Assumes all drives are identical. Real-world results vary by vendor, filesystem, snapshots, and compression settings.
How this NAS storage calculator helps
Buying drives for a NAS can feel straightforward until you start looking at usable space. You might install 4 drives rated at 12 TB each and expect 48 TB, but then discover your available capacity is much lower. That’s normal. This calculator gives you a practical estimate of what you can really use after RAID protection, filesystem overhead, and a healthy free-space buffer.
Raw capacity vs usable capacity
Raw capacity
Raw capacity is simply: number of drives × drive size. It does not account for redundancy, formatting, metadata, snapshots, or performance headroom.
Usable capacity
Usable capacity is what remains after your RAID layout reserves space for data protection and after filesystem overhead is applied. If you also keep free space (recommended), your “safe working capacity” is lower again. This is the number that matters for planning.
Quick RAID level guide
- JBOD: combines capacity, no redundancy. Fast to fill, risky for important data.
- RAID 0: maximum speed and full raw capacity, but zero fault tolerance.
- RAID 1: mirrors data for safety; high protection, lower capacity efficiency.
- RAID 5: one drive worth of parity, good balance for home and small office setups.
- RAID 6: two drives worth of parity, better for large arrays and large disks.
- RAID 10: mirrored pairs with striping; strong performance and solid resilience.
Why your NAS should keep free space
Running a NAS near 100% full can hurt performance and increase operational risk. Most admins aim to keep 10%–20% free for smoother writes, snapshot growth, rebuilds, and maintenance tasks. This calculator includes a reserve setting so your estimate reflects real-world operation instead of theoretical maximums.
Example planning scenarios
Home media NAS
A 4-bay system with 8 TB drives in RAID 5 gives strong capacity with one-drive fault tolerance. If your media library is growing quickly, model next year’s expected growth now to avoid replacing drives too soon.
Small business file server
For critical documents, RAID 6 is often worth the capacity tradeoff. Two-drive fault tolerance can reduce risk during rebuilds, especially with high-capacity drives where rebuild time may be long.
Important reminder: RAID is not backup
RAID helps with drive failure, but it does not protect against accidental deletion, ransomware, corruption, theft, or fire. Use a true backup strategy such as 3-2-1:
- 3 copies of your data
- 2 different storage media
- 1 copy offsite (or cloud)
Common sizing mistakes to avoid
- Planning from raw TB instead of usable TB
- Ignoring growth over 12–24 months
- Forgetting snapshot and versioning storage usage
- Running arrays almost full with no expansion path
- Assuming every RAID option has the same rebuild risk
Final takeaway
A good NAS plan is about capacity and resilience. Use this calculator to estimate realistic usable space, then add growth margin and backup coverage. If you do that before buying drives, you’ll spend less, avoid painful migrations, and keep your data safer over the long run.