Equal-size drives only

NAS RAID Capacity & Drive Count Calculator

Calculate a current layout or work backward from a usable-capacity target to the minimum drive count and total raw purchase capacity.

Calculation direction

Reading a target-capacity result

Required drives is the minimum count that satisfies the selected RAID rules and allowance. Total raw capacity is drive count × capacity per drive before parity or mirroring.

Options when bays are insufficient

If the selected RAID exceeds the NAS bay limit, the result shows the missing bays and the minimum per-drive capacity needed in the available bays. It also compares other RAID levels that fit with the same drives, but do not reduce fault tolerance based on capacity alone.

Product technical comparison

For each 8, 12, 16, 20, and 24 TB candidate, enter the exact model, CMR/SMR method, official compatibility, warranty, and AFR. Configuration review status marks missing facts for verification and excludes SMR or incompatible entries. The result keeps Empty bays and Same-RAID expansion in the same comparison.

Verify the exact model and official evidence together. Capacity-specific SKUs, recording methods, firmware, and compatibility scope can differ within one product family.

What the calculator assumes

Every drive has the same capacity. RAID 5 spends one drive-equivalent on parity, RAID 6 spends two, and RAID 10 mirrors half of the drives. Adding RAID 1 mirrors does not increase usable capacity.

RAID is not a backup. It does not protect you from deletion, ransomware, fire, theft, or failure of the entire NAS.

Mixed drives and vendor-specific RAID

Layouts such as SHR depend on drive sizes and implementation details. Recheck them with a vendor tool such as the official Synology RAID Calculator.

Compare RAID levels by fault tolerance and rebuild risk.

Worked examples

Two-bay photo NAS

Two 8 TB drives in RAID 1 with 5% reserve provide a planning estimate of 7.6 TB. It tolerates one drive failure but still needs backup for deletion, malware, theft, and fire.

Four-bay general NAS

Four 8 TB drives in RAID 5 with 5% reserve estimate 22.8 TB. Compare RAID 6 when rebuild time and a second failure matter more than capacity efficiency.

Four-bay critical storage

Four 12 TB drives in RAID 6 with 10% reserve estimate 21.6 TB, trading capacity for two-drive fault tolerance.

Last reviewed: 2026-08-21