RAID Calculator
Pick your drive count, per-drive capacity and RAID level, and I will work out the usable capacity, how many drives can fail without losing data, the storage efficiency, and what the array costs to build from live Amazon.com prices. Planning the array from scratch instead? Start with the NAS storage calculator to size it, then come back here to check the RAID geometry.
Every RAID level with 4 × 8 TB
| RAID level | Usable | Survives | Efficiency |
|---|---|---|---|
| RAID 0 | 32 TB | no failure | 100% |
| RAID 1 | 8 TB | 3 drives | 25% |
| RAID 5 | 24 TB | 1 drive | 75% |
| RAID 6 | 16 TB | 2 drives | 50% |
| RAID 10 | 16 TB | 1 drive | 50% |
Usable capacity assumes one array of equally sized drives. Fault tolerance is the number of drives that can fail at once without data loss; RAID 10 can survive more if the failures land in different mirror pairs, so the guaranteed floor is shown. Prices are the typical (25th-percentile) live price of a trustworthy 3.5″ HDD at the chosen capacity on Amazon.com; the buy button links the single cheapest listing. Links are affiliate links, disclosure.
Not sure how many terabytes you actually need first? The NAS Storage Calculator sizes the array from your Plex library, photos, backups and cameras, then this tool tells you the usable capacity and redundancy of the RAID you build with those drives.
How RAID capacity works
Every RAID level trades raw capacity for either speed or the ability to survive a failed drive. The usable capacity is a simple function of how many equally sized drives you have and which level you pick:
- RAID 0 stripes across every drive for full capacity and speed, but a single failure loses the whole array. Efficiency is 100%, fault tolerance is zero.
- RAID 1 mirrors the same data onto every drive, so usable capacity is one drive worth and the array survives every failure but the last one.
- RAID 5 (RAIDZ1) spreads one drive of parity across the set. Usable is drive count minus one, and it survives a single failure.
- RAID 6 (RAIDZ2) uses two drives of parity. Usable is drive count minus two, and it survives two simultaneous failures — the safer choice for large drives with long rebuild times.
- RAID 10 stripes across mirrored pairs, giving half the raw capacity with fast reads and writes; it survives at least one failure, and more if the losses fall in different pairs.
The build cost above is not a rule of thumb: it multiplies your drive count by the typical (25th-percentile) live price of a trustworthy internal 3.5-inch hard drive at the capacity you chose, drawn from the same price index that powers the cheapest $/TB ranking. RAID buyers usually need several matched drives at once, so the tool prices the whole array and links the cheapest listing to buy.