Skip to main content
StorageMath

Storage Capacity Planner

The Storage Capacity Planner walks you through the step-by-step reduction from raw drive capacity down to the gigabytes you can actually write data to. Enter your Number of Drives, Drive Capacity, and RAID Level, then adjust Hot Spares, System Reserve, and File System Overhead, and click Calculate Capacity to see exactly how each layer chips away at your usable storage.

Drive Count, Raw Size, RAID Level and File System: Planner Inputs

Raw, Formatted and Usable Capacity After All Overhead

Raw Capacity

—

After RAID

—

After Reserves

—

Final Usable

—

RAID, File System and Snapshot Overhead Summary

Hot Spare Loss

—

RAID Overhead

—

System Reserve

—

FS Overhead

—

Total Overhead

—

Sizing Storage for NAS, SAN, VM and Backup Workloads

Storage capacity planning requires understanding the difference between raw capacity and usable capacity. When vendors advertise a 10TB drive, that's the raw capacity measured in decimal gigabytes (1GB = 1,000,000,000 bytes). However, operating systems typically report capacity in binary gigabytes (1GiB = 1,073,741,824 bytes), immediately showing ~7% less space. Check out our IOPS and throughput calculator.

Beyond the decimal vs binary difference, several overhead factors reduce usable capacity. RAID protection consumes capacity for parity or mirroring. System reserves hold space for metadata, snapshots, and array operations. File system formatting creates additional overhead for directories, journals, and allocation tables. Learn more: convert TB to TiB and storage units.

For accurate capacity planning, always calculate usable capacity using realistic overhead percentages. Enterprise arrays typically reserve 5-15% for system operations, while file systems consume 1-5% depending on block size and feature set. Planning with these factors ensures you don't over-commit storage resources. See also: StorageMath calculators.

NTFS, ext4, ZFS, RAID and Snapshot Overhead: Typical Values

  • System Reserve (5-15%): Metadata, snapshots, thin provisioning overhead
  • File System (1-5%): NTFS ~3%, ext4 ~1-2%, ZFS ~3-5%
  • Hot Spares: 1 per 20 drives or 1-2 per RAID group recommended
  • Growth Buffer: Consider 20-30% for future growth