Why Your NAS Has Less Storage Than Advertised
You buy four 4TB drives expecting 16TB. Your NAS reports roughly 10-11TB usable. That gap is not a defect. It is the result of six distinct deductions, each with a documented reason.
See exactly how much usable storage your configuration delivers.
Layer 1: Binary vs Decimal Conversion (Always ~6.8% Loss)
Hard drive manufacturers advertise capacity in decimal terabytes: 1 TB = 1,000,000,000,000 bytes. Your operating system measures in binary tebibytes: 1 TiB = 1,099,511,627,776 bytes. That 9.95% difference means every "4TB" drive is actually 3.64 TiB as your NAS will report it.
| Advertised | Actual (TiB) | Loss |
|---|---|---|
| 2 TB | 1.82 TiB | −6.8% |
| 4 TB | 3.64 TiB | −6.8% |
| 8 TB | 7.28 TiB | −6.8% |
| 12 TB | 10.91 TiB | −6.8% |
| 16 TB | 14.55 TiB | −6.8% |
| 20 TB | 18.19 TiB | −6.8% |
This is unavoidable and applies to every drive in every NAS regardless of brand.
Layer 2: RAID Parity Overhead
RAID configurations trade raw capacity for redundancy. The formula depends on the RAID type:
- RAID 1 (mirror): You keep one drive's worth of data. Two 4TB drives in RAID 1 = 3.64 TiB usable.
- RAID 5: (n−1) drives usable. Four 4TB drives = 3 × 3.64 = 10.91 TiB before other deductions.
- RAID 6: (n−2) drives usable. Four drives = 2 × 3.64 = 7.28 TiB.
- SHR (Synology): Optimises across mixed drive sizes. Equal to RAID 5 with identical drives.
- RAIDZ1/Z2/Z3 (TrueNAS/ZFS): Similar to RAID 5/6, but with ZFS-specific overhead on top.
- UnRAID: One drive (the largest) is used for parity. Usable = sum of all other drives.
Layer 3: System Partitions and OS Overhead
Every NAS operating system reserves disk space for its own use. This varies by brand and is the most significant source of confusion because vendors do not always show it clearly in their own specifications.
- Synology DSM: ~10 GB per installed drive (vendor-published)
- QNAP QTS: ~70 GB per storage pool + 20 GB per static volume (vendor-published)
- UGREEN UGOS Pro: ~16 GB flat (vendor-published)
- TerraMaster TOS: ~8.3 GB per drive (vendor-published)
- TrueNAS: ~2 GiB swap per drive; ZFS adds further complexity
- UnRAID: 0 GB (OS runs from USB boot drive)
- ASUSTOR ADM: Varies by model. No vendor-published figure available
Layer 4: Filesystem Metadata
After RAID and system overhead, the filesystem itself reserves space for its internal data structures. These are estimated values (no filesystem publishes an exact per-volume figure):
- Btrfs: ~4% (default on Synology DSM, ASUSTOR ADM, TerraMaster TOS)
- ext4: ~2% (default on QNAP QTS, UGREEN UGOS Pro)
- ZFS: ~3.1% or 128 GiB cap, whichever is smaller (TrueNAS. Actual overhead is higher in practice)
- XFS: ~1% (default on UnRAID)
Layer 5: Snapshot Reserve
Some NAS operating systems reserve additional capacity for point-in-time snapshots. QNAP is the most significant: QTS defaults to reserving 20% of a storage pool for snapshots. This is adjustable in Storage Manager but is enabled by default on every new QNAP pool.
Other brands (Synology, UGREEN, TerraMaster) default to 0% snapshot reserve, though you can enable snapshots and they will consume available space.
Layer 6: Recommended Headroom
This is not a hard deduction. No space is actually reserved. But running a NAS near full capacity degrades performance and can prevent system operations (snapshot creation, indexing, app installations). The standard recommendation is to keep at least 10% free at all times. ZFS-based systems (TrueNAS) are more sensitive and perform best below 80% utilisation.
Putting It Together: A Real Example
Synology 4-bay, four 4TB drives, SHR, Btrfs:
- Advertised: 16.00 TB
- After binary conversion: 14.55 TiB
- After SHR (≈ RAID 5): 10.91 TiB
- After system partitions (10 GB × 4 drives): 10.87 TiB
- After Btrfs metadata (~4%): 10.44 TiB
- After 10% headroom: 9.39 TiB available for files
That is 58.7% of advertised capacity. And every deduction is documented and expected.
Frequently Asked Questions
Is there any way to get more of the advertised capacity?
Switching to ext4 instead of Btrfs saves ~2% metadata overhead. Reducing QNAP's snapshot reserve from 20% to 0% recovers significant space. Beyond that, the binary conversion and RAID parity losses are fixed. The RAID calculator's Exact Mode lets you input your actual overhead values if you know them from your NAS's Storage Manager.
Which RAID type gives the best usable capacity?
RAID 5 (or SHR with identical drives) gives the best balance of redundancy and usable capacity for most 4-bay NAS setups. RAID 6 uses two parity drives instead of one, giving more protection at the cost of ~25% less usable space. JBOD/RAID 0 give 100% capacity but with no redundancy. A single drive failure loses everything.
Why does my NAS show a different number than the calculator?
The calculator uses typical published values. Your NAS may have different firmware-level system overhead, use a different filesystem, or have snapshot reserves enabled. Use the calculator's Exact Mode and enter the values your NAS's Storage Manager actually reports.