In how many months will your NAS be full?
A NAS does not warn you as it fills: it slows down, then backups fail on a Sunday night. This tool converts your real monthly growth into a dated deadline — the number that turns “we should think about it” into a budget line.
Runway before saturation
18months
Deadline in sight: budget the expansion and watch the growth rate.
- Space still available
- 1.78TB
- Current fill level
- 50%
The method
Everything starts from USABLE capacity: what remains after RAID (one parity drive in RAID 5, half in a mirror) — the figure your NAS interface shows, not the one on the drive boxes.
Two reserves come off: the share kept for snapshots and the system, and the zone beyond the fill threshold — because a volume at 90% slows down and destabilises everything running on it. What remains is the space genuinely available for YOUR data.
Monthly growth does the rest: remaining space divided by that pace gives the runway in months. Measuring it over your last 3–6 months is the only honest method — gut estimates always run low.
In the formula: C the usable capacity (TB), r the reserve (%), s the threshold (%), U the used space (TB), g the growth (GB/month).
available = C × (1 − r/100) × s/100 · runway (months) = (available − U) × 1,000 / g
Good to know
- Growth is never linear: a new baby, a 4K camera or a phone upgrade doubles it overnight. Recalculate after every usage change.
- RAID protects against a drive failure, not deletion, ransomware or theft: a filling NAS must never be solved by deleting the offsite backup.
- Snapshots grow with how much data CHANGES, not how much you store: looping surveillance footage can inflate the reserve well past 15%.
- If cameras are filling this NAS, their exact volume is computed with the CCTV storage tool in this same category — and its result becomes your “monthly growth”.
- Expanding a RAID volume takes planning: depending on the system (SHR, ZFS, mdadm), adding a drive can be trivial… or require a full migration. Check before buying the drive.
FAQ
- Why not fill a NAS to 100%?
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Three reasons: modern filesystems (Btrfs, ZFS) need free space to write efficiently and self-heal — full, their performance collapses; services (backups, databases, cameras) fail without always warning when writes are refused; and no room remains for any repair manoeuvre. 85% is the consensus red line.
- How do I measure my real monthly growth?
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Most NAS keep a usage history (dashboard or reports): take the value from 6 months ago, subtract, divide by 6. Without history, note today’s usage and check again in a month — then come back and update this calculation. Avoid extrapolating an exceptional week (holidays, a migration).
- Better to add a drive or replace the small ones?
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If a bay is free, adding is simpler and cheaper. With full bays, replace drives ONE AT A TIME with bigger ones, letting the rebuild finish between each — capacity typically only expands once all are swapped. Either way: current backup first; rebuilds are the riskiest moment of a volume’s life.
- My NAS shows fewer TB than my drives add up to: normal?
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Yes, for three stacked reasons: RAID redundancy (a whole drive in RAID 5), the base-10 / base-2 gap (an “8 TB” drive is 7.3 TiB), and formatting. Four 8 TB drives in RAID 5 yield about 21.8 TiB usable. That is this tool’s “usable capacity” — take the interface’s figure.
- Should I delete data to push the deadline back?
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The big clean-up yields less than expected: you buy a few weeks of growth at the cost of hours of sorting and the risk of regretted deletions. The real levers: deduplicate photos, compress or offload cold archives, and above all cap what grows on its own (camera retention, recycle bins, backup versions).