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RAID Storage Usable Capacity

Calculates usable storage capacity for common RAID configurations from the number and size of drives used.

Written by the Vorzep Team

Computer ScienceData StructuresStorage

RAID Storage CalculatorUsable Capacity

Capacity = N × S
Select RAID level — then choose what to solve for
RAID Level:
Solve for:
GB
—
GB
Usable Capacity—
Small (<100 GB) Medium (100 GB–1 TB) Large (1–10 TB) Very Large (>10 TB)
RAID 0: N×S · RAID 1: S · RAID 5: (N−1)×S · All capacities in GB

Variables

SymbolQuantityUnit
Usable CapacityTotal usable storageTB
NNumber of drives
SCapacity of each driveTB

What it means

RAID (Redundant Array of Independent Disks) configurations provide different trade‑offs between performance, capacity, and redundancy. The usable storage capacity depends on the RAID level. RAID 0 (striping) uses all disks for data, giving capacity = N × S (total raw capacity), but no redundancy. RAID 1 (mirroring) duplicates data across two disks, so capacity = S (only one disk worth). RAID 5 (block‑level striping with parity) uses one disk worth of parity across N disks, so capacity = (N−1) × S, tolerating a single disk failure. RAID 6 uses two parity disks, capacity = (N−2) × S. These formulas are essential for storage planning, cost estimation, and designing fault‑tolerant systems. In practice, the usable capacity may be slightly less due to overhead and formatting. Understanding these capacities helps in choosing the right RAID level for performance, availability, and budget. The formula is widely used in server and data center design.

Worked example

RAID Storage Capacity – Two Examples

Real‑World
Scenario 1 – 4 Drives RAID 5: You have 4 drives of 2 TB each in RAID 5. What is the usable capacity?
ParameterValue
N (drives)4
S (size per drive)2 TB
RAID 5 capacity(N−1) × S = 3 × 2 = 6 TB
1RAID 5 uses 1 drive worth of capacity for parity.
2Usable = 3 × 2 = 6 TB
Result 6 TB usable ✓ 8 TB raw, 2 TB parity
Scenario 2 – 6 Drives RAID 10: You have 6 drives of 4 TB each in RAID 10. What is the usable capacity?
ParameterValue
N (drives)6
S (size per drive)4 TB
RAID 10 capacity(N/2) × S = 3 × 4 = 12 TB
1RAID 10 mirrors across pairs, so usable is half total capacity.
2Usable = 3 × 4 = 12 TB
Result 12 TB usable ✓ 24 TB raw, 12 TB redundancy
Key insight: Different RAID levels trade capacity for performance and fault tolerance – choose based on your needs.

Common mistakes

  • RAID levels: The formula given is for RAID 0, RAID 1, and RAID 5 only. Other levels (e.g., RAID 6, RAID 10) have different capacity formulas.
  • N disks: N is the total number of disks in the array.
  • S size: S is the capacity of a single disk (all disks assumed equal).
  • RAID 5 parity: One disk worth of capacity is lost to parity, so usable = (N−1)×S.
  • Hot spare: If a hot spare is included, it does not contribute to usable capacity.

Applications

RAID (Redundant Array of Independent Disks) usable capacity depends on the RAID level: RAID 0 gives N×S (all disks for data), RAID 1 gives S (mirrored), and RAID 5 gives (N−1)×S (one disk for parity). This capacity calculation is essential for storage system design, balancing performance, redundancy, and cost. Engineers use these formulas to choose the appropriate RAID level for database servers, file servers, and backup systems. RAID 0 provides maximum capacity and performance but no redundancy; RAID 1 offers fault tolerance at lower capacity; RAID 5 balances both. Understanding these capacities helps in planning storage arrays, estimating usable space, and ensuring data protection.

  • Design of storage arrays for servers
  • Database and file server capacity planning
  • Data protection and disaster recovery strategies
  • Performance optimisation with RAID levels
  • Cost‑benefit analysis of storage solutions