Equal-size drives onlyNAS 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.
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 purchase and five-year cost comparison
For each 8, 12, 16, 20, and 24 TB candidate, enter the exact model, price, CMR/SMR method, official compatibility, warranty, and AFR. Only fully checked CMR models with confirmed compatibility qualify for the 5-year expected cost ranking. Purchase 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.
The five-year expected cost is not a lifespan forecast. It is initial cost plus a simple expected value of initial cost × entered AFR × years after warranty. It excludes shipping, downtime, recovery, price changes, correlated failures, replacement inventory, and renewed warranty terms.
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.
Formula and units
With equal-size drives, RAID 0 uses all drives, RAID 1 one drive of capacity, RAID 5 loses one drive, RAID 6 loses two, and RAID 10 uses half. The planning reserve is then subtracted from that simple array capacity.
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.
How to interpret the result
This is a purchase-planning figure. Vendor reservations, filesystems, bad-block replacement, snapshots, and installed packages change the number shown by the NAS. If the reverse calculation exceeds the enclosure bays, compare larger drives or an expandable enclosure.
Error sources and limits
Mixed-size SHR layouts, special ZFS vdevs, and tiered pools are outside this simple model. RAID 10 fault tolerance depends on which mirror members fail. Use the exact vendor compatibility list and official capacity calculator for the final design.
Frequently asked questions
Is RAID a backup?
No. It reduces downtime but does not independently recover deletion, corruption, theft, or fire.
Why is the displayed capacity smaller?
TB versus TiB, filesystem metadata, and reserved space all contribute.
Can I mix drive sizes here?
No. This calculator assumes equal-size drives; use the vendor tool for mixed layouts.
Last reviewed: 2026-08-21