Why a 40TB NAS Does Not Give You 40TB Usable

The short answer: 40TB of drives is not 40TB of usable storage

If a NAS contains 40TB of raw drive capacity, the amount available for files depends mainly on its RAID layout:

Layout	Approximate capacity before filesystem overhead	Protection
No redundancy or RAID 0	40TB	None
RAID 1 across two equal drives	20TB	One drive can fail
RAID 5 or single-parity RAID	30TB	Typically one drive can fail
RAID 6 or dual-parity RAID	20TB	Typically two drives can fail
RAID 10 with four equal drives	20TB	Depends on which drives fail

These are simplified capacity estimates. The NAS operating system, filesystem, snapshots, reserved space, and the difference between TB and TiB reduce the amount shown as available even further.

A four-drive NAS with 4 x 10TB drives illustrates the point:

• Raw capacity: 40TB
• RAID 5 or single parity: approximately 30TB
• Decimal-to-binary display difference: 30TB is approximately 27.3TiB
• Filesystem and system overhead: varies by NAS and filesystem
• Snapshots, recycle bins, and reserved free space: consume additional capacity

In practice, you should not plan to fill the entire amount shown by the NAS. A sensible purchase starts with the capacity needed for data, then adds room for growth, snapshots, and recovery copies.

The four capacity losses to separate

Capacity confusion usually comes from treating several different numbers as if they were the same.

1. Raw drive capacity

Raw capacity is the sum of the advertised drive capacities:

Raw capacity = number of drives × capacity of each drive

For example:

4 × 10TB = 40TB raw

Drive manufacturers normally advertise capacity in decimal units:

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

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

• 1TiB = 1,099,511,627,776 bytes
• 1TiB is approximately 1.10TB

Therefore, 40TB in decimal is approximately 36.4TiB. This is not a missing-capacity problem; it is a difference in measurement units. Some NAS interfaces label binary values as TB, so check the units used by the specific system.

2. RAID or parity loss

RAID uses part of the raw capacity for mirroring or parity.

For equal-sized drives, simplified formulas are:

• RAID 1: usable capacity ≈ one drive
• RAID 5: usable capacity ≈ (number of drives - 1) × smallest drive
• RAID 6: usable capacity ≈ (number of drives - 2) × smallest drive
• RAID 10: usable capacity ≈ half of raw capacity
• RAID 0: usable capacity ≈ raw capacity, but there is no drive-failure protection

A parity array does not provide free storage. The parity information is distributed across the drives, and the equivalent of one or more drives is unavailable for ordinary files.

Mixed drive sizes can make the result worse. Many RAID implementations calculate each drive's contribution using the smallest drive in the array, although the exact behavior depends on the NAS platform. Do not assume that adding a larger drive immediately turns all of its extra capacity into usable space.

RAID also affects more than capacity:

• RAID 1 uses more raw capacity for redundancy but has a straightforward layout.
• RAID 5 provides more usable capacity than RAID 6 but has less parity protection.
• RAID 6 sacrifices another drive's worth of capacity for additional protection.
• RAID 10 can offer useful performance and rebuild characteristics, but its usable capacity is roughly half of raw capacity.
• ZFS configurations such as RAIDZ1 and RAIDZ2 have similar single- and dual-parity capacity concepts, but their exact usable result includes ZFS pool and dataset behavior.

RAID protects against some drive failures. It does not replace a backup.

3. Filesystem and NAS overhead

After RAID or parity is applied, the NAS creates a filesystem or storage pool. That layer needs space for structures such as:

• Filesystem metadata
• Directory and allocation information
• RAID or pool management data
• Checksums, depending on the filesystem
• Snapshot metadata
• System volumes and application data
• Reserved or unavailable pool space

There is no universal percentage that applies to every NAS. The amount depends on the operating system, filesystem, RAID implementation, number and size of files, snapshots, and installed applications.

ZFS deserves particular attention because its practical capacity is not simply the raw RAIDZ formula. Pool metadata, datasets, snapshots, record settings, and free-space requirements all affect the amount that should be used. ZFS pools are generally easier to manage when they retain substantial free space, rather than being filled to the last byte.

4. Free-space headroom and reserved capacity

“Usable” in a storage calculator often means space that could theoretically be allocated. It does not necessarily mean space you should fill with permanent data.

Free space is needed for:

• File creation and modification
• Temporary files and application processing
• Snapshot changes
• RAID or ZFS maintenance
• Rebuilds and resilvering
• Surveillance recording buffers
• Plex downloads, artwork, transcoding, or temporary media files
• Future growth

A common planning approach is to keep about 15% to 20% of the post-RAID capacity free, but this is a planning guideline rather than a NAS rule. A system with heavy snapshot use, many small files, or demanding applications may need more.

For a 30TB post-RAID pool:

30TB × 20% = 6TB headroom

30TB - 6TB = 24TB target working capacity

The NAS may show more than 24TB as available, but using 24TB as the planning limit leaves room for normal operation and growth.

Worked example: four 10TB drives

Suppose you are evaluating a four-bay NAS with four 10TB drives.

RAID 5 example

Step 1: Calculate raw capacity

4 × 10TB = 40TB raw

Step 2: Subtract single-parity capacity

40TB - 10TB = 30TB before filesystem overhead

Step 3: Account for unit display

30TB decimal ≈ 27.3TiB

Step 4: Allow for filesystem and NAS overhead

The exact amount depends on the NAS and filesystem. It cannot be calculated accurately without the platform's storage calculator or an actual pool.

Step 5: Reserve operating headroom

Using a 20% planning reserve on the 30TB theoretical post-RAID capacity:

30TB × 0.20 = 6TB

30TB - 6TB = 24TB target before variable overhead

The practical conclusion is that four 10TB drives in RAID 5 should not be purchased on the assumption that they provide 40TB of comfortable file storage. A more realistic planning number is below 24TB after allowing for overhead and headroom, with the exact NAS-reported figure varying by platform.

How other layouts change the same four-drive NAS

Layout	Approximate pre-overhead capacity	Main trade-off
RAID 0	40TB	Maximum capacity, no redundancy
RAID 5	30TB	One parity drive's capacity
RAID 6	20TB	Two parity drives' capacity
RAID 10	20TB	About half the raw capacity
RAID 1 pairs	20TB	Mirrored protection, half the raw capacity

These figures describe array capacity, not a backup. A failed drive can be replaced and the array rebuilt, but deleted, encrypted, corrupted, or overwritten files may not be recoverable from RAID alone.

Capacity planning for real NAS workloads

Personal files and shared folders

Estimate current data first, then include growth:

Required working capacity = current data + annual growth × planning years

Add space for versioning, snapshots, and temporary files separately. If a household currently has 8TB of data and adds 2TB per year, a three-year working estimate is:

8TB + (2TB × 3) = 14TB

That 14TB is not the raw drive requirement. It must fit below the NAS's practical post-RAID capacity while leaving headroom.

Plex and media libraries

Plex media is often predictable because movie and TV files have known sizes, but the library can grow quickly. Also account for:

• New media purchases or downloads
• Multiple versions or quality levels
• Subtitles and artwork
• Transcoding temporary files
• Download and import staging areas
• Backups of application metadata

SSD cache may improve some workloads, but it does not add meaningful bulk media capacity. Do not buy a smaller array expecting cache to solve a capacity shortage.

Surveillance recording

Surveillance storage is driven by camera count, resolution, frame rate, codec, motion activity, and retention period. A rough planning relationship is:

Required storage = recording bitrate × retention time

Include a safety margin because continuous recording can consume space more quickly than a basic estimate suggests. Surveillance data may also need a different retention policy from documents and photos.

Snapshots and versioned backups

Snapshots are useful for recovering from accidental deletion or ransomware, but changed blocks consume storage. A snapshot-heavy workload can require substantially more space than the current file set alone.

Set retention deliberately:

• Keep frequent short-term snapshots for recent mistakes.
• Keep fewer long-term snapshots for important historical versions.
• Monitor snapshot growth instead of assuming it is free.

RAID is not backup

RAID improves availability by allowing a system to continue operating after certain drive failures. It does not protect against:

• Accidental deletion
• Ransomware or malware
• A damaged or misconfigured filesystem
• Fire, theft, flood, or power events
• NAS theft
• Failed updates
• Synchronization mistakes that replicate deletions
• Multiple failures beyond the array's protection level

A backup should be a separate, recoverable copy. For important data, use a plan that includes another device, an offline or otherwise isolated copy, and preferably an off-site copy. The backup target must also be sized for the data, versions, and retention policy it will store.

If your NAS has 24TB of practical working capacity, a backup requirement is not automatically solved by buying a second “24TB” device. Its RAID layout, backup versions, filesystem overhead, and growth reserve also need to be calculated.

What expansion changes

Buying exactly enough capacity today can make future expansion expensive.

Before choosing a NAS, check:

• Whether additional bays can be added later
• Whether the platform supports replacing drives with larger ones
• Whether the array can expand after replacement
• Whether expansion requires replacing every drive
• Whether expansion is performed online or requires migration
• Whether expansion changes the RAID or RAIDZ layout
• Whether a second pool or volume is a better upgrade path
• Whether the NAS supports expansion shelves, if relevant

Drive replacement is not the same as capacity expansion. Replacing one drive with a larger model may provide no extra usable space until the required number of drives has been replaced and the pool has been expanded. Confirm the procedure in the NAS documentation before buying drives for a future upgrade.

NAS capacity sizing checklist

Use this checklist before selecting the NAS and drives:

1. Measure current data. Include shared folders, Plex media, surveillance recordings, computer backups, and application data.
2. Estimate growth. Use an annual estimate and choose a planning period, such as three to five years.
3. Choose the protection level. Compare RAID 1, RAID 5, RAID 6, RAID 10, or the platform's ZFS/RAIDZ options.
4. Calculate post-RAID capacity. Use the smallest-drive rule where applicable and do not count parity space as file space.
5. Convert units carefully. Expect decimal drive labels and possible TiB-style NAS displays.
6. Allow for overhead. Account for the filesystem, system volumes, metadata, snapshots, and reserved space.
7. Keep operating headroom. Plan to leave roughly 15% to 20% free unless the platform or workload requires more.
8. Plan backups separately. RAID capacity is not backup capacity.
9. Check expansion procedures. Confirm whether larger drives or additional bays can actually increase usable space.
10. Monitor the result. Set alerts for pool usage, drive health, snapshots, and backup failures.

The key buying lesson is simple: start with the amount of data you need to store, not the advertised total of the drives. A “40TB NAS” describes raw media capacity; the useful planning number comes after redundancy, units, overhead, headroom, and backup requirements are accounted for.

When you are ready to compare systems, Browse NAS and consider the available NAS and storage servers by bay count, expansion options, and the capacity their supported RAID layouts can actually provide.