Our home server cost breakdown puts a small NAS at roughly $200–$350 diskless, and a larger one at $500–$1,000+ — a wide enough range that "how many bays do I actually need" is worth answering properly before you buy, rather than defaulting to whatever's cheapest or most popular. This guide is a sizing framework, not a list of brands: work out your real numbers first, and the right box becomes obvious.
Start with usable capacity, not drive capacity
The number on the box (say, "4-bay, up to 80TB") is raw capacity — the sum of every drive slot filled with the largest drive it supports. Almost nobody runs a NAS without redundancy, and redundancy costs usable space:
- 2-bay NAS in RAID 1 (mirror): two drives hold identical copies of your data. Usable capacity = one drive's worth, regardless of how many drives you install. Two 4TB drives = 4TB usable, not 8TB.
- 4-bay NAS in RAID 5 / Synology SHR / similar: usable capacity = total capacity minus one drive's worth, since that one drive's worth of space is reserved for parity data that lets the array survive a single drive failure. Four 4TB drives = about 12TB usable.
- 4-bay+ in RAID 6 / SHR-2: minus two drives' worth of capacity, but survives two simultaneous drive failures — worth considering once you're storing something you really can't afford to lose and don't have an equally solid backup elsewhere.
This is the single most common source of "wait, why is my usable space so much smaller than advertised" confusion, and it's worth doing this math against your current library size before you ever open a shopping page.
Work out your real growth rate first
Rough, real-world reference points to estimate from:
- A modern smartphone photo is typically 3–8MB; a phone shooting 4K video runs roughly 130–400MB per minute of footage.
- A household actively taking photos and some video commonly adds 50–150GB per year to a photo/video library once everything (including duplicates and edits) is counted.
- A modest movie collection (a few hundred titles at reasonable quality, not maxed-out 4K remuxes) runs roughly 1–3TB total; a large 4K movie/TV collection can run into the tens of terabytes.
Take your current library size (check your existing cloud storage usage, e.g. Google Photos or iCloud, as a starting estimate for photos), multiply your annual growth rate by 3–5 years, and add that to today's number. That total — not today's number alone — is what you should be sizing usable capacity against.
2-bay vs 4-bay: what actually changes
| 2-bay | 4-bay+ | |
|---|---|---|
| Typical use case | Photo backup, small file sync, one modest media library | Growing media library, multiple use cases at once, families sharing one NAS |
| Redundancy options | RAID 1 (mirror) only — 50% usable capacity | RAID 5/SHR (n-1 drives usable) or RAID 6/SHR-2 (n-2, more failure tolerance) |
| Growing capacity later | Replace both drives with larger ones (rebuild pool) | Same, but a 4-bay gives more headroom before you need to touch existing drives at all |
| Typical diskless price | $150–$300 | $300–$600+ |
| Power draw | Lower (fewer drives spinning) | Higher, but better $/TB of usable, protected storage |
A practical decision framework
- If your realistic 3-5-year total is under roughly 4TB (a photo-focused household, light media use): a 2-bay NAS with two matched drives in RAID 1 is enough, and it's the cheaper, lower-power option.
- If your total lands in the 8–20TB range (an active media library, or photos plus a meaningful video collection): a 4-bay NAS in RAID 5/SHR gives you real headroom and lets you add capacity by swapping in larger drives one at a time later, without buying a second unit.
- If you're already past 20TB or expect to be soon (a large 4K media library, multiple contributors, or archival video): look at a 5-bay+ unit or a larger DIY build from the start, since replacing an undersized NAS entirely is more expensive and disruptive than buying slightly bigger the first time.
Don't forget the drives themselves
NAS enclosures are commonly sold diskless — the bay count and RAID capability is only half the buying decision. The drives you put in matter just as much for reliability, and NAS-rated drives are genuinely different from the desktop hard drives most people already have lying around; see our WD Red vs Seagate IronWolf comparison for which ones to actually buy and why plain desktop drives are a bad fit for a multi-bay array.
Software matters too, not just the enclosure
A prebuilt Synology or QNAP NAS gets you a polished app ecosystem and easy setup out of the box. A self-built NAS running TrueNAS SCALE on a mini PC or small tower with a drive cage gives you ZFS's stronger data-integrity guarantees and no vendor lock-in, at the cost of a slightly steeper setup. If you're still deciding between a dedicated NAS appliance and building your own, our Proxmox vs TrueNAS vs Unraid comparison is the right next read.