
Storage resilience planner fundamentals
Choosing protection
RAID 0 is a stripe and provides no fault tolerance. RAID 1 and RAID 10 trade half or more of raw capacity for mirrored copies and predictable recovery.
RAID 5 preserves more capacity but has no remaining redundancy during a rebuild. RAID 6 retains a second parity margin and is often easier to defend for large, slow-rebuilding drives.
Calculation method
Separate installed hardware, RAID protection, formatting, and operating reserve
Capacity is removed in a deliberate order: dedicated spares first, then parity or mirror protection, filesystem overhead, and finally operating reserve. Rebuild time uses sustainable throughput rather than a drive-label maximum.
Detailed calculation process and general formulas
N_active = N - N_spareC_raw = N x DC_RAID5 = (N_active - 1) x DC_RAID6 = (N_active - 2) x DC_RAID10 = floor(N_active / 2) x DC_formatted = C_RAID x (1 - o)C_operational = C_formatted x (1 - r)t_rebuild = D x 10^6 / (v x 3600)P_any = 1 - (1 - AFR)^N_active Every formula above uses the current calculator's own quantities. The live worked example later on this page substitutes the entered values and reconciles the result.
Reading the analysis
Capacity units and overhead
Drive labels use decimal TB while some operating systems display binary TiB. Treat that as a unit conversion, then model filesystem metadata separately.
Operating reserve is intentionally unused capacity. It should not be merged with formatting overhead because the two deductions support different decisions.
Improving the plan
Rebuild and degraded operation
A rebuild competes with production work, reads every surviving member, and may slow for error recovery. Benchmark healthy and degraded states.
Rebuild duration is an exposure window, not necessarily downtime. The service can still miss latency objectives while the volume remains online.
Decision scale
RAID is not backup
RAID does not protect against deletion, ransomware, logical corruption, controller defects, or site loss. Maintain independent recovery copies and test restores.
Document recovery point, recovery time, replacement stock, alert routing, and the person responsible for a failed-member response.
More marginMore resilience to variation Less marginMore sensitive to assumptions
The live result above supplies the current decision point.
Scenario comparison
Same hardware under alternative RAID policies
This comparison holds the other current inputs constant so the selected policy or demand assumption remains the variable under review.
| Layout | Usable before overhead | Operational capacity | Drive tolerance | Write penalty | Minimum members |
|---|
Worked example
Your complete storage resilience planner calculation, step by step
This example follows the values currently entered above and updates whenever an input changes.
Scope and limitations
Model limitations
This model assumes equal-size conventional RAID members. Use a product-specific planner for heterogeneous disks, declustered parity, erasure coding, object storage, thin-provisioned pools, or dynamic vendor reservations.
Key terminology
Storage resilience planner glossary
- N
- installed drive count; measured in drives.
- N_spare
- dedicated hot spares; measured in drives.
- D
- smallest usable member size; measured in TB/drive.
- o
- filesystem overhead; measured in decimal.
- r
- operating reserve; measured in decimal.
- v
- effective rebuild rate; measured in MB/s.
- AFR
- annual failure probability per member; measured in decimal/year.
Important note
RAID capacity and fault tolerance do not prove backup, restore, application availability, or data durability. Confirm vendor-specific behavior and maintain an independently tested recovery copy.
Frequently asked questions
Why is formatted capacity lower than drive labels?
Filesystems consume metadata, and operating systems may display binary TiB while vendors sell decimal TB.
Does a hot spare add another failure tolerance?
No. It shortens the delay before reconstruction but does not add parity or mirror protection while idle.
Why keep free-space reserve?
Snapshots, copy-on-write behavior, fragmentation, garbage collection, and rebalancing often require deliberate headroom.
Can mixed-size drives be used?
Conventional arrays are usually constrained by the smallest member. Model every member at that usable size or use the vendor planner.
Is rebuild time downtime?
Usually not, but it is a reduced-redundancy and often reduced-performance interval.