How RAID capacity is calculated
A NAS (network-attached storage) combines several hard drives into one storage pool using RAID. Depending on the RAID level, part of the raw capacity is used to store copies or parity information so that the array survives a drive failure. RaidRight calculates usable space for the most common levels and then subtracts a reserve for the file system, snapshots and free space.
| RAID level | Usable capacity | Drives that can fail | Minimum drives | Best for |
|---|
| RAID 0 | N × size | 0 | 2 | Speed, temporary data |
| RAID 1 | 1 × size | N − 1 | 2 | 2-bay NAS, simple redundancy |
| RAID 5 | (N − 1) × size | 1 | 3 | Balanced capacity and protection |
| RAID 6 | (N − 2) × size | 2 | 4 | Large drives, critical data |
| RAID 10 | N ÷ 2 × size | 1 per mirror | 4 | Databases, virtual machines |
TB vs TiB: why your NAS shows less space
Drive makers use decimal terabytes (1 TB = 1,000,000,000,000 bytes). Operating systems often show binary tebibytes (1 TiB = 1,099,511,627,776 bytes). An "8 TB" drive therefore appears as about 7.28 TiB. Nothing is missing – it's just a different unit.
Planning for growth
Business data typically grows 20 – 40 % per year, faster with video, design files or camera recordings. Size your NAS for at least three to five years. Storage performance and reliability also suffer when a volume is over 80 – 85 % full, so plan to expand before that point. A NAS with spare bays lets you add drives later; replacing drives one by one with larger models also expands most arrays.
Choosing a NAS for the office
- Bays: 2-bay for home offices, 4 – 6 bays for small businesses, 8 + bays or rack models for growing teams.
- CPU and RAM: a quad-core CPU and 8 GB+ RAM if you run virtual machines, Docker or surveillance.
- Networking: 2.5 GbE or 10 GbE ports so several users can open large files quickly.
- SSD cache: M.2 slots speed up databases and many small files.
- Drives: NAS-rated CMR drives designed for 24/7 operation and vibration in multi-bay enclosures.
- Software: snapshots (protection against ransomware), user permissions, cloud sync and backup apps.
- UPS: connect a UPS via USB so the NAS shuts down cleanly during power cuts.
RAID is not a backup
RAID protects against a drive failing. It does not protect against deleting a file, ransomware, theft, fire or the NAS itself failing. Use snapshots for quick recovery and keep at least one copy elsewhere: an external drive rotated off-site, a second NAS at another location, or a cloud backup service. The classic 3-2-1 rule – three copies, on two different media, one off-site – is still the best guideline.
Frequently asked questions
How much usable space does RAID 5 give?
The capacity of all drives minus one. Four 8 TB drives in RAID 5 give 24 TB usable (about 21.8 TiB).
RAID 5 or RAID 6?
RAID 6 survives two drive failures and is recommended for arrays with large (12 TB+) drives or more than six drives, because rebuilds take many hours.
Why does my 8 TB drive show 7.27 TB?
Operating systems report binary units (TiB). 8 TB equals 7.28 TiB – no capacity is lost.
How many drives do I need for RAID 10?
At least four, always in pairs. Half of the raw capacity is usable.
Is RAID a backup?
No. RAID only protects against hardware failure. You still need separate backups, ideally including an off-site copy.
About RaidRight
RaidRight is an independent, free calculator for planning network-attached storage (NAS) and RAID arrays in small offices and home offices. We are not affiliated with any NAS or hard drive manufacturer.
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The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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Last updated: October 28, 2026
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