RAID Storage Calculator
Compute usable capacity, fault tolerance, and expected IOPS for RAID 0, 1, 5, 6, 10, and JBOD. Choose a level, enter the number of drives and their capacity, and see the full storage profile.
RAID 0 — speed, zero safety
Stripes data across all drives with no parity or mirror. Read and write IOPS scale linearly with N, and sequential throughput is roughly N× a single drive. But any single drive failure loses the entire array, because the data of every file is scattered across all N drives. RAID 0 is used for scratch space, video editing cache, and anywhere raw speed matters more than durability.
RAID 1 — simplest redundancy
Mirrors every write to two or more drives. Usable capacity is S × (N/2) for even N, or S for the classic 2-drive configuration. Read IOPS scale as N (any mirror can serve a read), but write IOPS stay at N/2 because every write has to be sent to all mirrors. RAID 1 is the default for OS drives and small arrays where simplicity and reliability matter more than capacity.
RAID 5 — the classic trade-off
Stripes data across N−1 drives and stores one drive's worth of parity distributed across all drives. Usable capacity is (N−1) × S; efficiency climbs toward 100% as N grows (75% at N=4, 92% at N=12). It tolerates any one drive failure, but has a notorious vulnerability: rebuilding a large drive takes hours or days, during which a second failure destroys the array. For drives above 2 TB many shops avoid RAID 5 entirely.
RAID 6 — dual parity, safer rebuilds
Same striping as RAID 5 but with two parity blocks per stripe, so two drives can fail without data loss. Usable capacity is (N−2) × S. The write penalty is worse (6 I/Os per write vs 4 for RAID 5), so write performance is lower, but the extra safety margin makes RAID 6 the default for large drive arrays (8 TB and up) where rebuild windows are measured in days. Efficiency at N=6 is 67%; at N=12 it is 83%.
RAID 10 — best of both
Combines mirroring and striping: drives are paired into mirrors, and the mirrors are striped. Usable capacity is (N/2) × S — you lose half your raw capacity to redundancy. But RAID 10 gives you the best of both worlds: fast reads (any mirror can serve), fast writes (parallel writes to pairs), and short rebuild times (only one mirror needs to resync). It is the default for high-performance databases and any workload where both IOPS and reliability matter.
JBOD — just a bunch of disks
Not really RAID at all — the drives are presented individually or spanned into one logical volume. Usable capacity is N × S, same as RAID 0, but with no striping benefit and no redundancy whatsoever. A single drive failure loses only the data on that drive (if presented individually), or the entire span (if spanned). Useful for archives, cold storage, and cases where the OS manages the drives directly.
How to Use This Calculator
Pick a RAID level, set the number of drives and the capacity per drive, and the calculator computes the usable capacity, efficiency, fault tolerance, and IOPS estimate.
1. Choose a RAID level
Click one of the six RAID buttons. Each shows its formula and its fault tolerance in the label. The whole card recomputes based on your selection.
2. Enter N and S
Type a value or drag the slider for the number of drives and the capacity per drive. The calculator flags arrays that are too small for the selected RAID level (e.g., RAID 5 with only 2 drives).
3. Set drive IOPS and read/write mix
Use the drive IOPS field to describe your drive type (HDD, SSD, NVMe). Adjust the read/write mix to match your workload. The calculator uses the standard write-penalty model for each RAID level.
4. Read the result and verdict
The result card shows the featured value (usable capacity, efficiency %, or read IOPS) plus a badge describing the fault tolerance and level.
5. Inspect the layout diagram
The diagram shows the drives as a grid, with data blocks, parity blocks, and mirror copies marked. It visualises exactly how the selected RAID level distributes data.
6. Switch to Advanced for depth
Advanced mode adds an efficiency-by-drive-count chart, a sensitivity analysis, and companion metrics including write IOPS, blended IOPS, rebuild time estimate, and array warning flags.
What This Tool Offers
Built around the classic RAID capacity formulas — one calculator covering all common levels from JBOD and RAID 0 through RAID 10.
Six RAID levels
RAID 0, 1, 5, 6, 10, and JBOD with the correct usable capacity formula, minimum drive count, and fault tolerance for each.
IOPS model
Read IOPS scale linearly; write IOPS use the standard write-penalty model (×1 for RAID 0/1, ×4 for RAID 5, ×6 for RAID 6). Blended IOPS uses the read/write mix.
Drive layout diagram
A visual grid showing where data, parity, and mirror blocks sit on the drives, so you can see exactly how the level distributes data.
Array warnings
Flags too-few-drives configurations, RAID 5 with large drives (high rebuild risk), and RAID 10 with an odd number of drives.
Efficiency and IOPS charts
Usable-vs-raw capacity bars, plus an Advanced curve showing how efficiency approaches 100% as drive count grows.
Reference tables
A full lookup of all RAID levels at a glance: minimum drives, formula, capacity, tolerance, IOPS scaling, and typical use.
Reference Tables & Conversions
RAID levels at a glance, capacity for common configurations, and a full variable reference.
| Level | Min drives | Usable formula | Fault tolerance | Read IOPS | Write penalty | Typical use |
|---|---|---|---|---|---|---|
| RAID 0 | 2 | N × S | None | N × drive | 1× | Scratch, video cache |
| RAID 1 | 2 | (N/2) × S | 1 drive per mirror | N × drive | 2× | OS drives, small arrays |
| RAID 5 | 3 | (N−1) × S | 1 drive | N × drive | 4× | General file servers |
| RAID 6 | 4 | (N−2) × S | 2 drives | N × drive | 6× | Large arrays, big drives |
| RAID 10 | 4 (even) | (N/2) × S | 1 drive per mirror | N × drive | 2× | Databases, high IOPS |
| JBOD | 1 | N × S | None | N × drive | 1× | Archives, cold storage |
Usable Capacity Lookup — 4 TB Drives
Usable capacity in terabytes for common drive counts across each RAID level, using 4 TB drives.
| N | RAID 0 | RAID 1 | RAID 5 | RAID 6 | RAID 10 | JBOD |
|---|
Variable Reference
| Symbol | Meaning | Typical range | Notes |
|---|---|---|---|
| N | Number of drives | 1 – 128 | Must meet the level's minimum |
| S | Capacity per drive | 0.1 – 100 TB | Assumed identical for all drives |
| R | Raw capacity | N × S | Total before RAID overhead |
| U | Usable capacity | Level-dependent | What the OS sees as one volume |
| Eff | Storage efficiency | 25% – 100% | U / R × 100 |
| FT | Fault tolerance | 0 – 2 drives | Depends on RAID level |
| IOPSr | Read IOPS | N × drive | Striping scales reads |
| IOPSw | Write IOPS | (N × drive) / penalty | Penalty depends on level |
What Does RAID Usable Capacity Measure?
RAID (Redundant Array of Independent Disks) combines multiple physical drives into a single logical volume. The usable capacity is how much that volume presents to the operating system — always less than the raw total (N × S), because RAID reserves some of the space for parity or mirroring. Each RAID level makes a different trade-off between capacity, performance, and fault tolerance, and the calculator quantifies all three.
The RAID Capacity Formulas
The usable capacity of each level in terms of N (drive count) and S (per-drive capacity in TB):
RAID 1: U = (N / 2) × S
RAID 5: U = (N − 1) × S
RAID 6: U = (N − 2) × S
RAID 10: U = (N / 2) × S
JBOD: U = N × S
Storage efficiency is the ratio of usable to raw capacity:
Read IOPS scale linearly with N across all levels. Write IOPS apply a per-level write penalty that models the extra I/O operations required by parity or mirroring:
Write IOPS = (N × IOPSdrive) / penalty
Blended IOPS = read% × Read IOPS + write% × Write IOPS
Write penalties by level: RAID 0 = 1, RAID 1 = 2, RAID 5 = 4, RAID 6 = 6, RAID 10 = 2, JBOD = 1. The higher the penalty, the fewer write IOPS a given raw N can deliver.
RAID Variables
| Symbol | Meaning | Notes |
|---|---|---|
| N | Number of drives | All assumed identical capacity |
| S | Capacity per drive | In TB, GB, or any consistent unit |
| U | Usable capacity | What the OS sees as a single volume |
| R | Raw capacity | N × S, before RAID overhead |
| Eff | Efficiency | U / R × 100 |
| FT | Fault tolerance | Number of simultaneous drive failures tolerated |
| Penalty | Write penalty | Extra I/Os per host write (level-specific) |
Why RAID 5 Is Falling Out of Favour
For decades RAID 5 was the default choice for general file servers, offering a good balance of capacity (N−1 drives usable) and reasonable fault tolerance. Modern drive capacities have undermined that calculation. When a 4 TB drive fails and is replaced, the array has to rebuild the missing data by reading every block on every surviving drive — a process that can take 6–24 hours. During that entire rebuild window, the array is running with no redundancy: if a second drive fails, the whole array is lost. For a 12 TB drive the rebuild can take several days.
The risk isn't just theoretical — disk failures cluster. Drives from the same manufacturing batch, running in the same chassis with the same thermal profile, often fail close in time. And the probability of a bit error during the rebuild scales with the amount of data read: at 1014 bits (a typical SATA URE rate), a full 12 TB rebuild has a ~10% chance of hitting an uncorrectable error and failing the rebuild. This is why most storage vendors now recommend RAID 6 for arrays with drives larger than 2 TB.
The practical guidance has shifted: use RAID 6 or RAID 10 for any array with drives ≥ 4 TB, avoid RAID 5, and consider RAID 10 whenever write IOPS matter. RAID 5 still makes sense for small arrays of modest drives where the rebuild window is short. But the simple formula (N−1) × S hides a complex risk trade-off that modern storage engineers have to weigh explicitly.
RAID Calculation Examples
Usable = (4 − 1) × 4 = 12 TB
Efficiency = 12 / 16 = 75% · tolerates 1 drive failure
Usable = (8 − 2) × 8 = 48 TB
Efficiency = 48 / 64 = 75% · tolerates 2 drive failures
Usable = (8 / 2) × 2 = 8 TB
Efficiency = 8 / 16 = 50% · tolerates 1 failure per mirror
Read IOPS = 4 × 100,000 = 400,000
Write IOPS = 400,000 / 4 = 100,000
Blended = 0.70 × 400,000 + 0.30 × 100,000 = 310,000 IOPS
Frequently Asked Questions
It depends on the workload. For a home NAS with 2 drives, RAID 1 gives you simple redundancy with full capacity lost to mirroring — that is the trade. For 4+ drives in a home or small office NAS with drives up to 4 TB, RAID 5 is a reasonable balance. For 6+ drives or drives larger than 4 TB, RAID 6 is safer because rebuilds on large drives are slow and risky. For databases, virtualization hosts, and any workload with heavy random writes, RAID 10 is the standard choice — you pay 50% of raw capacity but get the best combination of IOPS and reliability. Avoid RAID 0 for anything you actually care about; it has no redundancy at all.
Because every block of data is stored twice. In RAID 10, drives are paired into mirrors and the mirrors are striped. Each pair contributes S/2 to the usable capacity (one full drive's worth of the pair is a mirror of the other). With N drives you have N/2 pairs, and each pair gives S/2 usable — total (N/2) × S, which is exactly half the raw. It is the simplest possible redundancy scheme and it costs half the capacity, but it gives the best IOPS and the shortest rebuild times.
Write penalty models the number of physical I/O operations required for one host write. RAID 0 and JBOD have a penalty of 1 (one write goes to one drive). RAID 1 and RAID 10 have a penalty of 2 (mirrored writes go to two drives). RAID 5 has a penalty of 4 because a small write requires: reading the old data, reading the old parity, writing the new data, and writing the new parity. RAID 6 has a penalty of 6 because it does all of that twice (two parities). The higher the penalty, the fewer write IOPS a given raw drive count can deliver — which is why RAID 5 and 6 are poor choices for write-heavy databases.
You can, but every drive in the array is treated as the size of the smallest one. Mixing a 4 TB drive with three 8 TB drives in RAID 5 gives you 3 × 4 = 12 TB usable, not 28 TB — the extra 4 TB on the larger drives is wasted. This is why RAID arrays are almost always built from identical drives. The only exception is JBOD, which can span drives of different sizes and use all of the capacity, at the cost of losing everything if any single drive fails.
When a drive fails in RAID 1, 5, 6, or 10, the array runs in degraded mode (no redundancy for RAID 5, one less for RAID 6, etc.) until a replacement drive is installed and the missing data is reconstructed from the surviving drives. This rebuild process reads all data from the surviving drives and writes the reconstructed blocks to the new drive. On a 4 TB HDD at 100 MB/s sequential, this takes about 11 hours; on a 12 TB drive, 33 hours; on a 20 TB drive, over 55 hours. During that entire window, the array is vulnerable: another drive failure can take it down. This is why RAID 5 with large drives is often discouraged in favour of RAID 6 or RAID 10.
No. This is the most common misconception about RAID. RAID protects against drive failure, but not against accidental deletion, malware, fire, theft, flood, or a controller failure that corrupts writes across all drives. RAID also propagates logical errors: if a file is corrupted on the host side, the corruption is written to every drive in the array simultaneously. The only protection against those threats is a separate backup — ideally with at least one copy off-site (the 3-2-1 rule: three copies, two media types, one off-site). Use RAID for uptime and performance, use backups for durability.
Hardware RAID uses a dedicated controller card with its own processor and cache, so it does not consume host CPU cycles and typically delivers better performance and more reliable behaviour under power loss (thanks to battery-backed cache). Software RAID (mdadm on Linux, Storage Spaces on Windows, ZFS, Btrfs) uses the host CPU and is more flexible, easier to migrate, and often more transparent. Modern CPUs are fast enough that software RAID often performs as well as hardware RAID for SATA/SAS workloads, and ZFS in particular offers features (checksums, snapshots, RAID-Z) that pure hardware RAID cannot match. For most users today, software RAID — especially ZFS — is the better default.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| Usable Capacity | Total usable storage | TB |
| N | Number of drives | |
| S | Capacity of each drive | TB |
What it means
RAID (Redundant Array of Independent Disks) configurations provide different trade‑offs between performance, capacity, and redundancy. The usable storage capacity depends on the RAID level. RAID 0 (striping) uses all disks for data, giving capacity = N × S (total raw capacity), but no redundancy. RAID 1 (mirroring) duplicates data across two disks, so capacity = S (only one disk worth). RAID 5 (block‑level striping with parity) uses one disk worth of parity across N disks, so capacity = (N−1) × S, tolerating a single disk failure. RAID 6 uses two parity disks, capacity = (N−2) × S. These formulas are essential for storage planning, cost estimation, and designing fault‑tolerant systems. In practice, the usable capacity may be slightly less due to overhead and formatting. Understanding these capacities helps in choosing the right RAID level for performance, availability, and budget. The formula is widely used in server and data center design.
Worked example
RAID Storage Capacity – Two Examples
Real‑World| Parameter | Value |
|---|---|
| N (drives) | 4 |
| S (size per drive) | 2 TB |
| RAID 5 capacity | (N−1) × S = 3 × 2 = 6 TB |
| Parameter | Value |
|---|---|
| N (drives) | 6 |
| S (size per drive) | 4 TB |
| RAID 10 capacity | (N/2) × S = 3 × 4 = 12 TB |
Common mistakes
- RAID levels: The formula given is for RAID 0, RAID 1, and RAID 5 only. Other levels (e.g., RAID 6, RAID 10) have different capacity formulas.
- N disks: N is the total number of disks in the array.
- S size: S is the capacity of a single disk (all disks assumed equal).
- RAID 5 parity: One disk worth of capacity is lost to parity, so usable = (N−1)×S.
- Hot spare: If a hot spare is included, it does not contribute to usable capacity.
Applications
RAID (Redundant Array of Independent Disks) usable capacity depends on the RAID level: RAID 0 gives N×S (all disks for data), RAID 1 gives S (mirrored), and RAID 5 gives (N−1)×S (one disk for parity). This capacity calculation is essential for storage system design, balancing performance, redundancy, and cost. Engineers use these formulas to choose the appropriate RAID level for database servers, file servers, and backup systems. RAID 0 provides maximum capacity and performance but no redundancy; RAID 1 offers fault tolerance at lower capacity; RAID 5 balances both. Understanding these capacities helps in planning storage arrays, estimating usable space, and ensuring data protection.
- Design of storage arrays for servers
- Database and file server capacity planning
- Data protection and disaster recovery strategies
- Performance optimisation with RAID levels
- Cost‑benefit analysis of storage solutions
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