How to Choose the Right RAID Level for Your NAS (and Calculate Usable Capacity)

Picking a RAID level for your home NAS or server is one of those decisions that feels simple until you actually sit down to do it. RAID 5 sounds like the safe default. RAID 6 sounds safer. RAID 10 sounds fast. But how much usable space do you actually get? And what happens when a drive fails during a rebuild?

This guide walks through the RAID levels that matter for NAS setups in 2026, the capacity math behind each one, and how to pick the right level for your use case.

RAID Levels at a Glance

Level	Min Drives	Usable Capacity	Fault Tolerance	Best For
RAID 0	2	N × drive size	None	Cache, temp data
RAID 1	2	50%	N-1 drives	System disk, critical data
RAID 5	3	(N-1) × drive size	1 drive	File server, home NAS
RAID 6	4	(N-2) × drive size	2 drives	Large storage, business
RAID 10	4 (even)	50%	1 per mirror pair	Databases, high-performance apps
RAID-Z1	3	(N-1) × drive size	1 drive	ZFS / TrueNAS home
RAID-Z2	4	(N-2) × drive size	2 drives	ZFS enterprise storage
RAID-Z3	5	(N-3) × drive size	3 drives	Very large capacity, data centers

The Capacity Math

The formulas are straightforward once you know them:

• RAID 0: All drives count. 4 × 4TB = 16TB usable. But one failure = all data lost.
• RAID 1: Mirror. Usable = smallest drive size. Adding more drives increases redundancy, not capacity.
• RAID 5: Usable = (N-1) × smallest drive. 4 × 4TB = 12TB usable, 1 drive of parity.
• RAID 6: Usable = (N-2) × smallest drive. 4 × 4TB = 8TB usable, 2 drives of parity.
• RAID 10: Usable = 50% of total. 4 × 4TB = 8TB usable. Fast reads and writes, but you lose half your raw capacity.
• RAID-Z1/Z2/Z3: Same math as RAID 5/6, but with ZFS checksums and copy-on-write.

The catch: traditional RAID uses the smallest drive as the baseline. If you mix a 2TB drive with three 4TB drives in RAID 5, you get (4-1) × 2TB = 6TB usable, and the extra 2TB on each 4TB drive sits unused. Synology's SHR (Hybrid RAID) handles mixed drives better by creating multiple virtual arrays, which is why SHR is popular for home NAS setups where you upgrade drives gradually.

How to Choose

Safety first: RAID 1, RAID 6, RAID 10, or RAID-Z2/Z3. If your data matters and you can't afford downtime, go with dual parity.

Performance first: RAID 0 or RAID 10. RAID 0 is fastest but has zero fault tolerance — only use it for data you can afford to lose. RAID 10 gives you both speed and redundancy.

Balanced: RAID 5 or RAID-Z1. Good capacity efficiency with single-drive protection. Best for home NAS and small file servers.

Enterprise / high availability: RAID 6, RAID 10, or RAID-Z2/Z3. When you have 8+ drives or drives larger than 8TB, dual parity becomes important because rebuild times get long, and the chance of a second drive failing during rebuild increases.

RAID 5 vs RAID 6: The Rebuild Problem

This is the decision most people get wrong. RAID 5 feels like enough — one drive can fail, you replace it, done. But with modern large drives (8TB, 12TB, 20TB), a RAID 5 rebuild can take days. During that rebuild, your array is running with zero redundancy. If a second drive has a latent bad sector (URE — Unrecoverable Read Error), the rebuild fails and you lose data.

Rule of thumb: if your drives are 8TB or larger, or you have more than 6 drives, use RAID 6 or RAID-Z2. The extra parity drive is cheap insurance.

Common Mistakes

1. "RAID is backup" — No, it's not. RAID protects against drive failure. It doesn't protect against accidental deletion, ransomware, fire, theft, or a second drive failing during rebuild. Always have a real backup (3-2-1 rule: 3 copies, 2 different media, 1 offsite).

2. "My 4TB drives give me 16TB in RAID 5" — No. 4 × 4TB in RAID 5 = (4-1) × 4TB = 12TB usable. One drive's worth goes to parity.

3. "The OS shows less space than I expected" — Drive manufacturers use decimal TB (1TB = 1,000GB). Operating systems use binary TiB (1TiB = 1,024GiB). A "4TB" drive shows as ~3.64TiB in your OS. Add filesystem overhead and system reserved space, and usable capacity is always less than the label.

4. "Adding drives to RAID 1 increases capacity" — No. RAID 1 is mirroring. Adding a third drive to RAID 1 gives you a third copy, not more space.

5. "Mixed drive sizes are fine in traditional RAID" — They work, but you waste space. Traditional RAID uses the smallest drive as the baseline. SHR or ZFS handle mixed sizes more efficiently.

Planning Your Array

Before buying drives, calculate:

1. How much usable space do you need? Add 20-30% headroom for growth.
2. How many drives can your NAS hold? A 4-bay NAS limits your options.
3. What's your budget? RAID 10 costs more per usable TB than RAID 5/6.
4. How critical is the data? If downtime is unacceptable, go dual parity.
5. Are you mixing drive sizes? If yes, consider SHR or plan to buy same-size drives.

A quick way to compare options: plug your drive sizes into a RAID calculator and see usable capacity, protection, and unused space for each level side by side. For example, this free RAID calculator supports up to 12 mixed-size drives, SHR, traditional RAID, and ZFS RAID-Z, with a visual breakdown of usable vs. protected vs. unused space. It's a handy way to sanity-check your plan before buying drives.

Bottom Line

There's no single "best" RAID level — it depends on your drives, your budget, and how much you value capacity vs. safety vs. speed. For most home NAS users, RAID 5 or SHR hits the sweet spot. For anything business-critical or with large drives, RAID 6 or RAID-Z2 is the safer bet. And whatever you choose, remember: RAID is not backup.