Why Usable Capacity Isn’t Just “Add Up the Drives”

RAID (Redundant Array of Independent Disks) combines multiple physical drives into one logical volume, but different RAID levels trade raw capacity for fault tolerance in very different ways. A four-drive array can deliver anywhere from one drive’s worth of usable space to nearly the full raw total, depending entirely on which RAID level you choose — this calculator shows the usable capacity and fault tolerance for your specific drive count and RAID level before you commit to a build.

The Trade-Off Between Levels

RAID 0 stripes data across all drives for maximum speed and capacity but offers zero redundancy — losing any one drive loses everything. RAID 1 mirrors data across drives, sacrificing capacity for simplicity and tolerance of multiple failures. RAID 5 and RAID 6 use distributed parity to tolerate one or two drive failures respectively while keeping most of the raw capacity usable, at the cost of needing at least 3 or 4 drives. RAID 10 stripes mirrored pairs together, combining speed and redundancy but using half your raw capacity.

Why Your OS Shows Less Than You Bought

Drive manufacturers rate capacity in decimal terabytes (1 TB = 10¹² bytes), but operating systems typically report storage in binary tebibytes (1 TiB = 2⁴⁰ bytes). That mismatch alone makes an “8TB” drive show up as roughly 7.28 TiB in your file browser — nothing is missing, it’s just a different unit. This calculator shows both figures so the difference doesn’t come as a surprise.

This tool is especially useful for:

• Editors and DITs sizing a RAID array for a project’s storage needs.
• Anyone building or buying a NAS and comparing RAID levels before purchase.
• IT and post-production teams planning fault tolerance against drive failure.