How Many Drives Do You Really Need?

Updated: 2026-04-29

Last updated March 2026 · 8 min read

The most common mistake in NAS planning is underestimating storage needs. The second most common is overbuying. Here is a practical framework to find the right number.

Try it yourself! Compare RAID configurations with your drive sizes.

Open RAID Calculator →

Step 1: Calculate Your Actual Data

Before choosing drives, measure what you have. On Windows, right-click your data folders and check properties. On Mac, use Finder's Get Info. On Linux, run du -sh. Most people are surprised – their actual data is often either much more or much less than they assumed.

Step 2: Estimate Growth

Think in terms of years, not months. A media library grows at roughly 2-5TB per year for active collectors. Photos and videos from phones add 100-500GB per year per family member. Work documents and backups are typically modest (under 1TB per year).

A good rule of thumb: take your current data, double it, then add 50%. That is your 3-5 year target. If you have 8TB today, plan for roughly 20TB usable capacity.

Step 3: Factor in RAID Overhead

Remember that RAID eats into your raw capacity. With RAID 5, you lose one drive's worth. With RAID 6 or SHR-2, you lose two. Unraid uses the largest drive for parity. To get 20TB usable in RAID 5, you need roughly 27TB raw (four 8TB drives, or three 12TB drives).

This is where our calculator becomes essential – plug in different drive combinations and see exactly what each RAID type gives you.

Try it yourself! Compare RAID configurations with your drive sizes.

Open RAID Calculator →

Step 4: Choose Your Drive Count

2 drives: Mirror only (RAID 1 or SHR-1). Simple, safe, limited capacity. Good for a first NAS or pure backup duty. Usable: one drive's worth.

3-4 drives: The sweet spot for most home users. RAID 5 or SHR-1 gives good capacity with single-drive protection. A 4×8TB array in RAID 5 gives 24TB usable.

5-6 drives: Consider RAID 6, SHR-2, or Unraid with dual parity. The risk of a second failure during rebuild increases with array size and drive capacity. 6×16TB in RAID 6 gives 64TB usable.

8+ drives: Definitely use dual-parity protection. At this scale, also consider splitting into multiple arrays or vdevs for better resilience and rebuild performance.

Step 5: The Upgrade Strategy

Buy the NAS or case with more bays than you need today. Filling a 4-bay NAS with 4 drives on day one leaves no room for expansion. Instead, start with 3 drives in a 4-bay or 4 drives in an 8-bay. You will thank yourself later.

With Unraid and SHR, you can add drives one at a time without rebuilding. With ZFS, you add entire vdevs. Plan your expansion path before buying.

Common Scenarios

Light user (documents, photos, small media): 2×4TB in RAID 1 = 4TB usable. Total cost under $200 including a used mini-PC.

Media enthusiast (1080p library, photos, some 4K): 4×8TB in RAID 5 = 24TB usable. Comfortable for several years of growth.

Serious homelab (4K library, VMs, Docker, backups): 6×16TB in RAID 6 = 64TB usable. Room for significant expansion.

Data hoarder (Linux ISOs, obviously): 8×20TB in Unraid = 140TB usable. You know who you are.