NAS Storage Calculator
Enter what you store today and how it grows. The tool separates three numbers that marketing pages routinely merge: the data you have, the data you will have, and the capacity you should buy — which is not the same as the sum of your drive labels.
Data today
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Projected (growth applied)
—
after the horizon you set
Recommended usable
—
projection + reserve
Raw capacity needed
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Configurations that reach this size (same RAID layout, different drive sizes):
| Drive size | Drives needed | Raw capacity | Usable capacity |
|---|
Usable capacity is what the volume reports after RAID. Operating systems show TiB, which reads about 10% lower again — that is unit conversion, not missing space.
How the calculation works
Photos multiply count by average file size. Video offers two paths: hours times bitrate (GB = hours × 3600 × Mbps ÷ 8 ÷ 1000, decimal), or a library size you already know. Documents and existing data are entered directly. The total is compounded by your growth rate across the planning horizon, then padded by the reserve percentage — that padded figure is the usable capacity to buy for.
Raw capacity then depends on the RAID layout: RAID 5 spends one drive on parity, RAID 6 spends two, RAID 10 spends half the bays, and RAID 1 pairs cap out at a single drive's size. The configuration table lists, for common drive sizes, how many drives reach your target and what the array would actually report as usable.
What this tool deliberately does not do: it does not guess your photo count, does not assume compression, and does not treat raw capacity as usable. If a vendor's chart implies otherwise, ask which of those three they are hiding.
Why is usable capacity smaller than the drive labels add up to?
Three reasons: RAID reserves capacity for redundancy (a mirror spends half, parity spends one or two drives), file systems and snapshots hold back a few percent, and drive makers label capacity in decimal TB while operating systems display binary TiB. A four-drive RAID 5 array of 8 TB drives offers roughly 21.8 TB usable, not 32 TB.
How much reserve should I plan for?
Twenty percent is a workable default for a general-purpose NAS: it covers file system overhead, snapshot space and the fact that arrays behave badly when nearly full. Surveillance volumes can run tighter; archival volumes can run looser.
Should I buy more drives or bigger drives?
Bigger drives cost less per bay but concentrate rebuild risk: when a 20 TB drive fails, the rebuild reads and writes every remaining sector for a day or more. Four mid-size drives spread that risk and let you grow one drive at a time. The configuration table above shows the same usable capacity at different drive sizes so you can weigh it.