RAID Storage Calculator
Usable storage capacity for common RAID levels.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Usable capacity
- 20 TB
- Raw capacity
- 24 TB
- Fault tolerance
- Survives 1 drive failure
Quick answer
RAID usable capacity depends on how much space goes to redundancy: RAID 0 = n × size, RAID 1 = one drive, RAID 5 = (n − 1) × size, RAID 6 = (n − 2) × size, and RAID 10 = half the raw space. Six 4 TB drives in RAID 5 give 20 TB usable of 24 TB raw and survive one drive failure.
What the RAID Storage calculator does
This calculator shows how much usable space an array of identical drives provides at each common RAID level, alongside the raw total and how many drive failures the array can survive. It covers the same ground as a RAID capacity calculator for a home NAS, a small-business file server or a storage shelf.
Use it when deciding between more capacity and more protection: RAID 5 and RAID 6 trade one or two drives' worth of space for redundancy, while RAID 10 gives up half the space for faster writes and rebuilds.
It assumes all drives are the same size. With mixed sizes, standard RAID treats every drive as the size of the smallest one. Capacities are in the decimal terabytes printed on the drive; your operating system may show a smaller number, as explained below. To convert between TB and TiB, use the data storage converter.
The formula
RAID 0: n × S RAID 1: S RAID 5: (n − 1) × S RAID 6: (n − 2) × S RAID 10: (n ÷ 2) × S
| Symbol | Meaning | Unit |
|---|---|---|
| n | Number of drives in the array | drives |
| S | Capacity of one drive (the smallest, if they differ) | TB |
RAID levels were defined in Patterson, Gibson and Katz's 1988 Berkeley paper and later standardized in practice by vendors and SNIA. Figures are before file-system overhead. Software layouts follow the same arithmetic: ZFS RAID-Z1 and RAID-Z2 give roughly the same usable space as RAID 5 and RAID 6. Hybrid schemes such as Synology SHR or Unraid handle mixed drive sizes differently and are not modeled here.
Worked examples
Four-bay home NAS
A home user fills a four-bay NAS with 8 TB drives in RAID 5 for photos and media.
Inputs
- Number of drives4
- Drive size8 TB
- RAID levelRAID 5
Result
- Usable capacity24 TB
- Raw capacity32 TB
- Fault toleranceSurvives 1 drive failure
24 TB of 32 TB is usable and one drive can fail. With drives this large, rebuilds take many hours, so keep a separate backup of anything irreplaceable.
Eight-bay server with RAID 6
A small business builds an eight-drive array of 16 TB disks and wants to survive two failures.
Inputs
- Number of drives8
- Drive size16 TB
- RAID levelRAID 6
Result
- Usable capacity96 TB
- Raw capacity128 TB
- Fault toleranceSurvives 2 drive failures
96 TB usable with two-drive tolerance, losing 25% of raw space to parity. RAID 6 is the usual choice for large drives because a second failure during a rebuild does not lose the array.
RAID 10 for a database or VM host
Six 2 TB SSDs for a database server where write speed matters more than capacity.
Inputs
- Number of drives6
- Drive size2 TB
- RAID levelRAID 10
Result
- Usable capacity6 TB
- Raw capacity12 TB
- Fault toleranceSurvives 1 failure per mirrored pair
Only 6 TB of 12 TB is usable, but writes avoid parity calculations and a rebuild only copies one mirror. Losing both drives of the same mirrored pair still loses the array.
RAID levels compared
The write penalty is the number of disk operations each random write causes, a rough guide to write performance on parity arrays.
| Level | Minimum drives | Usable space | Drive failures survived | Write penalty |
|---|---|---|---|---|
| RAID 0 | 2 | 100% | 0 | 1 |
| RAID 1 | 2 | 1 drive | All but one | 2 |
| RAID 5 | 3 | (n − 1) ÷ n | 1 | 4 |
| RAID 6 | 4 | (n − 2) ÷ n | 2 | 6 |
| RAID 10 | 4 | 50% | 1 per mirror pair (more if in different pairs) | 2 |
Why your NAS shows less space than the calculator
Drive makers use decimal units (1 TB = 10¹² bytes), while Windows and many NAS dashboards report binary units (1 TiB = 2⁴⁰ bytes) but often label them "TB". A 4 TB drive therefore appears as about 3.64 TiB, roughly 9% less. File systems, snapshots and reserved space take a further few percent. Plan for the displayed figure being around 10% lower than this calculator's result.
RAID 5 with large drives and rebuild risk
When a drive fails, the array must read every remaining drive end to end to rebuild. Many consumer drives are specified at one unrecoverable read error per 10¹⁴ bits read, about 12.5 TB, so rebuilding a large RAID 5 array carries a real chance of hitting an error with no redundancy left. That is why RAID 6 or RAID 10 is generally preferred for arrays of 8 TB-plus drives.
Whatever the level, RAID is not a backup: deletion, ransomware, corruption and controller faults affect every drive at once. Budget for an off-array copy, and use the download time calculator to estimate how long restoring from a cloud backup would take.
How to use the RAID Storage Calculator
- 1Enter the number of drives in the array, not counting hot spares.
- 2Enter the capacity of one drive in TB as printed on the label; with mixed sizes, enter the smallest.
- 3Choose the RAID level. RAID 5 needs at least 3 drives, RAID 6 at least 4 and RAID 10 an even number.
- 4Read usable capacity and fault tolerance, then allow about 10% for TiB display and file-system overhead.
Frequently asked questions
How much usable space do 4 drives give in RAID 5?
Three drives' worth. Four 8 TB drives give 24 TB usable, because one drive's capacity is used for distributed parity.
Is RAID 5 or RAID 6 better?
RAID 6 is safer because it survives two drive failures, which matters during long rebuilds of large drives. RAID 5 gives one more drive of usable space and slightly faster writes.
Is RAID 10 better than RAID 5?
RAID 10 is faster for random writes and rebuilds more quickly, but gives only 50% usable space. RAID 5 is more space-efficient and is typically used for bulk storage rather than databases.
Can I mix drive sizes in a RAID array?
You can, but standard RAID only uses the capacity of the smallest drive on every disk. Mixing a 4 TB and an 8 TB drive in RAID 1 gives 4 TB usable.
What happens if two drives fail in RAID 5?
The array is lost and data must be restored from backup. RAID 6 or a RAID 10 layout where the failures land in different mirror pairs would survive the same event.
References
- A Case for Redundant Arrays of Inexpensive Disks (RAID), Patterson, Gibson and Katz (1988) — ACM SIGMOD / University of California, Berkeley
- Common RAID Disk Data Format (DDF) Specification — Storage Networking Industry Association (SNIA)
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
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