Storage Tool

RAID Calculator 2026

Free RAID calculator for RAID 0, 1, 5, 6, and 10. Enter your drive count, capacity, and RAID type to instantly see usable storage, parity overhead, fault tolerance, estimated rebuild time, and cost per usable terabyte. Supports all major RAID levels used in Synology, QNAP, TrueNAS, and UnRAID builds — results update in real time.

Always use CMR drives in RAID arrays. SMR drives can cause rebuild stalls lasting days, dramatically increasing the risk of data loss. See our CMR vs SMR guide before purchasing.

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2-4 Bay NAS (Synology/QNAP)

34.199999999999996 TB is perfect for a 2-4 bay NAS. Works great for Plex, backups, or small office storage. RAID 5 or 1 recommended.

RAID Type

1 parity drive. Survives 1 drive failure. Good balance of efficiency and safety.

Popular Setups

Number of Drives (min: 3)

Drives:

Drive Capacity (TB)

Filesystem Overhead: 5%

Filesystems reserve space for metadata. ext4 ≈ 5%, BTRFS ≈ 10%.

Raw Capacity

36 TB

4 drives × 12TB

Usable Capacity

34.199999999999996 TB

71.2% efficiency

Fault Tolerance

1 drive

Can fail simultaneously

Data Loss Risk

MODERATE: Can survive 1 failure. Rebuild time: 6.8 days

Rebuild Time

Estimated rebuild after 1 drive failure: 6.8 days

During rebuild, array is vulnerable to second failure.

Best For

•Medium arrays (4-8 drives)

•Good capacity/redundancy balance

•⚠️ Risky with drives >12TB

Learn More

RAID Levels Explained Best Drives for Plex NAS Drive Recommendations What is a NAS?

RAID Type Comparison

RAID Type	Min Drives	Fault Tolerance	Efficiency	Best For
RAID 0	2	0 drives	100%	Speed only, no redundancy needed
RAID 1	2	1 drive	50%	2-drive mirror, critical data
RAID 5	3	1 drive	~67–94%	3–5 drive arrays, balanced
RAID 6	4	2 drives	~50–89%	6+ drive arrays, high safety
RAID 10	4	1 drive	50%	Performance + redundancy

Not sure which RAID level is right for you? Read our CMR vs SMR guide →

Understanding RAID: A Practical Guide

What Is RAID?

RAID (Redundant Array of Independent Disks) combines multiple physical hard drives into a single logical volume to improve performance, reliability, or both. Different RAID levels make different trade-offs between usable capacity, redundancy, and speed. The right RAID level depends on what you're storing, how much downtime you can tolerate, and your budget. RAID is widely used in network-attached storage (NAS) devices like Synology and QNAP, in homelabs running TrueNAS or UnRAID, and in enterprise servers.

How to Choose the Right RAID Level

2 drives, mirrored backup: Use RAID 1 for a simple one-to-one mirror. 50% efficiency, survives one failure.
3–5 drive NAS: RAID 5 gives the best balance of capacity (~67–80% efficiency) and one-drive fault tolerance. See our RAID levels guide for detailed comparisons.
6+ drive arrays: RAID 6 is strongly recommended. Rebuilds with modern 16TB+ drives can take days, and RAID 6 protects against a second failure during that window. This is especially critical for Plex media servers running 24/7.
Database / VM workloads: RAID 10 delivers excellent random I/O at the cost of 50% capacity efficiency.
Scratch / cache: RAID 0 maximizes speed and capacity but offers no redundancy — use only with separate backups.

Why Rebuild Time Matters

When a drive in a RAID 5 or RAID 6 array fails, the array enters a degraded state and must reconstruct the missing data from parity once a replacement drive is installed. With drives in the 16TB to 24TB range, this rebuild can take 24 to 96 hours of continuous reading from every other drive in the array. During this period, those drives are under sustained heavy load, which is precisely when a second failure becomes most likely. This is the core reason RAID 6 (which can tolerate two failures) is the recommended choice for any modern array of six or more large drives. Use this calculator to estimate rebuild times for your specific configuration — knowing the rebuild window helps you decide between RAID 5, 6, or 10.

RAID Is Not a Backup

RAID protects against hardware failure, not against the most common causes of data loss: accidental deletion, ransomware, software bugs, controller failure, fire, or theft. The 3-2-1 backup rule remains the gold standard: three copies of your data, on two different media types, with at least one copy stored offsite or in the cloud. Use RAID for uptime and to reduce the pain of a single drive failure — not as your only line of defense.

CMR vs SMR in RAID Arrays

Always use CMR (Conventional Magnetic Recording) drives in any RAID configuration. SMR (Shingled Magnetic Recording) drives use overlapping tracks that require background rewrite operations, which can cause sustained-write performance to collapse to a few MB/s. In a RAID rebuild, this often results in stalls that last days or fail entirely, potentially destroying the array. Stick to enterprise-class NAS drives (WD Red Plus/Pro, Seagate IronWolf/IronWolf Pro, Toshiba N300, WD Ultrastar, Seagate Exos) which are all CMR.

Frequently Asked Questions

How much usable space does RAID 5 give you?

RAID 5 gives you (N − 1) × drive size of usable storage, where N is the number of drives. For example, 4 × 8TB drives in RAID 5 yields 24TB usable (75% efficiency). One drive's worth of capacity is reserved for distributed parity across the array.

What is the minimum number of drives for RAID 6?

RAID 6 requires a minimum of 4 drives. It uses 2 drives worth of parity (distributed), so a 4-drive RAID 6 array yields only 2 drives of usable space (50% efficiency). The benefit is surviving two simultaneous drive failures, making it the recommended choice for arrays with 6 or more drives.

Is RAID 5 or RAID 6 better for NAS?

For arrays with 3–5 drives, RAID 5 is a reasonable choice with better efficiency. For 6+ drives, RAID 6 is strongly recommended. Rebuild times on large RAID 5 arrays can exceed 24–72 hours, during which a second failure means total data loss. RAID 6 tolerates that second failure, giving you time to replace the first failed drive safely.

Does RAID replace backups?

No. RAID protects against drive failure but not against accidental deletion, ransomware, fire, flood, or controller failure. Follow the 3-2-1 backup rule: 3 copies of data, on 2 different media types, with 1 offsite or cloud copy. RAID is a component of a backup strategy, not a replacement for one.

How long does a RAID 5 rebuild take with 16TB drives?

A RAID 5 rebuild with 16TB drives typically takes 30–96 hours depending on array load, controller speed, and drive performance. Larger drives mean longer rebuilds, which is why RAID 6 is recommended for any array using drives 8TB or larger.

Why are SMR drives bad for RAID?

SMR (Shingled Magnetic Recording) drives have device-managed write zones that cause severe performance degradation during sustained writes. In a RAID array, this causes rebuilds to stall for days or fail entirely. Always use CMR (Conventional Magnetic Recording) drives in any RAID configuration. See our CMR vs SMR guide for details.

What's the difference between RAID 10 and RAID 5?

RAID 10 stripes data across mirrored pairs, giving excellent read/write performance and surviving multiple drive failures (one per mirror). RAID 5 uses distributed parity, providing better storage efficiency (~75% vs 50%) but slower writes and longer rebuilds. RAID 10 is preferred for databases and high-IOPS workloads; RAID 5/6 is better for bulk storage.

Can I mix drive sizes in a RAID array?

Most traditional RAID arrays will use the smallest drive's capacity for all drives, wasting space on larger drives. Synology Hybrid RAID (SHR) and UnRAID's parity model can use mixed drive sizes more efficiently. For best results in standard RAID 5/6/10, use identical drives — same model, capacity, and CMR technology.