Practical guide
Estimate usable NAS capacity from a RAID layout
ArrayKeep connects a drive purchase question to a storage workload. First select the layout and reserve, then inspect the decimal TB and TiB results.
Start with equal-drive inputs
The default scenario uses four 8 TB drives and RAID 5. The simplified engine treats RAID 5 as drive count minus one drive equivalent, producing 24 TB of raw capacity.
RAID 5 raw TB = (drive count - 1) x drive capacity
The default 4 x 8 TB layout calculates 24 raw decimal TB before reserve and filesystem effects.
Apply a free-space reserve
A reserve keeps the planning target below the raw layout result. With the default 10% reserve, 24 raw TB becomes 21.6 usable decimal TB, also shown as about 19.65 TiB.
usable TB = raw TB x (1 - reserve percent / 100)
Keep layout assumptions explicit
This worksheet assumes equal-capacity drives and does not infer bays, filesystem behavior, snapshots, compression, or drive support. Replace the illustrative values with the proposed array details.
Check your own scenario
Replace the illustrative inputs with measured values and dated specifications.
Open ArrayKeep Storage PlannerMethod and references
Read the full calculation method and its limits
Method and examples reviewed 2026-09-30. Examples are illustrative; input values and specifications are not independently verified.