A RAID capacity calculator answers a specific question: how much logical storage can a selected array layout expose from its active member drives? The answer depends on drive count, member capacity, and the redundancy scheme. Adding the capacities printed on the drive labels gives installed raw capacity, not necessarily the space available for files. Mirroring stores additional copies, parity stores recovery information, and mismatched drives can leave capacity outside the common array geometry.

This planner models conventional equal-width RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10. Enter every active drive in decimal terabytes, separated by commas. For example, four equal eight-terabyte members are written as 8,8,8,8. An entry such as 8,8,12,12 explicitly represents different member sizes rather than assuming all four drives are identical. The planner supports up to sixty-four members and explains its smallest-drive assumption in both the form and results.

The main output is capacity before formatting. Supporting rows separate installed capacity, unused mixed-size capacity, and mirroring or parity allocation. This separation helps you explain where the difference comes from without calling every missing terabyte filesystem overhead. The failure-tolerance figure describes a drive-loss property of the selected layout under stated assumptions. It does not describe a backup policy, a reliability probability, a rebuild duration, or a promise that an application will remain available through every fault.

RAID 0 stripes data across members without redundancy. In this equal-width model, it exposes one smallest-drive-sized contribution from each member. It has the largest usable capacity for a given set of equal drives, but losing a member can make the striped array unusable. Use its capacity figure only alongside a clear acceptance of that risk. Capacity alone is not evidence that RAID 0 is suitable for the only copy of important information.

RAID 1 is modeled as a single mirror containing every entered drive. Two members hold two copies; four members hold four copies of the same logical address space. Four-way mirroring is not the same as two mirrored pairs striped together. If that paired layout is what you intend, choose RAID 10 instead. This distinction matters because a four-way RAID 1 mirror exposes only one member's capacity, while a four-drive RAID 10 array exposes two members' capacity.

RAID 5 uses distributed single parity, and RAID 6 uses distributed dual parity. Their capacity cost is equivalent to one and two member drives respectively, even though a dedicated physical parity drive is not required. RAID 10 combines two-drive mirrors with striping across the pairs. The calculator requires at least two members for RAID 0 or RAID 1, three for RAID 5, and four for RAID 6 or RAID 10. RAID 10 additionally requires an even count.

Manufacturers commonly label storage using decimal prefixes. One terabyte, abbreviated TB, means one trillion bytes. A tebibyte, abbreviated TiB, means two to the fortieth power bytes, or 1,099,511,627,776 bytes. Because a TiB contains more bytes than a TB, the numerical value expressed in TiB is smaller for the same physical quantity. The conversion is a change of unit, not evidence that parity or formatting consumed additional storage.

The calculator converts usable TB to bytes and then divides by the number of bytes in one TiB. A 24 TB RAID 5 estimate becomes approximately 21.828 TiB. Both numbers refer to exactly the same modeled usable capacity before additional reservations. Some operating systems and interfaces display binary-scaled values while using a decimal-looking label. Check the documentation or the underlying byte count before comparing a manufacturer specification with a storage-management screen.

Do not subtract the TB-to-TiB difference a second time as an overhead allowance. Start with a consistent byte quantity, translate it to whichever unit your planning documents use, and then separately account for actual storage reservations. When budgeting several arrays, write units next to every value rather than relying on the spreadsheet column name alone. This is particularly important when a vendor quote, a backup report, and an operating-system dashboard use different display conventions. The result card deliberately shows both TB and TiB so the conversion remains visible.

Begin with the capacity of each active member and the layout your actual system supports. Compare more than one RAID level using the same list of drives so that raw capacity stays constant. Review usable capacity and redundancy together, then check how the operating system reports the units. If the estimate looks unexpectedly small, first look for an accidental four-way RAID 1 selection, an undersized member, a dedicated spare included in the list, or a TB-versus-TiB comparison.

Next, consider operational behavior that the capacity equations do not predict. Rebuild time depends on member size, device speed, controller behavior, background load, error handling, and other implementation details. A layout with more usable space may have different degraded-mode performance or repair exposure. Similarly, identical nominal capacity does not establish identical write performance, fault isolation, or suitability for a transactional workload. Use vendor guidance and representative measurements rather than deriving performance from the number of capacity-equivalent parity members.

Finally, document the assumptions alongside your result: active member sizes, RAID level, smallest-drive policy, spare policy, units, and excluded reservations. Test backup restoration before making destructive changes, and keep a record of which physical devices belong to which array or mirror pair. Recalculate after proposed expansion, but confirm that the platform supports the transition rather than only the final geometry. A useful capacity plan is a reproducible calculation joined to a verified operational process, not simply the largest number that fits on a purchasing spreadsheet.