RAID Calculator
Calculate usable storage, fault tolerance, and space efficiency for RAID 0, 1, 5, 6, and 10 arrays with any number of drives.
RAID (Redundant Array of Independent Disks) combines multiple drives into a single logical volume for speed, redundancy, or both. This calculator shows the usable capacity, space efficiency, fault tolerance, and minimum drive count for RAID 0, 1, 5, 6, and 10 across any drive configuration you enter.
About RAID Calculator
RAID Levels at a Glance
| RAID Level | How It Works | Min Drives | Usable Capacity | Fault Tolerance | Read Speed | Write Speed |
|---|---|---|---|---|---|---|
| RAID 0 (Stripe) | Data split across all drives in blocks | 2 | N x drive size (100%) | None - one failure loses everything | Excellent (N x single drive) | Excellent (N x single drive) |
| RAID 1 (Mirror) | Identical copy on each drive | 2 | 1 x drive size (50%) | N-1 drives can fail | Good (reads from any mirror) | Normal (writes to all mirrors) |
| RAID 5 (Stripe + Parity) | Data striped with distributed parity | 3 | (N-1) x drive size | 1 drive | Good | Slower (parity calculation) |
| RAID 6 (Stripe + Double Parity) | Data striped with two parity blocks | 4 | (N-2) x drive size | 2 drives | Good | Slowest (double parity) |
| RAID 10 (Mirror + Stripe) | Striped across mirrored pairs | 4 (even) | N/2 x drive size (50%) | 1 per mirror pair | Excellent | Good |
Usable Capacity Formulas
The calculator applies these formulas to determine how much space you can actually use:
| RAID Level | Formula | Example: 4 x 4 TB Drives | Usable | Efficiency |
|---|---|---|---|---|
| RAID 0 | N x S | 4 x 4 TB | 16 TB | 100% |
| RAID 1 | S | 4 TB (mirrored 4 ways) | 4 TB | 25% |
| RAID 5 | (N - 1) x S | 3 x 4 TB | 12 TB | 75% |
| RAID 6 | (N - 2) x S | 2 x 4 TB | 8 TB | 50% |
| RAID 10 | (N / 2) x S | 2 x 4 TB | 8 TB | 50% |
N is the number of drives, S is the size of each drive. If drives have different sizes, most RAID controllers treat every drive as the size of the smallest one, wasting the extra capacity on larger drives. Use the storage converter if you need to translate between TB, GB, and other units.
How Space Efficiency Changes with Drive Count
RAID 5 and RAID 6 become more space-efficient as you add drives, because the fixed parity overhead is spread across more drives:
| Drive Count | RAID 0 | RAID 1 | RAID 5 | RAID 6 | RAID 10 |
|---|---|---|---|---|---|
| 2 drives | 100% | 50% | N/A | N/A | N/A |
| 3 drives | 100% | 33% | 67% | N/A | N/A |
| 4 drives | 100% | 25% | 75% | 50% | 50% |
| 6 drives | 100% | 17% | 83% | 67% | 50% |
| 8 drives | 100% | 13% | 88% | 75% | 50% |
| 12 drives | 100% | 8% | 92% | 83% | 50% |
RAID 10 stays at 50% regardless of drive count because every drive has exactly one mirror. RAID 5 with 8 drives reaches 88% efficiency, which is why it is popular for NAS devices with many bays.
Choosing the Right RAID Level
| Use Case | Recommended RAID | Why |
|---|---|---|
| Video editing scratch disk | RAID 0 | Maximum speed and capacity, data is temporary and backed up elsewhere |
| Operating system / boot drive | RAID 1 | Simple redundancy, fast reads, easy to recover from a failure |
| Home NAS / media server | RAID 5 | Good balance of capacity and protection for 3-5 drives |
| Business file server | RAID 6 | Survives two simultaneous failures, important for large arrays where rebuild times are long |
| Database server | RAID 10 | Best random I/O performance with redundancy |
| Enterprise storage (large arrays) | RAID 6 or RAID 60 | Double parity is essential when rebuild of a single large drive takes 12-24 hours |
RAID Rebuild Times and Risk
When a drive fails in a RAID 5 or 6 array, the controller reconstructs the missing data from parity during a rebuild. This process stresses the remaining drives and takes time proportional to drive capacity:
| Drive Size | Approximate Rebuild Time (RAID 5) | Risk During Rebuild |
|---|---|---|
| 1 TB HDD | 2-4 hours | Low - short window of vulnerability |
| 4 TB HDD | 8-16 hours | Moderate - sustained stress on remaining drives |
| 8 TB HDD | 16-36 hours | High - another drive failure during rebuild loses everything |
| 16 TB HDD | 24-72 hours | Very high - this is why RAID 6 exists for large drives |
| 4 TB SSD | 1-3 hours | Low - SSDs rebuild much faster |
With modern drive sizes (8 TB+), RAID 5 is increasingly risky because a second drive failure during the long rebuild window means total data loss. This is why RAID 6 has become the standard recommendation for arrays with large HDDs.
RAID Is Not a Backup
This is the single most important thing to understand about RAID. RAID protects against drive failure only. It does not protect against:
| Threat | RAID Protects? | What Protects You |
|---|---|---|
| Single drive failure | Yes (RAID 1/5/6/10) | RAID redundancy |
| Accidental file deletion | No | Backups, snapshots, versioning |
| Ransomware / malware | No - encrypts all drives simultaneously | Offline or immutable backups |
| Controller failure | No - may corrupt the array metadata | Backups, matching spare controller |
| Fire, flood, theft | No | Off-site backup (cloud or physical) |
| Silent data corruption (bit rot) | Not in standard RAID | ZFS, Btrfs, or other checksumming file systems |
| Multiple simultaneous drive failures | Depends on RAID level | RAID 6 or 10 for two failures, backups for more |
The 3-2-1 backup rule applies regardless of RAID: three copies of data, on two different media types, with one copy off-site.
Hardware RAID vs Software RAID
| Type | How It Works | Pros | Cons |
|---|---|---|---|
| Hardware RAID | Dedicated RAID controller card with its own processor and cache | Fastest performance, offloads CPU, battery-backed cache prevents data loss during power failure | Expensive, controller failure can make data inaccessible, locked to that controller model |
| Software RAID (mdadm, Windows Storage Spaces) | OS manages RAID using the CPU | Free, portable between systems, no special hardware needed | Uses CPU resources, no battery-backed write cache |
| ZFS / Btrfs | File system with built-in RAID-like functionality | Data checksumming (catches bit rot), flexible configuration, snapshots | Higher RAM requirements (ZFS wants 1 GB per TB), learning curve |
| Fake RAID (motherboard RAID) | BIOS-level RAID using the chipset | Included with motherboard, no extra hardware | Slower than true hardware RAID, limited features, tied to that chipset |
To estimate bandwidth when transferring files to and from your array, check the file transfer calculator. All calculations run in your browser with no data sent anywhere.
What Is a URE and Why Does It Matter for RAID 5?
An Unrecoverable Read Error (URE) is a sector the drive cannot read even after internal retries and ECC correction. Manufacturers specify URE rates on the drive's datasheet: consumer SATA drives are typically rated at 1 URE per 10^14 bits read (roughly one error every 12.5 TB), while enterprise SAS and nearline drives are rated at 1 per 10^15 bits (about one per 125 TB). SSDs in the enterprise class are usually 1 per 10^16 or better.
This matters because RAID 5 rebuilds require reading every byte on every surviving drive. If the controller hits even one URE during the rebuild, the rebuild fails and the array is lost. With a four-drive RAID 5 of 16 TB consumer HDDs, the rebuild must read 48 TB of data at a URE rate that expects an error roughly every 12.5 TB - statistically you would hit three or four UREs in a single rebuild pass. This is the "RAID 5 is dead" argument that has dominated storage forums since the late 2000s.
Worked example: You run a 5-drive RAID 5 with 8 TB consumer WD Red drives (10^14 URE spec). One drive fails. The rebuild must read 4 x 8 TB = 32 TB = 2.56 x 10^14 bits. Probability of at least one URE (approximating the Bernoulli chance per bit): 1 - (1 - 10^-14)^(2.56 x 10^14) ~= 1 - e^-2.56 ~= 92%. With enterprise drives rated 10^15, the same maths gives 1 - e^-0.256 ~= 23% - a big improvement but still not negligible. Swap to RAID 6 and a single URE during rebuild is absorbed by the second parity block, so the rebuild continues.
How RAID Levels Were Standardised
The five original RAID levels come from the 1988 paper "A Case for Redundant Arrays of Inexpensive Disks" by David Patterson, Garth Gibson, and Randy Katz at UC Berkeley. Their goal was to show that arrays of cheap PC-class disks could outperform a single IBM 3380 mainframe drive. The paper defined RAID levels 1 through 5; RAID 0 (pure striping, no redundancy) and RAID 6 (double parity) were added to the taxonomy afterwards as vendors formalised products. The industry body that oversees RAID today is the Storage Networking Industry Association (SNIA), whose Common RAID Disk Data Format (DDF) specifies on-disk metadata so arrays can be moved between vendors.
The "I" in RAID originally stood for "Inexpensive" but was quietly changed to "Independent" by the storage industry, partly because enterprise RAID hardware is rarely inexpensive.
Common Mistakes When Planning a RAID Array
| Mistake | What Happens | What to Do Instead |
|---|---|---|
| Mixing drives from the same manufacturing batch | Drives age identically, raising the chance of simultaneous failure | Buy from different retailers or batches and stagger install dates |
| Using desktop drives in a NAS | Desktop drives lack TLER and can be dropped from the array during normal error recovery | Use NAS-rated drives (WD Red, Seagate IronWolf, Toshiba N300) |
| RAID 5 on modern large HDDs | Rebuild times over 24 hours and high URE risk mean a second failure often kills the array | Use RAID 6, RAID 10, or ZFS RAIDZ2 for drives 8 TB and above |
| No hot spare in large arrays | Rebuild only starts when a human notices and swaps the drive, extending the vulnerability window | Configure at least one hot spare in any array of 6+ drives |
| Assuming RAID means backup | Ransomware, deletion, or controller corruption can wipe every drive at once | Keep a separate 3-2-1 backup independent of the array |
| Filling the array to 100% | RAID 5/6 performance collapses near full capacity and rebuilds take much longer | Plan to leave 15-20% free space |
Sources
Frequently Asked Questions
What is the difference between RAID 5 and RAID 6?
RAID 5 uses one drive's worth of space for parity and can survive one drive failure. RAID 6 uses two drives for parity, so it can survive two simultaneous failures. RAID 6 is safer for large arrays where the chance of a second failure during a rebuild is higher.
How much usable storage does RAID 10 give?
RAID 10 mirrors every drive, so you get exactly 50% of your total raw capacity as usable storage. Four 1 TB drives in RAID 10 give you 2 TB of usable space.
Is RAID 0 safe for everyday use?
RAID 0 offers no redundancy at all. If any single drive fails, you lose everything in the array. It is only suitable for temporary data, scratch disks, or situations where speed matters more than safety.
Can I mix different size drives in a RAID array?
Technically yes, but most RAID controllers will treat every drive as the size of the smallest one. A 1 TB drive and a 2 TB drive in RAID 1 gives you 1 TB of usable space, wasting half the larger drive.
Does RAID replace backups?
No. RAID protects against drive failure, but it does not protect against accidental deletion, ransomware, file corruption, or disasters. You still need a separate backup strategy.
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