raid 5 calculator

RAID 5 Capacity Calculator

Estimate usable storage, parity overhead, and efficiency for a RAID 5 array.

Number of drives (minimum 3)

Capacity per drive

Drive size unit

Hot spare drives (optional)

Filesystem/metadata overhead (%)

Note: This assumes equal-size drives. In practice, most controllers use the smallest drive size across the array.

What this RAID 5 calculator does

RAID 5 combines striping with distributed parity. That gives you improved read performance and protection from a single drive failure. The trade-off is that part of your raw capacity is used for parity data. This calculator helps you quickly answer common planning questions:

How much usable space will I get?
How much capacity is “lost” to parity?
How do hot spares affect total available storage?
What is my efficiency percentage?

RAID 5 capacity formula

For a RAID 5 array with equal drive sizes, usable capacity is generally:

Usable RAID 5 Capacity = (Number of Active Drives − 1) × Drive Size
where active drives = total drives − hot spares.

The equivalent of one drive is reserved for parity (distributed across all active drives, not on a single dedicated parity disk).

Quick example

If you have 6 drives, each 10 TB, and no hot spare:

Raw active capacity = 6 × 10 TB = 60 TB
Parity cost = 1 × 10 TB = 10 TB
Usable before filesystem overhead = 50 TB

Important RAID 5 planning details

1) Minimum drives required

RAID 5 needs at least 3 active drives. If you allocate too many drives as hot spares and active drives drop below 3, RAID 5 is not valid.

2) Fault tolerance

RAID 5 can survive exactly one failed drive. A second failure before rebuild completes can cause total array loss.

3) Rebuild risk on large disks

As drive sizes increase, rebuild windows become longer. Longer rebuilds mean the array spends more time in a degraded, riskier state. For very large arrays, many administrators prefer RAID 6 or RAID 10 depending on workload and risk tolerance.

4) Performance expectations

Reads: Often good due to striping.
Writes: Slower than RAID 0/10 for small random writes because parity must be updated.
Rebuild impact: Performance can drop significantly while rebuilding.

                        RAID is not backup. It helps availability, but it does not protect against accidental deletion, malware, corruption, site loss, or administrator error. Always keep independent backups.
                    

RAID 5 vs RAID 6 vs RAID 10 (short version)

RAID 5

Single-drive fault tolerance
Good capacity efficiency
Moderate write penalty due to parity calculations

RAID 6

Two-drive fault tolerance
Lower usable capacity than RAID 5
Better resilience for larger arrays

RAID 10

Mirroring + striping
High performance and faster rebuild behavior
Typically only ~50% raw capacity usable

Best practices when sizing a RAID 5 array

Use identical drives (same model and size if possible).
Keep firmware and controller updates current.
Monitor SMART/health metrics and rebuild times.
Prefer enterprise/NAS drives for 24/7 arrays.
Maintain backups plus tested recovery procedures.

FAQ

Does RAID 5 use one dedicated parity drive?

No. RAID 5 parity is distributed across all active drives.

Why is my real usable space lower than the calculator?

Operating systems report space differently (decimal vs binary units), and filesystems consume metadata space. Thin provisioning, snapshots, or vendor-specific reservation can reduce available capacity further.

Can I mix drive sizes?

You can, but effective capacity is usually limited by the smallest drive in the array. For predictable results, use matched drives.

Bottom line

A RAID 5 calculator is a fast way to estimate capacity and parity overhead before you buy hardware or expand an existing NAS/server. Use it for planning, but pair it with reliability strategy: proper backups, health monitoring, and a RAID level that fits your risk profile.