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To estimate how much space a NAS will actually have for files, calculate in stages: add the drives’ advertised capacities, account for the RAID or ZFS layout, then subtract the NAS’s system reservations, filesystem metadata, and any volume-size limits. The RAID result is not the same as file-available capacity, and the exact deductions depend on the NAS model and software.
Calculate capacity in four stages
- Find raw capacity. For equal-size drives, multiply the number of drives by the advertised capacity of one drive. For example, four 12 TB drives start with 48 TB of raw capacity. With mixed-size drives, list every drive separately and use the NAS vendor’s calculator rather than applying a single-drive-size formula.
- Apply the storage layout. For equal-size drives of capacity D, common planning estimates are (N−1) × D for one-disk parity, such as RAID 5 or RAIDZ1; (N−2) × D for two-disk parity, such as RAID 6 or RAIDZ2; and about half of raw capacity for two-way mirrors. N is the number of drives. These are estimates, not guarantees: drive-count requirements, implementation, mixed sizes, hot spares, and the NAS’s layout all matter. TrueNAS’s ZFS calculator accounts for ZFS-specific assumptions; Synology’s RAID guidance explains that capacity, protection, and performance are configuration tradeoffs.
- Subtract system and RAID reservations. The amount and accounting method vary by platform. For instance, Synology says each drive in a RAID reserves approximately 10 GB for system space. QNAP separately describes storage-reserved space and RAID metadata in its QTS and QuTS hero context. Do not treat one vendor’s deductions as universal.
- Subtract filesystem metadata and check volume limits. A filesystem may reserve space that is not available for file storage, and a pool’s capacity may exceed the maximum size of one volume. Check the exact NAS model’s supported RAID types, filesystems, and volume limits, then confirm the result shown by its storage manager.
The calculation is best treated as a planning estimate: file-available capacity ≈ raw drive capacity − redundancy cost − platform reservations − filesystem metadata − other configured reservations. The terms may be calculated at different layers, so avoid subtracting a reservation twice if the vendor’s calculator already includes it.
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Keep these capacity figures distinct
- Raw capacity: the sum of manufacturers’ advertised drive capacities.
- Post-layout or pool capacity: an estimate after accounting for redundancy or parity.
- Volume capacity: capacity assigned to a filesystem, subject to system reservations and model limits.
- File-available capacity: space remaining for stored files after filesystem metadata and other usage.
Vendor interfaces may use these labels differently. Check what a calculator’s displayed number represents before comparing it with a volume or file-storage figure. The vendor documentation cited here describes reductions at several stages but does not establish one universal TB-to-TiB conversion treatment.
How the deductions differ by NAS platform
Synology
Synology’s RAID Calculator estimates capacity available to create volumes, not final capacity for files. It says each RAID drive reserves approximately 10 GB for system space. The calculator lists a 4% metadata reservation for Btrfs volumes and 2% for ext4 volumes. Those percentages describe Synology’s published figures; they should not be applied to other NAS vendors or filesystem implementations.
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For different-sized drives in a Synology system, the company recommends SHR or SHR-2 to reduce wasted capacity compared with layouts that cannot make comparable use of the drive sizes. Check the model’s compatibility and supported configuration before choosing.
QNAP
QNAP’s FAQ, last modified August 27, 2026, says QTS and QuTS hero automatically reserve space for system operation and metadata. Its approximate examples include about 70 GB per storage pool or 20 GB per static volume for QNAP storage-reserved space; about 1% of each RAID group’s capacity, capped at 150 GB per group, for QNAP RAID metadata; and roughly 0.8%–1.8% of volumes over 100 GB for ext4 metadata. These are QNAP figures for the documented context, not universal rules. See QNAP’s explanation of unavailable disk capacity.
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TrueNAS and ZFS
TrueNAS’s ZFS Capacity Calculator exposes assumptions such as vdev type and width, spares, record size, ashift, disk swap size, and reservation. TrueNAS characterizes RAIDZ1 as tolerating one disk failure, RAIDZ2 as tolerating two, and mirrors as 50% capacity-efficient. Its calculator documentation was last modified March 2, 2026; use its inputs for a ZFS-specific estimate rather than relying only on a generic RAID formula.
Compare capacity alongside protection and model limits
A higher-capacity estimate does not by itself make a layout the right choice. Compare the usable-capacity estimate with the number of disk failures the layout is designed to tolerate, any documented performance distinctions, and whether the NAS supports the RAID type, filesystem, and desired volume size. A hot spare also affects the capacity available for data, so include it in the platform calculator.
Rank #3
- IronWolf internal hard drives are the ideal solution for up to 8-bay, multi-user NAS environments craving powerhouse performance.date transfer rate:6.0 gigabits_per_second
- Store more and work faster with a NAS-optimized hard drive providing 8TB and cache of up to 256MB
- Purpose built for NAS enclosures, IronWolf delivers less wear and tear, little to no noise/vibration, no lags or down time, increased file-sharing performance, and much more
- Easily monitor the health of drives using the integrated IronWolf Health Management system and enjoy long-term reliability with 1M hours MTBF
- Three-year limited product warranty protection plan and three year Rescue Data Recovery Services included
Supported options and maximum volume sizes are model-dependent. Synology’s DSM 7.4 storage-management specifications list supported RAID and filesystem options. As one model-specific example, the RS822RP+ specifications state a 108 TB maximum single volume and note that usable capacity will be lower depending on filesystem and metadata. That limit applies to that model, not to NAS devices generally.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Verify the estimate on the NAS
- Enter the exact drive capacities, drive count, layout, and any spare in the calculator for the NAS platform and model.
- Choose the intended filesystem and volume arrangement where the calculator supports those inputs; note whether its result is pool capacity, volume capacity, or file-available space.
- After configuration, check the NAS’s storage manager for the created pool and volume figures. If they differ from the estimate, check the selected filesystem, system or metadata reservations, spare configuration, and model limits.
Use the NAS maker’s own calculator as a planning aid—for example, Synology’s RAID Calculator or TrueNAS’s ZFS calculator—and treat the configured NAS’s displayed figures as the practical reference for that system.
Quick Recap
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- Available in capacities ranging from 2 to 22TB(1) | (1) 1GB = 1 billion bytes and 1TB = 1 trillion bytes. Actual user capacity may be less depending on operating environment.
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- Rated for 550TB/yr workload rate(2) | (2) Annualized Workload Rate = TB transferred x (8760 / recorded power-on hours). The maximum rated workload is specified for operating at typical temperature of 40C. Workload Rate will vary depending on your hardware and software components and configurations.
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Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
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