Plan NAS storage capacity by calculating usable space from disk count, size, and RAID level. Includes OS overhead and growth projections.
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 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 tool every NAS buyer should use before committing to a disk configuration.
Quantifying this parameter enables systematic comparison across environments, deployments, and time periods, revealing optimization opportunities that improve both performance and cost-effectiveness. This analytical approach supports proactive infrastructure management, helping teams avoid costly outages and maintain the service levels that users and business stakeholders depend on.
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. Data-driven tracking enables evidence-based infrastructure decisions, reducing the risk of over-provisioning costs or under-provisioning that leads to performance bottlenecks.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.