How to calculate Unraid array capacity
Add the capacities of the drives assigned to data. Keep parity drives out of that sum: they provide recovery information rather than space for your files.
For example, a 12 TB parity drive with 4 TB, 8 TB, and 12 TB data drives gives 24 TB of array-level data capacity. It doesn't shrink every data drive to 4 TB as a conventional equal-member RAID model would. The operating system will still report less file space after filesystem and other deductions.
Enter each drive under its intended role. The calculator won't silently choose a parity disk for you, because that choice also affects which drive you can add next.
Why must parity be at least as large as the data drives?
Parity must cover the largest data drive. With dual parity, both parity drives must meet that requirement. Extra space on a larger parity disk does not become data capacity.
| Mode | Modeled protection | Size requirement |
|---|---|---|
| No parity | No drive-failure protection from parity | No parity ceiling |
| Single parity | One drive failure | The parity drive must be at least as large as every data drive |
| Dual parity | Two drive failures | Both parity drives must be at least as large as every data drive |
These are layout boundaries, not a prediction that a recovery will succeed. Parity does not replace a separate backup of important files.
Can I add a larger data drive later?
Use the proposed-drive input to check that dependency without adding the drive to today's capacity. If the proposal exceeds the current parity limit, the result identifies the undersized parity drives.
For example, adding a 16 TB data drive to the earlier 12 TB parity setup requires a parity upgrade first. The calculator identifies that requirement; it doesn't perform a parity swap, estimate its duration, or confirm that the real array is ready.
Follow the official adding-disks and replacing-disks instructions for the actual operation.
What this Main Array estimate leaves out
The result is not filesystem-visible free space. It excludes filesystem metadata, partition alignment, reservations, cache pools, share allocation, snapshots, and application behavior. It also does not predict throughput or inspect disk health.
TB and TiB are different units for bytes, not different levels of overhead. Keep the input units consistent with the drive labels. If you're planning ZFS vdevs instead of an Unraid Main Array, use the ZFS capacity calculator.
What happens to the drive list?
Calculation stays in your browser and does not save the list to browser storage. Share is explicit. Its URL contains the drive capacities and roles, so anyone with that link can inspect the configuration.