What RAID level should I use for my NAS?

For 2 drives, RAID 1 (mirror) is the only option. For 3+ drives, RAID 5 offers good capacity with single-drive fault tolerance. For 4+ drives with large disks (8 TB+), RAID 6 provides dual-drive protection during lengthy rebuilds.

How much does the NAS OS use?

Synology DSM uses about 2–2.5 GB, QNAP QTS uses about 2–4 GB, and TrueNAS uses 8–16 GB for boot. Some systems use a separate boot drive. The percentage impact on large arrays is minimal but worth noting.

Should I use Btrfs or ext4?

Btrfs offers data checksumming, snapshots, and self-healing capabilities at the cost of slightly more overhead (3–5%). Ext4 is simpler with lower overhead (1–2%). For critical data, Btrfs's integrity features are worth the small space trade-off.

When do I need to expand my NAS?

Start planning expansion when you hit 70% utilization. Most NAS units allow adding drives to an existing volume or replacing drives with larger ones. The expansion process can take days for large volumes, so plan ahead.

How accurate are growth projections?

Linear growth projections are a reasonable starting point, but many environments see accelerating growth (more cameras, more users, more data sources). Check actual growth quarterly and adjust projections. Adding a 20% buffer is prudent.

Can I mix different drive sizes?

Traditional RAID uses the smallest drive's capacity for all members. SHR (Synology) and FlexRAID can use mixed sizes more efficiently. If using traditional RAID, buy all drives the same size and ideally from different production batches.

NAS Capacity Planner

Plan NAS storage capacity by calculating usable space from disk count, size, and RAID level. Includes OS overhead and growth projections.

NAS Configuration

Number of Disks

Disk Size

RAID Level

Alert at %

Input

% full

Overhead & Growth

OS Overhead

Filesystem Overhead

Monthly Data Growth

TB/month

Raw Capacity

32.0 TB

4 x 8 TB

After RAID

24.0 TB

RAID 5 @ 75%

After OS/FS

23.5 TB

2% OS + 3% FS

Usable Capacity

22.8 TB

available for data

Time Until Full

3.8 years

at 0.5 TB/month

Alert Threshold

16.0 TB

32 months from now

First-Year Growth Timeline

Month	Data Used	Remaining	Capacity %
1	0.50 TB	22.31 TB	2%
2	1.00 TB	21.81 TB	4%
3	1.50 TB	21.31 TB	7%
4	2.00 TB	20.81 TB	9%
5	2.50 TB	20.31 TB	11%
6	3.00 TB	19.81 TB	13%

Expansion Options

Option	Added Capacity	New Total	New Timeline
Add 2 x 8 TB	+11.4 TB	34.2 TB	5.7 yrs
Add 4 x 8 TB (full shelf)	+22.8 TB	45.6 TB	7.6 yrs
Double disk size (replace all)	+22.8 TB	45.6 TB	7.6 yrs

Best Practices: Keep NAS utilization below 85% for best performance. Start expansion planning at 70% full. Always maintain a separate backup copy of critical data. Monitor free space weekly using NAS alerts.

Planning notes, formulas, and examples

About the NAS Capacity Planner

Planning NAS capacity means accounting for RAID overhead, OS reserved space, filesystem overhead, and future data growth. A 4-bay NAS with 4 TB drives in RAID 5 doesn't give you 16 TB—after parity, OS space, and filesystem overhead, you'll have about 10.5–11 TB usable. And that number shrinks relative to your needs every month as data grows.

This calculator models the full capacity picture: start with disk count and size, apply RAID efficiency, subtract OS and filesystem overhead, then project how long your available capacity will last given your data growth rate. It's the calculator every NAS buyer should use before committing to a disk configuration.

When This Page Helps

NAS capacity is significantly less than raw disk totals due to RAID, OS, and filesystem overhead. This planner shows your actual usable space and how long it will last, preventing the frustration of running out sooner than expected.

How to Use the Inputs

Enter the number of disk bays and disk size.
Select the RAID level for your configuration.
Enter OS overhead (typically 1–2% or a fixed amount).
Enter the filesystem overhead percentage.
Optionally enter your monthly data growth for timeline projection.
Review usable capacity and estimated time to full.

Formula used

raw = disks × disk_size; raid_usable = raw × RAID_efficiency; usable = raid_usable × (1 − os_overhead/100) × (1 − fs_overhead/100); months_until_full = usable / monthly_growth

Example Calculation

Result: 22.8 TB usable, ~45 months until full

4 × 8 TB = 32 TB raw. RAID 5 with 4 drives: (4−1) × 8 = 24 TB. After 2% OS overhead: 23.5 TB. After 3% filesystem overhead: 22.8 TB usable. At 0.5 TB/month growth, that's approximately 45 months (3.75 years) until the drives are full.

Tips & Best Practices

Buy the largest drives your budget allows—adding bays later is more disruptive than replacing drives.
SHR (Synology Hybrid RAID) can use mixed-size drives more efficiently than traditional RAID.
Leave 10–15% of NAS capacity free for performance—Btrfs and ZFS slow down above 85% utilization.
NAS OS (DSM, QTS, TrueNAS) typically reserves 1–2% of total capacity for system use.
RAID is not backup—always maintain a separate backup of critical NAS data.
Consider NVMe cache drives to accelerate IO without increasing bulk capacity.

NAS for Home vs. Business

Home NAS setups typically use 2–4 bays with consumer drives. Business NAS uses 8–24+ bays with enterprise drives and often includes 10GbE networking, SSD caching, and UPS integration. The sizing principles are the same, but businesses should plan for higher availability and faster growth.

Drive Selection Tips

Choose NAS-rated drives (WD Red, Seagate IronWolf) designed for 24/7 operation and RAID compatibility. Desktop drives may work initially but have higher failure rates in RAID environments. Enterprise drives offer the best reliability but at a premium price.

Monitoring and Alerts

Set up free-space alerts at 70%, 80%, and 90% utilization. Many NAS operating systems can send email or push notifications. Automated monitoring prevents the scenario where a NAS fills up silently and stops accepting new files.

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

For 2 drives, RAID 1 (mirror) is the only option. For 3+ drives, RAID 5 offers good capacity with single-drive fault tolerance. For 4+ drives with large disks (8 TB+), RAID 6 provides dual-drive protection during lengthy rebuilds.