RAID Calculator – Usable Capacity
Enter your RAID level, drive count and drive size to find the usable capacity, raw capacity, array efficiency, fault tolerance and estimated speed of your RAID array. Covers RAID 0, 1, 5, 6, 10, 50, 60 and JBOD.
Find the Usable Capacity of Your RAID Array
RAID Usable Capacity Formula
Where:
- \(U\) — usable capacity of the array
- \(n\) — number of drives in the array
- \(s\) — size of one drive (all drives assumed to be identical)
- \(g\) — number of equal sub-groups (RAID 50 and RAID 60 only)
Usable capacity is the space left for data after redundancy is set aside: the number of drives' worth of capacity that survives, multiplied by the drive size. RAID 0 and JBOD keep everything, mirroring levels give up half the drives, and parity levels sacrifice one or two drives' worth per group.
RAID Usable Capacity – Calculation Example
Example: a RAID 5 array built from 6 identical 4 TB drives — the calculator's default values.
\[ U = (n-1) \cdot s = (6-1) \times 4\ \text{TB} = 20\ \text{TB} \]
\[ \text{efficiency} = \frac{20\ \text{TB}}{6 \times 4\ \text{TB}} \times 100\% \approx 83.33\% \]
Result: 20 TB usable (about 18.19 TiB) out of 24 TB raw, with tolerance for a single drive failure. Estimated sequential speed: 6× read and 1.25× write relative to one drive.
How to Use the RAID Calculator
- Select the RAID level (RAID 0, 1, 5, 6, 10, 50, 60 or JBOD).
- Enter the Number of drives in the array, from 1 to 256.
- Enter the Drive size of a single drive and pick GB or TB as the unit.
- For RAID 50 or RAID 60, also set the Number of groups. Each group must be equally sized, with at least 3 drives per RAID 50 group or 4 per RAID 60 group, and the total drive count must divide evenly by the group count.
- Read the results: usable capacity in TB and TiB, raw capacity, array efficiency, fault tolerance, estimated read/write speed and the minimum drive count for your level.
What Is Usable RAID Capacity?
Raw capacity is simply every drive added together: 6 drives of 4 TB give 24 TB raw. Usable capacity is what your operating system and file system can actually hold, because every RAID level spends some of that space on redundancy — parity blocks, mirror copies or both. A 6-drive RAID 5 array stores one drive's worth of parity, so only 20 TB reaches your data. This is why a freshly built array always shows less space than the box labels promised, no matter how it is configured.
The array efficiency figure expresses the same idea as a percentage: usable divided by raw. RAID 0 and JBOD sit at 100% because nothing is reserved; RAID 1 and RAID 10 settle at 50% because every byte is written twice; RAID 5 approaches 100% as drives are added ((n−1)/n), and RAID 6 trades a second parity drive for better protection, giving (n−2)/n. Compare the efficiency of different levels at your drive count to see exactly what each level of safety costs you in storage.
Fault tolerance counts how many drives can fail without losing data. RAID 5 tolerates one failure, RAID 6 tolerates two, and RAID 10 tolerates one failure per mirror pair (a second failure survives only if it hits a different pair). RAID 50 and RAID 60 tolerate one and two failures per group, so total tolerance grows with the group count. RAID 0 and JBOD tolerate none at all: one dead drive takes the whole volume with it. Tolerance is what you buy with the capacity you sacrifice.
The relative read and write speeds are idealized estimates for sequential workloads, shown as multiples of a single drive's speed. Reads scale with the number of drives in most levels, while parity writes carry a penalty: each RAID 5 write touches about 4 disk operations (the "4-write penalty") and each RAID 6 write about 6, which is why RAID 5's write estimate is (n−1)/4. Real speed depends on the controller, cache, stripe size and workload, so treat these numbers as a rough comparison between levels rather than a benchmark.
RAID Levels at a Glance
| Level | Capacity | Fault tolerance | Min. drives |
|---|---|---|---|
| RAID 0 | \(n \cdot s\) | None | 2 |
| RAID 1 | \(s\) | \(n-1\) drives | 2 |
| RAID 5 | \((n-1) \cdot s\) | 1 drive | 3 |
| RAID 6 | \((n-2) \cdot s\) | 2 drives | 4 |
| RAID 10 | \(\frac{n}{2} \cdot s\) | One per mirror | 4 (even) |
| RAID 50 | \((n-g) \cdot s\) | One per group | \(3g\) |
| RAID 60 | \((n-2g) \cdot s\) | Two per group | \(4g\) |
| JBOD | \(n \cdot s\) | None | 1 |
Assumptions, Limitations and Practical Tips
- Identical drives. The calculator assumes all drives are the same size. With mixed sizes, every drive is effectively capped at the smallest one, and usable capacity drops accordingly.
- Decimal units. Drive sizes use decimal storage units (1 TB = 1000 GB). Your operating system may report the result in binary TiB, where 1 TiB = 240 bytes — that is why 20 TB shows as roughly 18.19 TiB.
- File system overhead. Formatting, metadata and the file system's own structures consume a few percent of usable capacity. Leave headroom; do not expect to store a full 20 TB of files on that 20 TB array.
- Hot spares. A spare drive reduces the drive count available for data, so subtract it from \(n\) if it stands idle and does not participate in the array.
- RAID is not a backup. Redundancy protects against drive failure, not against deletion, corruption, ransomware or theft. Keep true backups of anything that matters.
- Rebuild risk. With large modern drives, rebuilding a degraded array takes many hours and stresses the surviving drives — one reason RAID 6 is preferred over RAID 5 for arrays of more than about 6–8 large drives.
- Securing the array. A storage box often holds your most important data in one place: protect the admin account with a strong password from a password generator, and give it a predictable address on your network with a planned IP subnet.
Frequently Asked Questions
How do I calculate RAID capacity by hand?
Multiply the smallest drive size by the number of drives whose space survives redundancy, then subtract nothing else. For RAID 5: \((n-1) \times s\). RAID 6: \((n-2) \times s\). RAID 1: just \(s\), no matter how many mirror copies exist. RAID 10: \((n/2) \times s\). For RAID 50/60, apply the same rule per group and add the groups together: \((n-g) \times s\) and \((n-2g) \times s\).
Why does my array show fewer TB in Windows or Linux than on the label?
Drive manufacturers use decimal units (1 TB = 10¹²abytes), while operating systems typically display binary units, where 1 TiB = 2⁴⁰ ≈ 1.0995 × 10¹² bytes. To convert, divide the decimal value by 1.0995: 20 TB becomes about 18.19 TiB. No capacity is missing — it is the same number of bytes expressed differently. Some systems label TiB as "TB", which adds to the confusion.
What if my drives are different sizes?
With mixed drive sizes in one array, every drive is limited to the size of the smallest one. Four drives of 4 TB plus four of 2 TB in RAID 5 act like eight 2 TB drives, giving \((8-1) \times 2 = 14\) TB usable. The unused portion of larger drives is wasted. Some controllers and file systems (ZFS, btrfs, SHR on Synology) can use that leftover space, so check your specific implementation.
Should I count my hot spare drive in the number of drives?
No — leave a standby hot spare out of the drive count. It holds no data and no parity, so it contributes nothing to usable capacity or speed. Enter only the drives actively participating in the array. If the spare later replaces a failed drive and joins the array, recalculate with it included: usable capacity stays the same, but rebuild risk rises in the meantime.
Is RAID 5 still safe enough for today's large drives?
RAID 5 remains common for small arrays, but many administrators prefer RAID 6 once you pass about 6–8 drives of 8 TB or more. Rebuilds on large drives take many hours or days and stress surviving drives; a second failure during that window loses a RAID 5 array. RAID 6 tolerates two failures at the cost of one more drive's worth of capacity — weigh that price against your rebuild risk and backup situation.
Why can't I actually fill the full usable capacity with files?
The usable figure is the size of the raw volume. Formatting then deducts the file system's own structures: metadata, journals, allocation tables and reserved blocks typically consume 1–4%. On top of that, small files waste space in clusters, volume snapshots reserve free room, and thin provisioning can inflate apparent sizes. Expect the mountable file system to offer a few percent less than the calculator's usable capacity.
Which RAID level gives the best capacity for a fixed number of drives?
If pure capacity matters most, RAID 0 or JBOD keep 100% of raw space, but a single drive failure destroys everything, so they only suit disposable data. Among protected levels, RAID 5 is the most efficient: \((n-1)/n\), so 12 drives keep about 91.7%. RAID 6 gives up one more drive, RAID 10 always costs half the raw space, and RAID 1 is the least efficient since only one drive's worth survives regardless of mirror count.