NAS Storage Calculator: How to Estimate Usable Capacity

Usable capacity: the short answer

Start with the capacity of the smallest drive, account for the RAID layout, subtract filesystem and pool overhead, then reserve free space for normal operation and growth.

A practical planning formula is:

Planned usable capacity = RAID capacity × (1 - filesystem/pool overhead) × (1 - free-space headroom)

This is a planning estimate, not an exact promise. The result can vary by RAID implementation, filesystem, drive mix, snapshots, reserved space, and how the NAS reports decimal terabytes versus binary tebibytes.

For example, four matching 12 TB drives in RAID 5 provide approximately:

• Raw capacity: 4 × 12 TB = 48 TB
• RAID 5 capacity before overhead: (4 - 1) × 12 TB = 36 TB
• Approximate binary display: 36 TB × 0.9095 = 32.7 TiB
• After an illustrative 5% pool/filesystem allowance: about 34.2 TB, or 31.1 TiB
• After reserving 20% free space: about 27.4 TB, or 24.9 TiB for planned data

The 5% and 20% figures in this example are planning assumptions, not universal NAS requirements. Check the documentation for your NAS OS and filesystem before making a purchase decision.

Step 1: Calculate raw drive capacity

Raw capacity is the sum of the advertised capacities of all installed drives:

Raw capacity = drive count × drive capacity

For mixed-capacity drives, add the capacities individually, but do not assume every RAID layout can use all of that total. Many RAID systems limit each drive to the capacity of the smallest drive in the array.

For a group of four drives rated at 12 TB each:

4 × 12 TB = 48 TB raw capacity

Raw capacity is not the space available for files. RAID protection, filesystem structures, snapshots, reserved space, and free-space requirements reduce it.

TB versus TiB

Drive manufacturers normally label capacity in decimal units:

• 1 TB = 1,000,000,000,000 bytes
• 1 PB = 1,000 TB

Operating systems and NAS interfaces may display binary units:

• 1 TiB = 1,099,511,627,776 bytes
• 1 TB ≈ 0.9095 TiB

To make a rough conversion:

Capacity in TiB = capacity in TB × 0.9095

A NAS interface may label a binary value as “TB,” so compare byte totals when an exact comparison matters.

Step 2: Subtract RAID or parity capacity

RAID determines how much capacity is used for redundancy. The following formulas are useful estimates for matching drives.

Layout	Approximate usable capacity before filesystem overhead	Protection and trade-off
RAID 0	N × S	No drive-failure protection
RAID 1, two drives	S	One copy of the data; roughly 50% raw capacity
RAID 10	(N ÷ 2) × S	Mirrored pairs; capacity is roughly half of raw
RAID 5	(N - 1) × S	One drive’s worth of parity
RAID 6	(N - 2) × S	Two drives’ worth of parity
Single-disk or JBOD	Varies	No single array-level redundancy

In this table:

• N = number of drives
• S = capacity of the smallest drive, or the usable size allocated per drive

RAID 5 example

With four 12 TB drives:

RAID 5 capacity = (4 - 1) × 12 TB = 36 TB

The remaining 12 TB is parity capacity. It is not lost in the sense of being unused; it allows the array to reconstruct data after a qualifying drive failure.

RAID 5 can look attractive for capacity, but drive replacement and rebuild considerations become increasingly important as drive sizes and array size increase. Do not select it solely because its capacity is higher than RAID 6 or RAID 10.

RAID 6 example

With six 12 TB drives:

RAID 6 capacity = (6 - 2) × 12 TB = 48 TB

Raw capacity is 72 TB, but two drive-equivalents are used for dual parity. RAID 6 sacrifices more capacity than RAID 5 while providing protection against two drive failures in the same array.

RAID 10 example

With four 12 TB drives:

RAID 10 capacity = (4 ÷ 2) × 12 TB = 24 TB

RAID 10 uses mirrored pairs and stripes data across those pairs. It generally provides less capacity than RAID 5 or RAID 6 with the same drives, but its failure behavior and rebuild characteristics may suit performance-sensitive workloads.

RAID 1 and larger mirrors

A two-drive RAID 1 mirror provides approximately the capacity of one drive:

RAID 1 capacity = 12 TB

Some NAS platforms support larger mirror or nested layouts. Their exact usable capacity depends on the implementation, so use the NAS vendor’s calculator or storage-pool documentation rather than assuming every mirror behaves like a two-drive RAID 1.

ZFS, RAIDZ, and vendor-specific layouts

ZFS RAIDZ capacity is not always accurately represented by the simple RAID 5 or RAID 6 formulas. Record size, sector size, vdev width, parity layout, metadata, and pool allocation all affect the result. ZFS also distinguishes between vdevs and the pool built from them.

For RAIDZ, use the ZFS or NAS platform’s capacity estimator when available. Treat a simple (N - 1) × S or (N - 2) × S calculation as a rough upper bound, not an exact result.

The same caution applies to vendor-specific flexible RAID systems, hybrid pools, data reduction, compression, deduplication, and thin provisioning.

Step 3: Account for filesystem and pool overhead

After RAID, the NAS filesystem needs space for structures such as:

• Metadata and allocation information
• Directories and file records
• Checksums or integrity data
• Copy-on-write behavior
• Snapshots and snapshot reservations
• Storage-pool reservations
• System volumes and configuration data
• Rebuild or expansion requirements

A more complete estimate is:

Filesystem-adjusted capacity = RAID capacity × (1 - overhead percentage)

Exact overhead is workload-dependent. A pool containing millions of small files can behave differently from one containing large media files. Snapshots can consume substantial additional space if data changes frequently, even though snapshots initially share blocks with the original files.

Do not treat a NAS interface’s “free space” number as the same thing as safe long-term capacity. Determine whether it includes snapshots, reserved pool space, system partitions, recycle bins, and other hidden or allocated storage.

Step 4: Reserve free-space headroom

A NAS should not normally be planned to run at 100% full. Free space may be needed for:

• Copy-on-write updates
• Snapshots
• Temporary files
• File transfers and transcoding
• RAID or RAIDZ expansion
• Scrubs, checks, and repairs
• Database and application activity
• Future growth before the next upgrade

Calculate the space available for planned data like this:

Planned data capacity = filesystem-adjusted capacity × (1 - headroom percentage)

There is no single correct headroom percentage. The appropriate amount depends on the filesystem, workload, snapshot policy, and expansion plan. A media archive with large static files may have different needs from a NAS running virtual machines, databases, containers, or active surveillance recording.

If your NAS vendor publishes a minimum free-space recommendation, use it. Otherwise, choose a conservative operating target and monitor the pool rather than waiting for an emergency warning.

Worked NAS capacity example

Suppose you are evaluating four matching 12 TB drives for a NAS that will store media, documents, and computer backups.

1. Raw capacity

4 × 12 TB = 48 TB raw

2. RAID capacity

If configured as RAID 5:

(4 - 1) × 12 TB = 36 TB before filesystem overhead

This is approximately:

36 TB × 0.9095 = 32.7 TiB

3. Illustrative filesystem allowance

If you use a 5% planning allowance for filesystem and pool overhead:

36 TB × 0.95 = 34.2 TB

In binary units:

34.2 TB × 0.9095 ≈ 31.1 TiB

This is only an estimate. The actual NAS-reported value may differ.

4. Free-space headroom

If you decide to reserve 20% of that adjusted capacity:

34.2 TB × 0.80 = 27.4 TB

The resulting planning target is approximately:

• 27.4 TB, decimal
• 24.9 TiB, binary

You should treat roughly 27.4 TB as the amount available for planned data in this example—not 36 TB and certainly not the original 48 TB raw capacity.

If snapshots, recycle bins, application data, or surveillance retention are also stored on the pool, their space must come from this same planning target.

Capacity planning for growth

Estimate current requirements first, then project forward:

Future data requirement = current data × (1 + annual growth rate)^years

For example, if you currently have 14 TB of data and expect 25% annual growth for three years:

14 TB × 1.25^3 ≈ 27.3 TB

That figure represents projected data, not the required raw drive capacity. You still need to account for RAID, filesystem overhead, headroom, snapshots, and backup copies.

Add non-file data to the estimate:

• Plex and media: original files, remuxes, optimized versions, subtitles, artwork, and temporary transcode space
• Surveillance: camera count, resolution, frame rate, codec, motion detection, and retention days
• Backups: full backups, incremental versions, deleted-file retention, and backup metadata
• Snapshots: how often they are taken and how much data changes between snapshots
• Applications: databases, containers, virtual machines, logs, and downloads
• Expansion margin: capacity needed before adding or replacing drives

A useful project formula is:

Required RAID capacity = desired planned data capacity ÷ ((1 - overhead) × (1 - headroom))

Then select the drive count and RAID layout that provide at least that much RAID capacity.

RAID is not backup

RAID protects availability against some drive failures. It does not protect your data from:

• Accidental deletion
• Ransomware or malware
• Corrupted files
• A failed RAID rebuild
• Theft or fire
• NAS theft or physical damage
• Incorrect synchronization
• Administrative mistakes

A RAID 5 array can still lose all data if the array fails or its contents are deleted. A RAID 1 mirror also reproduces deletions and corruption across both copies.

Plan backup capacity separately:

Backup target capacity = data to protect + required versions + backup overhead

For important data, keep at least one backup copy on a separate device, and consider an off-site or cloud copy. The backup target does not have to use the same RAID layout as the primary NAS, but it must have enough capacity for your retention policy.

Capacity implications for Plex and surveillance

Plex

Plex storage is usually dominated by the media library, not the NAS operating system. Estimate:

• Existing movie and TV files
• Future additions
• Duplicate formats or remuxes
• Optimized versions
• Subtitle and artwork data
• Temporary transcoding space

Transcoding space is often best placed on suitable local storage rather than treated as permanent media capacity. The right choice depends on the NAS, Plex configuration, and number of simultaneous users.

Surveillance

Surveillance capacity is driven by recording bitrate and retention:

Storage in bytes = bitrate in bits per second ÷ 8 × seconds recorded

A practical daily estimate is:

Daily storage = bitrate in Mbps × 10.8 GB

This is a rough decimal estimate for continuous recording by one camera at a constant bitrate. Multiply it by camera count and retention days:

Surveillance storage = daily storage per camera × camera count × retention days

Motion-only recording, variable bitrate, audio, camera codec, event frequency, and retention rules can change the actual result. Keep surveillance recording capacity separate from critical file capacity where possible, because cameras can consume free space quickly.

SSD cache and usable capacity

An SSD cache generally does not increase the NAS pool’s usable capacity. It may improve access patterns in suitable workloads, but it adds cost and another component that must be monitored.

SSD cache is not a substitute for:

• More hard-drive capacity
• A larger RAID or RAIDZ layout
• Adequate free space
• A backup
• Sufficient network bandwidth

For a media archive dominated by large sequential reads and writes, adding cache may produce little benefit. For metadata-heavy or frequently accessed workloads, the result depends on the NAS software, cache mode, SSD endurance, memory, and workload pattern. Confirm that the NAS supports the intended cache type and protection before buying SSDs for this purpose.

Power and expansion consequences

More drives increase raw capacity, but they also affect:

• Drive and system power consumption
• Heat and cooling requirements
• Noise
• Replacement cost
• Rebuild or resilver duration
• Backup size
• Expansion complexity

Before selecting a smaller NAS that appears to meet today’s capacity target, check how it expands:

• Can you add drives to the existing pool?
• Must all drives be replaced before capacity increases?
• Can a second pool or volume be added?
• Does expansion change the RAID or RAIDZ layout?
• Are expansion operations supported without deleting data?
• Is the network fast enough to use the additional capacity?

A two-bay NAS with mirrored drives may be simple to manage, while a larger chassis may offer better long-term expansion. The best choice depends on whether your priority is low cost, redundancy, performance, backup capacity, or avoiding a full migration later.

NAS sizing checklist

Use this checklist before buying drives or a NAS:

• [ ] List current data by category: media, documents, photos, backups, applications, and surveillance.
• [ ] Estimate annual growth and the number of years before your next expansion.
• [ ] Include snapshots, recycle bins, versioned backups, and temporary working space.
• [ ] Convert advertised drive capacity consistently between TB and TiB.
• [ ] Choose a RAID, RAIDZ, mirror, or other layout based on failure protection—not capacity alone.
• [ ] Use the smallest drive as the capacity limit when the RAID system requires equal-sized members.
• [ ] Subtract filesystem and pool overhead using the NAS vendor’s documentation where available.
• [ ] Reserve free-space headroom for your workload.
• [ ] Calculate backup capacity separately from primary NAS capacity.
• [ ] Check Plex transcoding and media-growth requirements if applicable.
• [ ] Calculate surveillance storage from bitrate, camera count, and retention days.
• [ ] Confirm drive-bay count, supported expansion methods, network speed, and power requirements.
• [ ] Verify the NAS’s actual capacity calculator before placing an order.

For the next step, Browse NAS and compare the available drive-bay and expansion options against your calculated target. You can also review NAS and storage servers when you need a larger platform for more drives, backups, Plex, or surveillance.