RAID Calculator

Planning a new RAID calculator-assisted storage build means knowing exactly how much usable capacity your disk array will deliver before you spend a cent on hardware. Whether you are setting up a home server, a NAS device, or a full enterprise storage deployment, the difference between raw disk size and what your operating system actually sees for data can be dramatic — and this interactive tool gives you that answer instantly across every major RAID type.

Choose Your RAID Configuration with This Free RAID Calculator

The first decision in any RAID planning exercise is selecting the right raid type for your workload. Each RAID level balances three competing priorities — read/write speed, data redundancy, and usable space — in a different way. Understanding those trade-offs lets you approach capacity planning confidently rather than guessing after the hardware is already racked. Below is a breakdown of every major RAID level this tool supports, along with the minimum disk count, resilience level, and storage efficiency each one provides.

Understanding Your RAID Array Options

RAID 0 — Striping

Minimum drives: 2. Drives allowed to fail: 0. Efficiency: 100%. Data is split across all media in the array for maximum read/write speed. A single device failure destroys the entire array with no possibility of data recovery. Best for scratch volumes, rendering caches, or non-critical transient data where speed is the only priority. Also referred to as raid0 in shorthand notation.

RAID 1 — Mirroring

Minimum drives: 2. Drives allowed to fail: 1. Efficiency: 50%. Every device is duplicated to at least one other unit. If one fails, your data survives. Usable space equals the size of a single unit regardless of how many you add. Commonly used in home server and small NAS builds where dependability matters more than raw space. Also written raid1.

RAID 4 — Dedicated Parity

Minimum drives: 3. Drives allowed to fail: 1. Efficiency: (N-1)/N. One unit is permanently dedicated to error-correction data. Reads are fast; writes create a bottleneck on that unit. Largely superseded by RAID 5 in modern storage configurations.

RAID 5 — Striping with Distributed Parity

Minimum drives: 3. Drives allowed to fail: 1. Efficiency: (N-1)/N. Error-correction data is distributed across all units. One device worth of space is consumed for resilience. A popular choice for NAS, homelab machines, and small IT infrastructure deployments needing a balance of space, speed, and protection. Also written raid5.

RAID 6 — Dual Distributed Parity

Minimum drives: 4. Drives allowed to fail: 2. Efficiency: (N-2)/N. Two units worth of error-correction data means your array survives two simultaneous failures — crucial in large machine pools where rebuild times are long enough for a second failure to occur. Also written raid6.

RAID 10 — Striped Mirror

Minimum drives: 4. Drives allowed to fail: up to 1 per mirror pair. Efficiency: 50%. Combines duplication and striping. You get the speed of RAID 0 with the protection of RAID 1. Efficiency is always 50% regardless of device count. Often the first choice for database machines and high-throughput workloads. Also written raid10.

RAID 50 — RAID 5 + Striping

Minimum drives: 6. Drives allowed to fail: 1 per RAID 5 sub-group. Efficiency: varies. Nested layout combining multiple RAID 5 groups into a stripe set. Used in SAN and enterprise environments to balance performance, space, and protection across a larger number of units. Also written raid 50.

RAID 60 — RAID 6 + Striping

Minimum drives: 8. Drives allowed to fail: 2 per RAID 6 sub-group. Efficiency: varies. The most resilient nested layout widely deployed in production. Each RAID 6 sub-array can absorb two independent failures, making raid 60 ideal for maximum protection in large disk configurations with strict software specifications. Also written raid60.

JBOD — Just a Bunch of Disks

Minimum drives: 1. Drives allowed to fail: 0. Efficiency: 100%. No protective mechanisms whatsoever. The available space equals the total of all units combined. Useful for temporary expansion or archiving with independent copies elsewhere, but JBOD provides zero protection — a single device failure means loss of data on that unit.

SHR and SHR-2 (Synology Hybrid RAID)

This raid calculator also supports SHR and SHR-2, proprietary Synology hybrid levels designed to minimize unused space when combining units of varying sizes. SHR tolerates 1 device failure; SHR-2 tolerates 2. If you plan to use media of different capacities in a Synology NAS, SHR or SHR-2 will maximize your actual available space compared to standard layouts.

RAID Level Comparison — Minimum Drives, Fault Tolerance & Storage Efficiency

RAID Level	Min. Drives	Drives Allowed to Fail	Storage Efficiency	Primary Benefit
RAID 0	2	0	100%	Maximum speed
RAID 1	2	1	50%	Full mirroring
RAID 5	3	1	(N−1)/N	Balanced capacity & redundancy
RAID 6	4	2	(N−2)/N	Dual fault tolerance
RAID 10	4	1 per pair	50%	Speed + mirroring
RAID 50	6	1 per group	~67–83%	Performance at scale
RAID 60	8	2 per group	~50–75%	Maximum protection capacity
JBOD	1	0	100%	Maximum raw capacity

Selecting Drive Capacity and Drive Type for Your RAID Levels

Once you have chosen your raid type, you need to enter the count of units and the individual size per unit as part of your disk configuration. You can select media as either hard disk (HDD) or solid-state (SSD) — the math is the same, but your choice matters for performance, dependability expectations, and the MTTDL (mean time to data loss) projection. When working with a synology raid calculator or similar NAS-specific estimator, you may also be prompted to confirm which units appear on the vendor approval list for your specific model, since some models may not support media of certain sizes even if the total space is achievable through other combinations or by pairing with expansion units.

Keep in mind that the number you see printed on a unit's label — what manufacturers advertise on the retail box — uses decimal (base-10) notation, where 1 TB equals 1,000,000,000,000 bytes. Your operating system, however, uses binary calculations (base-2), where 1 TiB equals 1,099,511,627,776 bytes. This gap means a "4 TB" hard unit delivers roughly 3.64 TiB as seen by your file system. Our estimator applies this conversion automatically so you get a realistic picture of available space rather than the inflated advertised figure.

• A minimum of 2 units is required for any protected storage configuration.
• Most NAS enclosures and machine platforms support a maximum of 24 bays per chassis — check your datasheet for your specific model's limit.
• When mixing units of varying sizes in standard RAID levels, the usable slots are governed by the smallest unit in the group — wasted space on larger units is unavoidable. Consider SHR if you have mismatched sizes.
• Recommended models from Synology are generated based on the count and per-unit size of your selected media — refer to supported hardware lists and the compatibility matrix for your product before purchasing.
• For combinations that push toward very large array sizes, note that many RAID implementations cap the maximum capacity at 512 TB for a single volume.

Interpreting the Synology RAID Calculator and Capacity Results

After you select your media and choose a RAID level, this raid capacity calculator produces several output figures that require careful interpretation. Storage planners and IT teams frequently confuse raw space, usable space, and net available space — and each number tells a different part of the story. Understanding what your results mean is just as important as the calculations themselves, because it determines how much real-world space you can plan to use during storage planning.

Raw Capacity vs. Usable Capacity vs. Available Capacity

Raw capacity is simply the sum of all units before any RAID overhead is applied. It is the theoretical maximum if you used every byte. In practice, RAID protection — whether through error-correction data, duplication, or dedicated units — consumes a portion of that total, leaving you with a smaller usable capacity figure. This is what the raid disk space utilization calculator displays as your working volume.

Beyond RAID overhead, there are additional layers of reduction. The reserved capacity for system is the space set aside for the system partition, a virtual-memory area, and any other OS data the platform needs. On platforms like Synology DSM, each unit in the array must reserve approximately 10 GB of space for the system partition and swap area — this reserved amount grows proportionally with your unit count. For example, in an 8-unit array, approximately 80 GB goes to OS partition space alone before you can create volumes for data.

When you create volumes, the file system itself consumes additional overhead. A volume formatted with the Btrfs file system reserves 4% capacity for index structures — meaning Btrfs metadata accounts for roughly 4% capacity of your formatted volume. A volume using the ext4 file system reserves 2% capacity for its own index overhead. As a result, the net available space — the room truly available for your files — is always somewhat less than the estimated space the tool reports. The Synology Storage Manager will show you the precise net available space once your volume is live, and it will typically be slightly lower than the planning estimate you see here.

Reserved Capacity for System — What Gets Set Aside

The reserved capacity for system covers several distinct areas that sit outside your user volume:

• System partition: Stores the OS, firmware, and core platform data. Each unit contributes roughly 2–3 GB to this area.
• Swap partition: Used as overflow memory when RAM is fully utilized. Allocated per unit; size varies by platform.
• Transient files: Logs, crash dumps, and short-lived operational files written by system processes.
• Index overhead: Btrfs and ext4 file system structures, journal space, and block group reservations consumed when volumes are created.

This reserved space does not appear in the free space column of your volume manager — it is simply absent from the available figure presented by your array manager and can sometimes surprise users who expect to see 100% of the usable space from the RAID calculation available for data.

RAID Type Resilience and Device Failure Scenarios

Resilience level tells you how many simultaneous device failures your array can absorb before suffering data loss. This is arguably the most important output of any raid reliability calculator. A higher resilience figure comes at the cost of space efficiency — there is no free lunch in RAID planning. IT teams balancing a large count of units in a machine environment must account for failure probability over the array's lifetime and plan around MTTDL figures appropriate for their unit count and workload.

Remember that RAID itself is not a safeguard strategy. An array protects against device failure, but not against accidental deletion, ransomware, operator error, controller failure, or site-level disasters. Pairing your storage configuration with a separate, offline copy is essential for true data protection. Many free data recovery scenarios that arise in practice — and even professional data recovery training curricula — trace back to arrays that survived a device failure only to be lost to a second failure during the rebuild window. This is exactly why RAID 6, RAID 60, SHR-2, and similar dual-parity layouts exist: to protect you during that vulnerable rebuild period.

Worked Examples Using the RAID Capacity Calculator

The following worked examples show precisely how to interpret the raid disk space utilization calculator outputs for three common real-world configurations. Each example uses the capacity calculation formulas that underpin the interactive tool.

Example 1 — 4 × 4 TB HDDs in RAID 5

This is one of the most popular configurations for small NAS and homelab deployments, offering a practical balance of space and protection during storage planning.

1. Raw capacity: Raw = 4 × 4 TB = 16 TB
2. RAID 5 usable capacity formula: Usable = (N - 1) × Size = (4-1) × 4 TB = 12 TB
3. Resilience: 1 unit — the array can survive 1 device failure without data loss.
4. Space efficiency: (12)/(16) × 100 = 75%
5. System reserved: Approximately 4 × 10 GB = 40 GB for the OS partition and virtual-memory area.
6. Net available space after Btrfs index overhead: Approximately 12 TB - 40 GB - (4% × 12 TB) ≈ 11.44 TB for user data.

Example 2 — 6 × 2 TB SSDs in RAID 6

All-flash arrays with solid-state media are increasingly common in enterprise environments and high-performance NAS. This layout adds a second layer of error-correction — critical when using high-density combinations where a rebuild could take hours.

1. Raw capacity: Raw = 6 × 2 TB = 12 TB
2. RAID 6 usable capacity formula: Usable = (N - 2) × Size = (6-2) × 2 TB = 8 TB
3. Resilience: 2 units — the array survives 2 simultaneous failures.
4. Space efficiency: (8)/(12) × 100 ≈ 67%
5. System reserved: Approximately 6 × 10 GB = 60 GB across the OS partition, virtual-memory area, and transient files.
6. Net available space after ext4 index overhead: Approximately 8 TB - 60 GB - (2% × 8 TB) ≈ 7.78 TB for user data.

Example 3 — 8 × 8 TB HDDs in RAID 10

RAID 10 is the preferred layout for database machines, virtualization hosts, and any workload demanding both high throughput and reliable protection. Eight large hard units in this arrangement produce a particularly instructive result.

1. Raw capacity: Raw = 8 × 8 TB = 64 TB
2. RAID 10 usable capacity formula: Usable = (N)/(2) × Size = (8)/(2) × 8 TB = 32 TB
3. Resilience: Up to 4 units can fail — provided each failed unit comes from a different mirror pair. If both units in any single mirror pair fail simultaneously, the array fails regardless.
4. Space efficiency: (32)/(64) × 100 = 50%
5. System reserved: Approximately 8 × 10 GB = 80 GB for OS data, platform partitions, and virtual-memory area.
6. Practical usable data space: After OS partition space and Btrfs 4% index reservation, roughly 30.5 TB of the 32 TB logical volume is available for network volumes or data shares.

Results Summary — Three Example RAID Configurations

Configuration	Raw Capacity	Usable Capacity	Reserved (System)	Drives Allowed to Fail	Efficiency
4 × 4 TB HDD — RAID 5	16 TB	12 TB	~40 GB	1	75%
6 × 2 TB SSD — RAID 6	12 TB	8 TB	~60 GB	2	67%
8 × 8 TB HDD — RAID 10	64 TB	32 TB	~80 GB	Up to 4 (1/pair)	50%

Additional Considerations for RAID Disk Space Utilization Calculator Results

Beyond the headline numbers, several real-world factors affect the final volume space your team will work with day to day. The calculation this tool performs represents the planning estimate — the figure you are most likely to see reported by your array manager before a volume is created. The net available space visible inside a created volume will be slightly lower, reduced by the file system's own index structures, journal overhead, and the OS disk management reservations. Good storage planning accounts for all of these layers.

RAID Type Trade-offs for Specific Use Cases

Choosing between raid types is ultimately a risk management decision. For a home machine or homelab with 4–6 units where device failure is infrequent and rebuild times are short, RAID 5 offers an excellent balance. For a business environment — whether a SAN, NAS cluster, or direct-attached machine — RAID 6 or RAID 60 provide the additional resilience that extended rebuild windows on large units demand. Infrastructure teams running 24 or more units often segment their array into multiple groups to manage rebuild risk across a given storage configuration.

Media type also matters beyond raw space. A solid-state RAID 10 array in a database machine delivers very different IOPS characteristics than a spinning RAID 6 array used for cold network archiving — even if the space and resilience figures match. Use the raid reliability calculator companion tool (see ServeTheHome's implementation which is adding MTTDL to the raid parameters) to model the statistical probability of data loss across different unit counts and RAID levels before finalizing your layout.

Drives, Compatibility, and Maximum Logical Drive Capacity

For Synology NAS users, the synology storage manager is the authoritative source for actual volume space once your array is live. The planning estimate from a free tool like this one gives you the design number; Storage Manager gives you the operational number. Always cross-reference your selected media against the Synology Products Compatibility List — some models have a compatibility matrix that restricts specific media sizes even when the count and combined space would otherwise be achievable. Expansion units can extend bay count beyond the base chassis, but each unit's media still counts toward the total count of media in the logical volume and the maximum capacity your platform supports (commonly capped at 512 TB for a single volume on many current platforms).

Finally, keep in mind that calculate raid results are always approximate array sizes — the infortrend raid calculator, the synology raid calculator, and this tool all produce approximate results based on standardized assumptions about binary calculations and system overhead. Your specific machine, firmware version, disk configuration, and choice of compatible media will introduce minor variations in the final net usable space. Always treat the output of any tool as a confident planning estimate rather than a guaranteed byte-precise figure — and pair your RAID protection with a tested, separate archival strategy to safeguard your investment against the risks that no RAID level can address.