RAID Storage Calculator – Calculate Usable Capacity and Drive Redundancy

RAID Storage Calculator – Calculate Usable Capacity and Drive Redundancy

When setting up a Network Attached Storage (NAS) unit or enterprise server, combining multiple hard drives into a RAID (Redundant Array of Independent Disks) array balances storage capacity, data performance, and hardware fault tolerance. However, because RAID reserves disk space for parity data or mirror copies, usable storage is always less than the total raw aggregate storage of the drives.

This calculator computes Usable Array Storage, Parity/Mirror Overhead Loss, Drive Fault Tolerance Limits, and Total Raw Aggregate Storage across 5 primary RAID levels.

RAID Level Formulas & Architectural Comparison

The calculator evaluates storage pools assuming identical capacity per hard drive (C) across N physical drives:

1. RAID 0 (Striping)

Data is split evenly across all drives without parity or duplication.

• Usable Capacity: N × C (100% efficiency)
• Fault Tolerance: 0 drives (Any single drive failure destroys the entire array).

2. RAID 1 (Mirroring)

Data is duplicated identically across all drives (minimum 2 drives).

• Usable Capacity: C
• Fault Tolerance: N - 1 drives.

3. RAID 5 (Single Distributed Parity)

Block-level striping with single distributed parity (minimum 3 drives).

• Usable Capacity: (N - 1) × C
• Fault Tolerance: 1 drive.

4. RAID 6 (Double Distributed Parity)

Block-level striping with dual distributed parity (minimum 4 drives).

• Usable Capacity: (N - 2) × C
• Fault Tolerance: 2 simultaneous drives.

5. RAID 10 (Stripe of Mirrors / RAID 1+0)

Combines RAID 1 mirroring with RAID 0 striping (requires an even number of drives, minimum 4).

• Usable Capacity: ≤ft( (N / 2) ) × C (50% efficiency)
• Fault Tolerance: 1 drive per mirror pair (up to (N / 2) drives, provided no single mirrored pair loses both drives).

Array Capacity & Redundancy Benchmark Matrix (4 x 2 TB Drives Example)

The table below compares array performance for four 2 TB hard drives (8 TB total raw aggregate storage):

Verified Step-by-Step Worked Example

Let's calculate usable storage and parity loss for a 4-drive RAID 5 array using 2 TB hard drives:

Step 1: Calculate Total Raw Aggregate Storage

Total Raw Storage = 4 drives × 2 TB = 8 TB

Step 2: Compute RAID 5 Usable Capacity

Usable Capacity = (N - 1) × C = (4 - 1) × 2 TB = 3 × 2 = 6 TB

Step 3: Compute Parity Loss Overhead

Parity Loss = Total Raw - Usable = 8 TB - 6 TB = 2 TB

Step 4: Determine Fault Tolerance Limit

Fault Tolerance = 1 Drive Failure

Second Worked Example: 6-Drive RAID 10 Array

Let's calculate usable storage for a 6-drive RAID 10 array using 3 TB hard drives:

Step 1: Calculate Total Raw Aggregate Storage

Total Raw Storage = 6 drives × 3 TB = 18 TB

Step 2: Compute RAID 10 Usable Capacity

Usable Capacity = ≤ft((N / 2)) × C = ≤ft((6 / 2)) × 3 TB = 3 × 3 = 9 TB

Step 3: Compute Mirror Loss Overhead

Mirror Loss = Total Raw - Usable = 18 TB - 9 TB = 9 TB

Step 4: Determine Fault Tolerance Limit

Fault Tolerance = 1 drive per mirror pair, up to 3 drives total

Decimal (TB) vs. Binary (TiB) Storage Discrepancy

To project download transfer times for filling disk arrays, check our Download Time Calculator or model hosting throughput with the Website Bandwidth Calculator.

Frequently Asked Questions (FAQ)

• Q1: Why is RAID 6 recommended over RAID 5 for large 12 TB+ hard drives?
• A1: During a RAID 5 drive rebuild, reading multi-terabyte drives subjects remaining disks to intense read stress. The statistical chance of encountering an Unrecoverable Read Error (URE) or a second drive failure during rebuild is high. RAID 6's double parity prevents array collapse.
• Q2: What happens if a second drive fails during a RAID 5 rebuild?
• A2: In a RAID 5 array, if a second drive fails before the replacement drive finishes rebuilding parity, the entire volume goes offline and data is lost.