RAID Capacity and Sizing Calculator

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RAID layout:

Supported layouts cover RAID 0, 1, 5, 6, 10, 50, and 60 for capacity planning.

Installed drives: {{ normalizedInput.installedDrives }} drives

Use the planned drive count in the shelf or array.

drives

Drive size:

Choose TB/GB for decimal vendor sizes or TiB/GiB for binary operating-system values.

Dedicated hot spares:

Use 0 when every installed drive is active in the RAID set.

spares

Parity groups:

Each group must have enough active drives for the selected nested parity layout.

groups

Target usable capacity:

Set the capacity the finished array should safely provide to workloads.

Reserve headroom: {{ formatPercent(normalizedInput.reservePct, 0) }}

Common planning targets keep 10-20% unallocated; 0% means no safety headroom.

Metadata/system overhead:

Leave at 0 for generic RAID math; set a vendor or filesystem reserve when known.

Chassis slot limit:

Use 0 when no chassis or expansion limit should be checked.

slots

Cost per drive:

Leave at 0 to omit cost guidance from the ledger and JSON.

Display precision:

Capacity math uses bytes internally; this only changes displayed rounding.

Metric	Value	Read	Copy
{{ row.metric }}	{{ row.value }}	{{ row.read }}

Layout	Planned usable	Efficiency	Fault tolerance	Target read	Copy
{{ row.layout }}	{{ row.plannedUsable }}	{{ row.efficiency }}	{{ row.faultTolerance }}	{{ row.targetRead }}

Path	Requirement	Result	Status	Copy
{{ row.path }}	{{ row.requirement }}	{{ row.result }}	{{ row.status }}

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RAID capacity sizing starts with the installed drive shelf and then removes capacity for inactive spares, parity or mirrors, storage-system overhead, and intentional reserve. The same set of drives can produce very different usable capacity depending on whether the layout is striped, mirrored, single-parity, dual-parity, or grouped as RAID 50 or RAID 60.

Capacity planning also has a protection dimension. RAID 0 maximizes capacity but has no drive-failure tolerance; RAID 5 spends one drive per parity group; RAID 6 spends two; RAID 10 spends roughly half of its engaged drives on mirrored copies. A good sizing pass makes those tradeoffs visible before disks, chassis slots, or target usable capacity are committed.

RAID capacity flow from installed raw capacity through active drives, parity or mirror capacity, overhead, and reserve to planned usable capacity

RAID does not replace backup, replication, snapshots, or recovery testing. It is a local availability and capacity layout choice, and its rebuild exposure grows as drive sizes and group widths grow.

How to Use This Tool:

Choose the RAID layout that matches the planned array: RAID 0, RAID 1, RAID 5, RAID 6, RAID 10, RAID 50, or RAID 60.
Enter installed drive count, drive size, and the size unit shown on the drives or procurement sheet. Decimal TB/GB and binary TiB/GiB are intentionally separate.
Subtract dedicated hot spares when they are not part of normal data or parity groups.
For RAID 50 and RAID 60, set the number of parity groups. Group width determines how many active drives are engaged and whether any drives are left outside the grouped layout.
Enter target usable capacity, reserve headroom, metadata or platform overhead, optional chassis slot limit, and optional cost per drive.
Review Sizing Ledger, RAID Alternatives, the capacity chart, the drive-count ladder, and the JSON export when the plan needs to be shared or audited.

Interpreting Results:

Planned usable capacity is the capacity left after RAID layout math, storage overhead, and reserve. Compare it with Target gap to see whether the design meets the requested usable target.

Efficiency compares planned usable capacity with installed raw capacity. It drops when mirrors, parity, hot spares, metadata overhead, or reserve increase. A lower efficiency can still be the right choice when fault tolerance or operational headroom matters.

Minimum drive count and Required drive size are planning aids. They answer two different questions: how many drives of the chosen size are needed, and how large each drive would need to be if the drive count stayed fixed.

Warnings deserve review before procurement. Odd active drive counts in RAID 10, unusable leftovers in RAID 50 or RAID 60, target shortfall, RAID 0 exposure, and chassis slot pressure are all signs that the layout should be checked against operational requirements.

Technical Details:

The sizing model assumes equal-size drives and calculates capacity from the smallest effective drive size. Hot spares are removed before RAID math. RAID 50 and RAID 60 divide active drives into parity groups; leftover active drives are reported as idle because they cannot be evenly engaged in the selected group count.

Formula Core:

\begin{array}{lcl} C_{raw} & = & N \times S \\ N_{active} & = & N - N_{spare} \\ C_{usable-pre} & = & N_{data} \times S \\ C_{usable} & = & C_{usable-pre} \times (1 - o) \times (1 - r) \\ Efficiency & = & \frac{C_{usable}}{C_{raw}} \end{array}

Here N is installed drive count, S is drive size in bytes, o is metadata or platform overhead, and r is reserve headroom. The display unit is converted only after byte-level arithmetic is complete.

RAID level data drive rules
Layout	Data-drive rule	Capacity meaning
RAID 0	`active drives`	All active capacity is data capacity, with no parity or mirror protection.
RAID 1	`1`	One drive worth of capacity is usable; the rest are mirror copies.
RAID 5	`active drives - 1`	One drive worth of capacity is spent on distributed parity.
RAID 6	`active drives - 2`	Two drive equivalents are spent on dual parity.
RAID 10	`engaged even drives / 2`	Mirrored pairs are striped; an odd active drive is not engaged.
RAID 50	`groups x (group width - 1)`	Each RAID 5 group contributes one parity drive equivalent.
RAID 60	`groups x (group width - 2)`	Each RAID 6 group contributes two parity drive equivalents.

Worked substitution: 8 installed 12 TB drives in RAID 6 with no hot spares have 6 data-drive equivalents. Before reserve, capacity is 72 TB. With 10% reserve and no metadata overhead, planned usable capacity is 64.8 TB, leaving about 4.8 TB above a 60 TB target.

Accuracy Notes:

Mixed-size arrays usually behave as if every drive were the smallest effective drive. Enter the common usable drive size, not the largest label size.
Controller metadata, filesystem allocation, snapshots, compression, deduplication, thin provisioning, and vendor reserve can change real usable capacity.
RAID protects against some drive failures, not deletion, corruption, ransomware, site loss, operator error, or backup failure.
Large drives and wide parity groups can lengthen rebuilds and increase degraded-operation exposure. Capacity fit is not the same as availability design.
Decimal drive labels and binary operating-system displays differ. A 12 TB drive is not the same quantity as 12 TiB.

Worked Examples:

Dual-parity storage shelf. Eight 12 TB drives in RAID 6 provide six data-drive equivalents. After a 10% reserve, planned usable capacity is about 64.8 TB, enough for a 60 TB target before filesystem-specific effects.

RAID 10 database tier. Ten active 7.68 TB SSDs in RAID 10 produce five drive equivalents before reserve. The efficiency is lower than RAID 5 or RAID 6, but mirrored pairs can be a deliberate choice for write-heavy workloads and faster rebuild behavior.

Grouped parity with leftovers. Twenty-four drives in RAID 60 across four groups engage all drives as four six-drive groups. Twenty-five active drives with the same four groups leave one active drive outside the grouped layout, so the warning should be resolved before buying chassis slots around that count.

FAQ:

Why does the drive label unit change the result?

Drive vendors commonly use decimal TB or GB, while many operating systems display binary TiB or GiB. The same bytes look smaller in binary units, so the calculator keeps those units separate.

Should hot spares be included in usable capacity?

Dedicated hot spares are not active data, parity, or mirror members during normal operation. They are available for rebuild workflow, so the calculator removes them before RAID math.

Why is RAID 10 shown with idle capacity for odd active counts?

RAID 10 is built from mirrored pairs. If the active drive count is odd, one drive cannot form a complete pair and is treated as not engaged.

Does meeting the target capacity mean the design is production-ready?

No. Capacity is only one check. Review controller support, rebuild policy, backup design, monitoring, workload performance, replacement-drive availability, and recovery objectives.

Glossary:

Raw capacity: Total installed drive capacity before RAID layout, spares, overhead, and reserve are removed.
Active drive: A drive participating in the RAID layout after dedicated hot spares are removed.
Parity: Redundant information that allows data reconstruction after one or more drive failures, depending on RAID level.
Hot spare: A standby drive reserved for rebuild workflow instead of normal data storage.
Reserve headroom: Usable capacity intentionally left unallocated for growth, snapshots, operational margin, or policy.

References:

Data Protection Best Practices, SNIA.
Managing RAID, Red Hat Enterprise Linux documentation.
What is Erasure Coding?, SNIA.