# RAID calculator (RAID 0, 1, 5, 6 and 10)

> Calculate usable RAID capacity, parity or mirror overhead and how many drive failures RAID 0, 1, 5, 6 and 10 survive, in TB and the TiB Windows shows.

Interactive version: https://www.calcopenly.com/programming/raid-storage-calculator
Subject: Programming and tech calculators

Usable capacity is the drive size times the number of drives' worth of space that holds data: n for RAID 0 striping, 1 for a RAID 1 mirror, n − 1 for RAID 5 single parity, n − 2 for RAID 6 double parity and n ÷ 2 for RAID 10 striped mirrors. The rest of the raw space holds copies or parity, which is what lets the array keep running after a drive fails.

People use it to size a NAS or server before buying drives. The default, four 8 TB drives in RAID 5, gives 24 TB usable out of 32 TB raw, 75% efficiency, and survives one failed drive. In Windows the same space appears as 21.83 TB, because Windows divides by 2^40 (the binary TiB) but prints "TB".

The results assume identical drives; with mixed sizes, each drive counts as the smallest one. File-system metadata, hot spares and reserved space are not subtracted, so the room left for files is slightly lower.

## Inputs

- **RAID level** (options: RAID 0, RAID 1, RAID 5, RAID 6, RAID 10)
- **Number of drives**
- **Size of each drive**: Drive makers label sizes in decimal TB (10¹² bytes).

## Results

- Usable capacity (TB) — main result
- Usable in binary units (as Windows and df -h show it) (TiB)
- Raw capacity (TB)
- Used for redundancy (TB)
- Space efficiency
- Drive failures always survived
- Fault tolerance
- Disk I/Os per small random write
- One drive in binary units (TiB)

## Formula

$$
\text{usable} = s \times \begin{cases} n & \text{RAID 0} \\ 1 & \text{RAID 1} \\ n-1 & \text{RAID 5} \\ n-2 & \text{RAID 6} \\ n/2 & \text{RAID 10}\end{cases}
$$

## Worked examples

### RAID 5 with 4 × 8 TB

- RAID level: RAID 5
- Number of drives: 4
- Size of each drive: 8 TB
- **Usable capacity: 24 TB**
- **Usable in binary units (as Windows and df -h show it): 21.83 TiB**
- **Space efficiency: 75.00%**
- **Drive failures always survived: 1**
- **One drive in binary units: 7.28 TiB**
- Checked against: Python 3.8 decimal: (4 − 1) × 8 = 24 TB; 24e12 / 2**40 = 21.8279 TiB; 8e12 / 2**40 = 7.2760 TiB

### RAID 6 with 6 × 4 TB

- RAID level: RAID 6
- Number of drives: 6
- Size of each drive: 4 TB
- **Usable capacity: 16 TB**
- **Used for redundancy: 8 TB**
- **Space efficiency: 66.67%**
- **Drive failures always survived: 2**
- **Disk I/Os per small random write: 6**
- Checked against: Python 3.8: (6 − 2) × 4 = 16; 16/24 = 66.67%

### RAID 10 with 4 × 2 TB

- RAID level: RAID 10
- Number of drives: 4
- Size of each drive: 2 TB
- **Usable capacity: 4 TB**
- **Space efficiency: 50.00%**
- **Drive failures always survived: 1**
- **Fault tolerance: 1 drive guaranteed; up to 2 if each failure is in a different mirror pair**
- Checked against: Python 3.8: 4 / 2 × 2 = 4 TB; with 2 mirror pairs one failure per pair is survivable, but two in the same pair are not

### Two-drive mirror

- RAID level: RAID 1
- Number of drives: 2
- Size of each drive: 1 TB
- **Usable capacity: 1 TB**
- **Space efficiency: 50.00%**
- **Drive failures always survived: 1**
- Checked against: Definition: every member holds a full copy

### RAID 0 has no redundancy (edge)

- RAID level: RAID 0
- Number of drives: 3
- Size of each drive: 500 GB
- **Usable capacity: 1.5 TB**
- **Used for redundancy: 0 TB**
- **Space efficiency: 100.00%**
- **Drive failures always survived: 0**
- Checked against: Python 3.8: 3 × 500 GB = 1.5 TB

### RAID 5 at its 3-drive minimum (edge)

- RAID level: RAID 5
- Number of drives: 3
- Size of each drive: 4 TiB
- **Usable in binary units (as Windows and df -h show it): 8.00 TiB**
- **Usable capacity: 8.796 TB**
- **Space efficiency: 66.67%**
- Checked against: Python 3.8: 2 × 4 × 2**40 / 1e12 = 8.796093 TB

## Questions

### How much usable space does RAID 5 give?

RAID 5 gives (n − 1) × drive size, because one drive's worth of space holds parity. Four 8 TB drives give 24 TB (75%) and eight give 56 TB (87.5%). It needs at least 3 drives and survives exactly one failure. Efficiency rises as you add drives, but so does the amount of data a rebuild must read without hitting a second failure.

### What is the difference between RAID 5 and RAID 6?

RAID 6 stores two independent parities (P and Q) instead of one, so it survives any 2 failed drives at the cost of a second drive's capacity. With six 4 TB drives, RAID 5 gives 20 TB and RAID 6 gives 16 TB. A small random write costs 4 disk I/Os on RAID 5 and 6 on RAID 6, so RAID 6 writes are slower.

### Is RAID 10 better than RAID 5?

RAID 10 is faster and rebuilds more safely, while RAID 5 stores more. RAID 10 always keeps 50% of raw space, so 4 × 8 TB gives 16 TB against 24 TB for RAID 5. A small random write costs 2 disk I/Os instead of 4, and a rebuild copies one mirror partner instead of reading every drive. RAID 10 survives 1 failure for certain and up to half its drives if each is in a different pair.

### Why does an 8 TB drive show as 7.28 TB in Windows?

Drive makers use decimal units, 1 TB = 10^12 bytes, while Windows divides by 2^40 = 1,099,511,627,776 bytes and still labels the result TB. 8 × 10^12 ÷ 2^40 = 7.28, so no space is missing; the gap is about 9% at terabyte scale. IEC 80000-13 names the binary unit the tebibyte (TiB). Macs have reported decimal units since OS X Snow Leopard.

### Is RAID a backup?

No. RAID keeps an array running through drive failures, but deletions, ransomware encryption and file-system corruption reach every drive at once, and RAID 0 has no redundancy at all. Keep separate backups: CISA recommends the 3-2-1 rule, which means 3 copies of important files, on 2 different media types, with 1 copy stored off-site.

### How accurate is the RAID calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “RAID 5 with 4 × 8 TB” is checked against Python 3.8 decimal: (4 − 1) × 8 = 24 TB; 24e12 / 2**40 = 21.8279 TiB; 8e12 / 2**40 = 7.2760 TiB.

### Where does the method come from?

SNIA Common RAID Disk Data Format (DDF) Specification, rev. 2.0 — RAID level definitions; Patterson, Gibson & Katz (1988), A Case for Redundant Arrays of Inexpensive Disks (RAID), SIGMOD; IEC 80000-13:2008 — binary prefixes (1 TiB = 2⁴⁰ bytes).

## Sources

- [SNIA Common RAID Disk Data Format (DDF) Specification, rev. 2.0 — RAID level definitions](https://www.snia.org/tech_activities/standards/curr_standards/ddf)
- [Patterson, Gibson & Katz (1988), A Case for Redundant Arrays of Inexpensive Disks (RAID), SIGMOD](https://doi.org/10.1145/50202.50214)
- IEC 80000-13:2008 — binary prefixes (1 TiB = 2⁴⁰ bytes)