How Much NAS Storage Do I Need?

The short answer: size for usable capacity, not raw drive capacity

You need enough usable NAS capacity for:

1. Your current data
2. Expected growth during the NAS's service life
3. Temporary working space
4. Snapshots, file versions, or recycle bins
5. Free-space headroom
6. Any storage reserved for applications such as Plex or surveillance

A useful planning formula is:

Required usable capacity = (current data + expected growth + working space + snapshot/versioning allowance) / (1 - desired free-space percentage)

Only after calculating that target should you choose the number and size of drives, RAID level, or ZFS layout.

For most home and small-business systems, do not plan to fill the storage pool completely. Keeping roughly 15% to 25% free is a practical starting point, although the right amount depends on the filesystem, snapshot policy, applications, and workload. Treat that percentage as planning headroom rather than guaranteed usable space.

To compare NAS models and drive-bay options, Browse NAS or review [NAS and storage servers](/products/?cat=nas-storage-servers).

Raw capacity is not usable capacity

Drive manufacturers normally quote capacity in decimal units:

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

Operating systems and storage tools may display capacity using binary units:

• 1 TiB = 1,099,511,627,776 bytes
• 1 PiB = 1,024 TiB

Therefore, a drive labeled 12 TB will appear as fewer than 12 TiB in many interfaces, even before RAID, parity, formatting, or filesystem reservations. This is a unit conversion, not lost drive capacity.

A simple approximation is:

Decimal TB / 1.0995 = approximate TiB

When comparing a NAS specification with the capacity shown in its management interface, check whether both figures use TB or TiB.

Account for RAID or parity

RAID protects against certain drive failures and can improve availability, but it reduces the capacity available for files. The amount lost depends on the layout.

Common planning approximations include:

Layout	Approximate usable capacity before filesystem overhead
RAID 0 or striped pool	Sum of all drives
RAID 1 with two drives	Capacity of one drive
RAID 5	Sum of drives minus one drive's capacity
RAID 6	Sum of drives minus two drives' capacity
RAID 10	Approximately half of the raw capacity
Single-parity ZFS layout, such as RAIDZ1	Often approximated as total capacity minus one drive, subject to layout details
Double-parity ZFS layout, such as RAIDZ2	Often approximated as total capacity minus two drives, subject to layout details

These are sizing estimates, not exact capacity guarantees. Actual results can vary with:

• Drive-size differences
• RAID implementation
• ZFS vdev layout
• Metadata and parity layout
• Hot spares
• Reserved pool space
• Storage efficiency settings
• Thin provisioning or deduplication

If a pool contains drives of different sizes, many RAID systems use the smallest drive as the effective size for each member. Mixing drive sizes can therefore leave some capacity unused.

RAID level changes the buying decision

Choosing a larger RAID level is not simply a matter of giving up capacity. It also changes fault tolerance, rebuild behavior, performance characteristics, and expansion options.

• RAID 1 is straightforward for a small two-drive NAS but provides limited capacity efficiency.
• RAID 5 or single parity offers more usable capacity but only one-drive fault tolerance.
• RAID 6 or double parity sacrifices more capacity for protection against two drive failures.
• RAID 10 sacrifices roughly half of raw capacity but is often considered for workloads that benefit from mirrored pairs and striping.
• ZFS adds checksumming and copy-on-write behavior, but its usable capacity depends on vdev design rather than only the number of drives.

Do not choose a RAID level based only on the capacity percentage. Consider the number of drives, replacement plan, rebuild exposure, workload, and whether the NAS will be expanded later.

Subtract filesystem overhead and reserve free space

After RAID or parity, the pool still needs space for filesystem structures and system data. The exact amount depends on the NAS operating system, filesystem, volume configuration, snapshots, metadata, and applications. Because this varies, do not apply an invented universal filesystem-overhead percentage.

ZFS, for example, uses copy-on-write and stores checksums and metadata. Snapshots initially consume little additional space, but they retain older blocks as files change or are deleted. A snapshot schedule can therefore consume substantial capacity over time.

Practical planning rules:

• Use the NAS interface's reported usable capacity when available.
• Do not treat the advertised raw capacity as file storage.
• Keep deliberate free-space headroom instead of waiting for an alert.
• Budget separately for snapshots, version history, recycle bins, and temporary files.
• Avoid assuming compression or deduplication will reduce the required drive capacity unless you have measured your own data.

A useful second-stage formula is:

Practical file capacity = post-RAID capacity - filesystem reservations - planned snapshot/versioning space

Or, when you are working backward from a data target:

Post-RAID capacity needed = required practical file capacity / (1 - free-space percentage)

Worked example: sizing a six-drive NAS

Suppose you currently have:

• 18 TB of documents, photos, media, and backups stored on other devices
• 6 TB of expected growth over the next few years
• 4 TB reserved for working files, snapshots, and version history

Your initial practical requirement is:

18 TB + 6 TB + 4 TB = 28 TB

If you want to keep 20% of the post-filesystem pool free:

28 TB / (1 - 0.20) = 35 TB

You therefore need at least about 35 TB of capacity after RAID, filesystem reservations, and other unavoidable deductions. In practice, choose additional margin because the 4 TB allowance is only an estimate and because actual pool overhead depends on the NAS platform.

Now consider six 12 TB drives:

• Raw capacity: 6 x 12 TB = 72 TB
• Double parity approximation: 72 TB - 2 x 12 TB = 48 TB
• Approximate binary display: 48 TB / 1.0995 = about 43.7 TiB

The 48 TB figure is only a nominal pre-filesystem estimate. It is not the amount you should plan to fill with files. After filesystem reservations and a 20% free-space policy, the practical limit is lower. This layout appears to meet the 35 TB planning target, but you should confirm the NAS's reported usable capacity and account for snapshots, hot spares, and the platform's expansion rules before buying.

The same raw 72 TB in a different layout could produce a very different usable result. A two-drive mirror, for example, would make only one drive's capacity available, while a striped pool could expose almost all raw capacity but would not protect against a drive failure.

Include growth instead of buying for today's data

NAS capacity decisions often fail because they measure only today's files. Estimate growth by category:

Data type	Useful growth estimate
Photos and phone videos	Current annual increase multiplied by the planned ownership period
Documents	Current annual increase plus project or business expansion
Media library	Existing library plus likely additions, conversions, and duplicate formats
Computer backups	Number of devices, backup frequency, retention period, and changed-data rate
Surveillance footage	Cameras, recording resolution, frame rate, motion activity, and retention days
Virtual machines and containers	Current virtual disk sizes plus snapshots and application data
Creative projects	Active project files, source footage, exports, and archive copies

A simple growth formula is:

Projected data = current data + (average monthly growth x number of months)

If growth is irregular, use a conservative estimate rather than an unusually quiet month. A NAS that is already close to full is harder to expand safely than one with unused bays and planned capacity headroom.

Do not forget backup capacity

RAID is not a backup.

RAID can keep a system operating after certain drive failures, depending on the layout. It does not protect against:

• Accidental deletion
• Ransomware or malware
• Corrupted files replicated across the array
• Theft or fire
• Failed NAS hardware
• Incorrect permissions or synchronization
• A user deleting the only copy

Plan backup storage separately from the NAS's primary usable capacity. A 3-2-1 strategy is a useful framework:

• Keep at least three copies of important data
• Use at least two different storage types or locations
• Keep at least one copy off-site

The backup target may need less capacity than the primary NAS if it excludes replaceable media, but it may need more if it stores multiple historical versions. Capacity planning should specify both:

Primary NAS capacity and Backup capacity

Do not count a backup disk, USB drive, or cloud destination as extra usable space in the main NAS pool.

Account for Plex, surveillance, and applications

Plex and media storage

Plex capacity is driven mainly by the size and number of media files, not by the number of users. Plan for:

• Movies and TV episodes
• Music and photos
• Duplicate versions or different quality levels
• Subtitle and artwork files
• Temporary transcode space
• Downloads and incomplete transfers

Transcode space is usually working space rather than long-term library capacity, but it still needs to be available on the NAS or another configured location. If you use an SSD for application data or transcoding, that SSD does not replace capacity for the media library.

Surveillance storage

Surveillance can consume capacity continuously. A rough estimate is:

Required video storage = cameras x average bitrate x seconds per day x retention days

Because network video is commonly measured in bits per second:

Storage bytes = bitrate in bits per second / 8 x recording seconds

Actual retention depends on resolution, codec, frame rate, motion recording, audio, and camera activity. Use the camera or surveillance software's storage calculator when available rather than assuming every camera records at its maximum rate all day.

SSD cache usually does not solve a capacity shortage

SSD cache can help some workloads, but it is not a substitute for larger hard drives.

A cache may improve repeated reads or absorb certain writes, depending on the NAS software and workload. It does not create a reliable copy of your media, documents, or backups. Cache behavior and requirements vary by NAS platform, and some workloads see little benefit.

If your problem is insufficient space, prioritize:

1. More usable hard-drive capacity
2. A suitable RAID or ZFS layout
3. Backup capacity
4. Network upgrades where they are actually a bottleneck
5. SSD cache only after measuring the workload

Consider networking and expansion separately

Network speed does not increase the storage pool's capacity. A faster network can make large transfers and multiple-user access more practical, but it does not compensate for an undersized NAS.

Expansion is also not automatic. Before choosing a small initial array, confirm:

• Whether the NAS supports adding drives to the existing pool
• Whether drives must be replaced one at a time
• Whether expansion requires matching or larger drives
• Whether the RAID implementation can reshape safely
• Whether ZFS expansion applies to the existing vdev or requires another vdev
• Whether adding a separate pool is acceptable for your applications
• Whether a free bay is useful without a supported expansion procedure

Replacing every drive with larger models may be possible on some systems, but it takes time and requires a backup. Never treat a planned migration or RAID reshape as your only copy of important data.

NAS sizing checklist

Use this checklist before selecting a NAS and its drives:

• [ ] Measure the current size of each data category.
• [ ] Estimate monthly or annual growth.
• [ ] Add working space for downloads, edits, exports, and temporary files.
• [ ] Add allowance for snapshots, file versions, and recycle bins.
• [ ] Decide how much free-space headroom you want to preserve.
• [ ] Convert the result into a required post-RAID usable capacity.
• [ ] Choose RAID, RAIDZ, or another layout based on fault tolerance as well as capacity.
• [ ] Account for filesystem overhead and platform-specific reservations.
• [ ] Check whether different drive sizes will reduce usable capacity.
• [ ] Budget separately for backups and historical versions.
• [ ] Add Plex transcode space or surveillance retention requirements where relevant.
• [ ] Check the NAS's drive-bay count and real expansion method.
• [ ] Confirm network requirements for the number of users and devices.
• [ ] Verify the NAS's reported usable capacity before purchase.
• [ ] Keep the drive models, backup plan, and replacement budget in scope.

The safest buying decision is usually the one that meets your future practical usable-capacity target after RAID and headroom, not the one with the largest raw TB number on the box.