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RAID Calculator

Estimate usable storage and nominal failure tolerance for RAID 0, 1, 5, 6, and 10 arrays.

Results

Usable capacity (TB)
50.00
Drive failures tolerated
1.00

Overview

If you are planning a NAS, homelab, or small server, one of the first questions is simple: how much usable storage do I really get after RAID overhead? Raw disk math is easy, but real arrays trade some of that raw space for mirroring or parity, so a `6 x 10 TB` build does not automatically give you `60 TB` to work with. This RAID calculator turns disk count, per-disk capacity, and RAID level into a fast planning answer for usable space and nominal drive-failure tolerance.

The route is designed for the core storage-planning job behind searches like `raid calculator`, `raid capacity calculator`, `raid 5 calculator`, and `raid 6 calculator`. It helps you compare RAID 0, 1, 5, 6, and 10 before you commit to a layout, order disks, or explain the trade-off to a client or teammate. The result is a clearer first-pass decision on whether you should optimize for capacity, parity protection, or mirrored resilience.

How to use this calculator

  1. Count how many disks you plan to include in the array (excluding hot spares) and note the raw capacity of each disk in terabytes. If disks differ in size, plan around the smallest disk and use that size here.
  2. Enter the disk count and disk size in the inputs.
  3. Select a RAID level from the list (RAID 0, 1, 5, 6, or 10) based on the level of redundancy and performance you want.
  4. Review the calculated usable capacity in terabytes and the nominal number of drive failures tolerated for that level.
  5. Experiment with different disk counts, disk sizes, and RAID levels to see how capacity and fault tolerance change before you commit to a layout.

Inputs explained

Disk count
The total number of drives participating in the RAID array, not including hot spares. For example, a 6‑disk RAID 6 uses all six drives for data and parity; hot spares would be counted separately.
Disk size (TB)
The raw capacity of each drive in terabytes. For mixed‑size arrays, most RAID implementations treat all drives as if they were the size of the smallest disk, so use the smallest capacity for conservative estimates.
RAID level
The redundancy scheme that determines how data and parity are laid out across disks. Higher RAID levels generally sacrifice more raw capacity to gain additional fault tolerance.

Outputs explained

Usable capacity (TB)
The approximate storage space left for data after the chosen RAID level reserves capacity for mirroring or parity. This is a planning figure, not the final filesystem-visible capacity, which will usually be a little lower.
Drive failures tolerated
The typical number of disk failures the chosen RAID layout can survive before the array is considered lost or at serious risk. This is most straightforward for RAID 0, 1, 5, and 6; RAID 10 depends on which specific mirrored pair fails.

How it works

Different RAID levels protect data by dedicating parts of the array to parity (error‑correcting information) or by fully mirroring data across drives. The more redundancy you choose, the less raw space is available for actual data.

For RAID 0, there is no redundancy—data is striped across all disks. Usable capacity is simply the full sum of all disks, and any disk failure takes the array down.

For RAID 1, disks are mirrored. In the simplest case, two disks mirror each other, so usable capacity is equivalent to the capacity of a single disk. In larger mirrored sets, this generalizes to half of the total raw capacity: usable = N/2 disks.

RAID 5 uses block‑level striping with single‑disk parity. One disk’s worth of capacity is effectively reserved for parity, so usable capacity is (N − 1) disks worth, and the array can typically tolerate one drive failure.

RAID 6 extends this to dual parity. Two disks’ worth of space are dedicated to parity, so usable capacity is (N − 2) disks, and the array can usually survive up to two simultaneous drive failures.

RAID 10 (also called 1+0) combines mirroring and striping: pairs of disks are mirrored, and those mirrors are striped together. In simple layouts, usable capacity is half of the raw capacity (N/2 disks), with fault tolerance depending on which drives fail within each mirrored pair.

The calculator applies these rules to your inputs, multiplies the number of usable disks by the per‑disk size in terabytes, and returns both usable TB and the typical number of drive failures the array can sustain without losing data.

Formula

Let N = disk count and S = smallest usable disk size in TB.

RAID 0:
  UsableDisks = N
  FaultTolerance = 0 (any single disk failure loses the array)

RAID 1 (simple mirroring):
  UsableDisks ≈ N ÷ 2
  FaultTolerance ≈ 1 disk per mirror pair

RAID 5 (single parity):
  UsableDisks = N − 1
  FaultTolerance = 1 disk

RAID 6 (dual parity):
  UsableDisks = N − 2
  FaultTolerance = 2 disks

RAID 10 (striped mirrors):
  UsableDisks ≈ N ÷ 2
  FaultTolerance ≈ 1 disk per mirror pair in the common case

UsableCapacityTB = UsableDisks × S

When to use it

  • Planning a home or small‑office NAS build and comparing RAID 5 vs RAID 6 vs RAID 10 for a given number of bays and drive size.
  • Helping non‑technical stakeholders understand why a 6‑disk, 10 TB RAID 6 array doesn’t deliver 60 TB of usable space and how much is “lost” to parity.
  • Evaluating whether adding more disks or moving to a different RAID level would provide enough usable capacity for a backup target or media library.
  • Checking how many drives you can afford to lose at each RAID level before the array goes offline, as part of a risk assessment or DR plan.

Tips & cautions

  • When mixing drive sizes, design for the smallest drive in the set. Larger drives will typically be truncated to match the smallest, leaving some capacity unused.
  • Consider hot spares as a separate design choice. Each spare reduces initial usable capacity but can significantly improve resilience by enabling faster automatic rebuilds.
  • Large arrays built from very high‑capacity disks (for example, 12–20 TB) often benefit from RAID 6 or RAID 10 rather than RAID 5, because rebuilds are slower and the chance of encountering an unrecoverable read error (URE) during rebuild is higher.
  • Remember that file systems, metadata, RAID controller overhead, and manufacturer capacity rounding (TB vs TiB) all reduce real‑world usable space beyond what this simple model reports.
  • If you are comparing vendor-specific layouts such as Synology Hybrid RAID or ZFS RAIDZ, use this route as a baseline only. Those systems can expose different expansion and capacity behavior than the standard RAID levels modeled here.
  • The calculator assumes equal‑size disks and standard RAID layouts. It does not account for advanced schemes, nested arrays beyond RAID 10, or vendor‑specific optimizations like SHR, erasure coding, or distributed parity.
  • Hot spares, filesystem overhead, sector size differences, and RAID controller metadata are not modeled; actual usable capacity will be lower than the raw calculation.
  • RAID 10 fault tolerance is simplified. In reality, a RAID 10 array may survive more than one drive failure if the failed drives are in different mirror pairs, or may fail after two if both are in the same pair.
  • Performance, rebuild time, and risk of data loss from UREs or controller failure are outside the scope of this tool—it focuses strictly on capacity and nominal fault tolerance.

Worked examples

6 × 10 TB drives in RAID 6

  • DiskCount = 6; DiskSizeTB = 10; RAID level = 6.
  • UsableDisks = N − 2 = 6 − 2 = 4.
  • UsableCapacityTB = 4 × 10 = 40 TB.
  • FaultTolerance = 2 drive failures before the array is at risk.
  • Interpretation: you trade away 20 TB of raw capacity to gain dual‑disk redundancy.

8 × 8 TB drives in RAID 10

  • DiskCount = 8; DiskSizeTB = 8; RAID level = 10.
  • UsableDisks ≈ N ÷ 2 = 8 ÷ 2 = 4.
  • UsableCapacityTB = 4 × 8 = 32 TB.
  • FaultTolerance ≈ 1 disk per mirror pair; multiple failures may be survivable if they hit different pairs.
  • Interpretation: you gain very strong performance and good redundancy, but usable capacity is half of raw.

4 × 4 TB drives in RAID 5 vs RAID 6

  • For RAID 5: UsableDisks = N − 1 = 3 → UsableCapacityTB = 3 × 4 = 12 TB; FaultTolerance = 1 disk.
  • For RAID 6: UsableDisks = N − 2 = 2 → UsableCapacityTB = 2 × 4 = 8 TB; FaultTolerance = 2 disks.
  • Interpretation: RAID 6 sacrifices 4 TB of usable capacity compared to RAID 5 in exchange for being able to survive a second drive failure.

Deep dive

This RAID calculator shows how much usable storage you get from a set of disks under RAID 0, 1, 5, 6, or 10 and how many drives can fail before the array is in danger. By entering a disk count, per-disk size, and RAID level, you get a quick, intuitive view of capacity and redundancy trade-offs.

It works well as both a generic `raid calculator` and a more specific `raid capacity calculator`, especially for NAS and server planning where parity and mirroring overhead are easy to underestimate.

The route also supports level-specific comparison intent such as `raid 5 calculator`, `raid 6 calculator`, and `raid 10 calculator` by making the capacity formulas explicit and showing how the fault-tolerance trade-offs change.

Because the assumptions are visible, you can use this tool to explain RAID layouts to teammates who are less familiar with storage design and want to know why `8 x 10 TB` does not mean `80 TB` of final usable space.

Methodology & assumptions

  • The route reads three inputs: disk count, disk size in terabytes, and RAID level.
  • Disk count is clamped to a whole number of at least `1`, and disk size is clamped to a non-negative value.
  • For RAID 0, the calculator assigns all disks to usable capacity and sets nominal fault tolerance to `0`.
  • For RAID 1 and RAID 10, it approximates usable disks as `floor(diskCount / 2)` to reflect mirrored capacity, and reports a simplified nominal fault-tolerance value of `1` because real survivability depends on which mirror pair fails.
  • For RAID 5, it reserves one disk's worth of capacity for parity and reports `1` tolerated disk failure.
  • For RAID 6, it reserves two disks' worth of capacity for parity and reports up to `2` tolerated disk failures, capped so the result never exceeds the number of remaining disks.
  • Usable capacity is then calculated as `usableDisks * diskSizeTb`, which is why mixed-disk arrays should be modeled using the smallest drive size.
  • The route intentionally models standard RAID only. It does not attempt to emulate vendor-specific schemes such as SHR or RAIDZ, nor does it include filesystem, hot-spare, or metadata overhead in the number.
  • Copy, examples, and formulas on the route are kept aligned with the `raidCapacityCalculator` implementation in `src/lib/calculators/calculations.ts`.

Sources

FAQs

Does the calculator include hot spares in the capacity calculation?
No. Hot spares are not counted as part of the usable array capacity. If you plan to dedicate one or more drives as hot spares, subtract them from the disk count before entering your values.
What happens if my drives have different capacities?
Most traditional RAID implementations use the smallest drive size as the baseline and effectively waste any additional space on larger drives. For conservative planning, use the smallest drive size in the calculator.
Does this tool account for filesystem or formatting overhead?
No. File systems, metadata, alignment, and manufacturer capacity conventions (TB vs TiB) all reduce real-world usable space. Plan on losing a few additional percent beyond what the RAID math alone suggests.
Is RAID the same as a backup?
No. RAID protects against drive failure but does not protect against accidental deletion, ransomware, file corruption, or catastrophic array failure. You should always maintain separate, tested backups.
How accurate is the RAID 10 fault-tolerance estimate?
RAID 10 fault tolerance depends on which specific disks fail. The rule of thumb is one drive per mirror pair, but in some failure patterns more disks can fail without data loss, while in others two failures in the same pair can be catastrophic. Treat the reported number as a typical, not worst-case, guideline.
What is the minimum number of disks for RAID 5, RAID 6, and RAID 10?
The usual minimums are 3 disks for RAID 5, 4 disks for RAID 6, and 4 disks for RAID 10. Vendor implementations can add their own constraints, but those minimums are the standard starting point when planning capacity and redundancy.
Why does my final NAS capacity still look smaller than this calculator says?
Because RAID math is only one layer of overhead. Filesystem metadata, snapshots, manufacturer TB-versus-TiB reporting, reserved space, and vendor-specific volume management all reduce the final capacity you see in the operating system. Treat this route as the RAID-level estimate before those additional deductions.

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This RAID capacity calculator provides simplified estimates of usable storage and nominal drive fault tolerance for standard RAID levels using equal-size disks. It does not account for vendor-specific implementations, filesystem overhead, hot spares, or detailed failure scenarios. Always verify capacity and fault-tolerance characteristics with your RAID controller documentation, storage vendor, and backup strategy before deploying a production system.