RAID Calculator
What is RAID?
RAID stands for Redundant Array of Independent Disks. It is a technology that combines multiple physical storage drives into a single logical unit to improve performance, fault tolerance, or both. First defined at the University of California, Berkeley in 1987, RAID has become an essential standard in servers, NAS systems, and professional workstations worldwide.
Depending on the RAID level you choose, you can benefit from three core techniques: striping (splitting data across multiple disks for faster reads and writes), mirroring (identical duplication of data across two or more drives for redundancy), and parity (mathematical calculations that allow lost data to be reconstructed without full duplication). Each RAID level combines these techniques differently to balance capacity, speed, and protection.
⚠️ A critical point many overlook: RAID is not a substitute for backups. RAID protects against physical disk failures, but not against human error, ransomware, accidental deletion, or natural disasters. The high availability RAID provides is complementary to —never a replacement for— a solid off-site backup strategy.
RAID can be implemented in two ways. Hardware RAID uses a dedicated controller card that manages the array independently from the OS, offering better performance and advanced features like write caching and battery backup. Software RAID offloads processing to the system CPU through the OS (mdadm on Linux, Storage Spaces on Windows, ZFS). It is more affordable and flexible but consumes processor resources. The right choice depends on your budget, performance needs, and how much control you require.
RAID Level Comparison Table
This table summarizes the key characteristics of the most commonly used RAID levels. Use it as a quick reference when planning your setup:
| Level | Min. Drives | Usable Capacity | Fault Tolerance | Read Speed | Write Speed | Efficiency | Best for |
|---|---|---|---|---|---|---|---|
| RAID 0 | 2 | N × capacity | None | N × (max) | N × (max) | 100 % | Gaming, video editing, temp cache |
| RAID 1 | 2 | Single disk capacity | 1 disk | 2× (theoretical) | 1× (single disk) | 50 % | Critical small systems, OS drives |
| RAID 5 | 3 | (N − 1) × capacity | 1 disk | (N − 1)× | 1/4× (penalty 4 RMW) | 67 % – 94 % | General servers, home NAS |
| RAID 6 | 4 | (N − 2) × capacity | 2 disks | (N − 2)× | 1/6× (penalty 6 dual RMW) | 50 % – 88 % | Archives, backups, critical data |
| RAID 10 | 4 | (N / 2) × capacity | 1 per mirror set | N × (max) | N/2 × | 50 % | Databases, VMs, mission-critical apps |
* N = number of disks in the array. Speeds are theoretical maximums; real-world performance depends on controller, drive type, and workload.
RAID Write Penalty & IO Penalty Explained
Write penalty is one of the most important concepts in RAID performance. It represents the physical I/O overhead a RAID level introduces for every logical write. For each write operation the system sends, the RAID subsystem must perform multiple physical operations on the disks.
Understanding write penalty is critical for sizing your RAID array performance, especially for small random write workloads (databases, virtualization, email). A 100 % sequential workload barely notices the penalty; a 100 % random one feels it at full force.
Write Penalty by RAID Level
| RAID Level | Write Penalty | Operations per write | Performance Impact |
|---|---|---|---|
| RAID 0 | 1 | 1 direct write | No overhead |
| RAID 1 | 2 | 2 writes (one per mirror) | Moderate |
| RAID 5 | 4 | 2 reads + 2 writes (RMW) | High |
| RAID 6 | 6 | 3 reads + 3 writes (dual RMW) | Very high |
| RAID 10 | 2 | 2 writes (one per mirror) | Moderate |
* RMW = Read-Modify-Write. Penalty assumes small random writes.
How to calculate real-world performance
To estimate your RAID array's effective throughput:
For example, with 4 SSD drives delivering 10,000 IOPS each:
- RAID 0: (4 × 10,000) / 1 = 40,000 IOPS
- RAID 5: (4 × 10,000) / 4 = 10,000 IOPS
- RAID 6: (4 × 10,000) / 6 = ~6,667 IOPS
- RAID 10: (4 × 10,000) / 2 = 20,000 IOPS
💡 Key insight: Write penalty is why RAID 10 is preferred for databases. With a penalty of 2 vs. 4 for RAID 5, it delivers twice the effective write IOPS.
Which RAID Level Should You Choose?
Choosing the right RAID level depends on your priorities: performance, redundancy, capacity, or a combination of these. Below we break down each RAID level to help you decide.
RAID 0 — Maximum performance, zero redundancy
RAID 0 uses pure striping: it splits data into blocks and distributes them across all disks in the array. This enables simultaneous read and write operations, multiplying throughput by the number of drives. With 4 disks, you get 4× the speed of a single drive and 100 % of total capacity.
The trade-off is severe: any single disk failure destroys the entire array. There is no redundancy, no parity, no mirroring. RAID 0 is only recommended for disposable or easily replaceable data, such as temporary caches, video editing scratch disks, or gaming drives where speed matters more than persistence. Never use RAID 0 for important data without an independent backup.
RAID 1 — Simple mirroring for critical systems
RAID 1 replicates all data identically across two or more disks (mirroring). If one drive fails, the system keeps running uninterrupted on the mirrored drive. Reads can benefit from two simultaneous sources (up to 2× theoretical), but writes perform at single-disk speed since every block must be written to all mirrors.
Its main drawback is cost per gigabyte: with 2× 1 TB drives, you only get 1 TB usable (50 % efficiency). RAID 1 is ideal for operating system drives, small servers, or any scenario where simplicity and immediate recovery are top priorities. As the easiest RAID level to set up and maintain, it is an excellent choice for low-drive-count environments.
RAID 5 — The sweet spot between performance and redundancy
RAID 5 combines striping with distributed parity. Data and parity information are spread across all drives so that no single disk is dedicated exclusively to parity. This allows the array to tolerate one disk failure without data loss while utilizing up to 94 % of total capacity in large configurations.
With a minimum of 3 drives, RAID 5 is the most popular choice for general-purpose servers, home and office NAS, and shared file storage. Reads benefit from striping at (N−1)×, but writes are slower than RAID 0 or RAID 10 due to parity calculation overhead. An important consideration: the larger modern drives are (12 TB, 20 TB), the longer the rebuild time, widening the window of vulnerability during recovery.
RAID 6 — Double parity for maximum protection
RAID 6 extends RAID 5 by adding a second parity block, allowing up to two disks to fail simultaneously without data loss. Usable capacity is (N−2) × disk capacity, with efficiency ranging from 50 % (4 disks) to 88 % (16+ disks).
This extra protection comes at a cost: writes are significantly slower than RAID 5 due to double parity calculation, and a minimum of 4 drives is required. RAID 6 is the natural choice for archival systems, backup targets, and any scenario where large-capacity drives (>10 TB) prolong rebuild times, increasing the risk of a second failure. It is the preferred RAID level in enterprise storage and data centers that cannot afford data loss during reconstruction.
RAID 10 — Performance and redundancy without compromise
RAID 10 (also called RAID 1+0) nests two levels: it creates mirrored pairs (RAID 1) and then stripes across them (RAID 0). The result is an array that delivers both the speed of RAID 0 and the redundancy of RAID 1. It requires a minimum of 4 disks, always in even numbers.
Capacity efficiency is 50 % (with 4× 1 TB drives you get 2 TB usable), but in return you get read speed up to N× and write speed up to N/2×. Fault tolerance is up to 1 disk per mirrored pair. RAID 10 is the preferred choice for transactional databases (MySQL, PostgreSQL, SQL Server), hypervisors and virtual machines, and any application where both speed and availability are critical and the budget allows it.
How to Use This RAID Calculator
Using our RAID calculator is straightforward. In three steps you will know the usable capacity, fault tolerance, and performance of any configuration:
- Select the RAID level — Choose from RAID 0, 1, 5, 6, or 10. The calculator will automatically show the minimum disk requirements and guide you if your selection is incompatible.
- Enter the disk size — Input the capacity of a single drive. The calculator assumes all disks are the same size. If you mix sizes, use the smallest drive's capacity.
- Enter the number of disks — Specify how many drives will make up the array. The calculator will validate that the count is compatible with your chosen RAID level and display instant results.
Worked Example: RAID 6 vs RAID 10
To understand the real-world differences between RAID levels, let us compare two common configurations using 6 disks of 1 TB each.
RAID 6 Configuration (6 disks × 1 TB)
- Usable capacity: (6 − 2) × 1 TB = 4 TB
- Fault tolerance: Up to 2 disk failures
- Read speed: Up to 4× that of a single disk (N − 2)
- Write speed: Significantly slower due to dual parity calculation
- Efficiency: 66.7 %
RAID 10 Configuration (6 disks × 1 TB)
- Usable capacity: (6 / 2) × 1 TB = 3 TB
- Fault tolerance: Up to 1 disk per mirrored pair (max 3 if one per mirror)
- Read speed: Up to 6× that of a single disk
- Write speed: Up to 3× (half the disks striped)
- Efficiency: 50 %
Which one should you pick?
Choose RAID 6 when capacity efficiency and dual-disk fault tolerance matter most. With 6 drives, RAID 6 gives you 4 TB usable — a full 1 TB more than RAID 10 — while protecting against any two simultaneous drive failures. It is ideal for archival storage, media repositories, and backup targets where write speed is less critical.
Choose RAID 10 when performance is paramount. RAID 10 delivers up to 6× read and 3× write speed, and its rebuilds are dramatically faster — instead of recomputing parity across the entire array, it simply copies data from the surviving mirror. This makes RAID 10 the go-to choice for high-transaction databases, virtual machine stores, and any write-intensive workload.
The bottom line: with 6 identical drives, RAID 6 offers 33 % more usable capacity and stronger fault tolerance; RAID 10 offers superior speed and faster recovery. Your workload determines the winner.
RAID-Z: ZFS Alternative to Traditional RAID
If you have researched advanced storage, you have likely heard about ZFS and its RAID implementation: RAID-Z. It is not a standard RAID level but an implementation integrated into the ZFS filesystem that solves several limitations of conventional RAID.
RAID-Z Variants
| Variant | Equivalent | Min. Disks | Tolerance | Usable Capacity |
|---|---|---|---|---|
| RAID-Z1 | RAID 5 | 3 | 1 disk | (N − 1) × capacity |
| RAID-Z2 | RAID 6 | 4 | 2 disks | (N − 2) × capacity |
| RAID-Z3 | — ZFS exclusive | 5 | 3 disks | (N − 3) × capacity |
Key advantages of RAID-Z
No write hole: ZFS never overwrites data in place. It writes new versions to free blocks and updates metadata atomically. If the system crashes, original data remains intact — eliminating the RAID 5/6 write hole.
Checksums and self-healing: Every block stores its own checksum. If a block becomes corrupt (bit rot), ZFS detects and repairs it automatically using parity or mirror. Traditional RAID cannot do this.
No controller needed: RAID-Z runs in software on direct-attached disks (JBOD). No expensive hardware RAID controller required.
RAID-Z limitations
- Requires more RAM (1 GB per TB of storage recommended)
- Cannot add disks one at a time to an existing vdev
- Expansion requires adding full vdevs or replacing disks
- Less flexible than traditional hot-swap RAID controllers
Frequently Asked Questions About RAID
Does RAID replace backups?
No, absolutely not. This is perhaps the most common misconception about RAID. RAID protects against disk hardware failures, but not against human error (accidental deletion), ransomware attacks, software corruption, viruses, theft, or natural disasters. A true backup means a copy of your data on separate media, ideally in a different physical location (the 3-2-1 rule: 3 copies, 2 different media types, 1 off-site). RAID and backups are complementary, not interchangeable.
Can I mix different size drives in RAID?
Technically yes, but it is not recommended. On most RAID controllers, array capacity is limited to the size of the smallest disk. If you mix 4 TB and 2 TB drives, the 4 TB drives will be treated as 2 TB, wasting 2 TB of capacity. To avoid this waste, it is best to use drives of the same model and capacity throughout the array. Some advanced systems like Synology Hybrid RAID or ZFS handle mixed sizes better, but this is not standard practice in traditional RAID setups.
How long does RAID rebuild take?
Rebuild time depends on several factors: disk size, RAID level, controller speed, system load during rebuild, and drive type (HDD vs SSD). As a rough guide, rebuilding a RAID 5 with 4 TB HDDs can take between 6 and 24 hours. With 12 TB+ drives, it can stretch to several days. During rebuild, the array operates at reduced performance and remains vulnerable to a second failure. NVMe SSDs can cut this time dramatically to minutes or hours. RAID 10 rebuilds faster than RAID 5 or 6 because it only requires copying data from the surviving mirror, with no parity calculations needed.
What is the safest RAID level?
If by "safest" you mean maximum protection against disk failure, RAID 6 is the safest standard level, tolerating up to two simultaneous drive failures without data loss. For maximum protection combined with performance, RAID 10 is a strong alternative. Beyond traditional RAID levels, systems like ZFS RAID-Z3 (triple parity) or configurations with dedicated hot spares push safety even further. Remember: no RAID level is "safe" on its own — true safety comes from combining RAID with off-site backups, proactive disk monitoring (SMART), and a disaster recovery plan.
Frequently Asked Questions
Which RAID type should I choose?
How many disks do I need for RAID 5?
What is the difference between RAID 5 and RAID 6?
What is RAID 10 and when should I use it?
Does RAID replace backups?
Can I mix different size drives in RAID?
How long does RAID rebuild take?
What is the safest RAID level?
What is RAID write penalty?
Is RAID-Z better than RAID 5 or RAID 6?
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