- In TiB (what the OS shows)
- Raw capacity
- Efficiency
- Survives failure of
RAID capacity calculator formulas
Every RAID level trades space for protection. With N drives and S as the size of the smallest drive, these are the formulas the calculator uses. The last column shows four 8 TB drives as an example.
| Level | Min drives | Usable capacity | Survives | 4 x 8 TB |
|---|---|---|---|---|
| RAID 0 | 2 | N x S | Nothing | 32 TB |
| RAID 1 | 2 | S | All but one drive | 8 TB |
| RAID 5 | 3 | (N - 1) x S | 1 drive | 24 TB |
| RAID 6 | 4 | (N - 2) x S | 2 drives | 16 TB |
| RAID 10 | 4 (even) | (N / 2) x S | 1 per mirrored pair | 16 TB |
| JBOD | 1 | Sum of all drives | Nothing | 32 TB |
RAID 5 calculator: worked examples
For RAID 5, subtract one drive and multiply by the smallest drive. Three 4 TB drives give (3 - 1) x 4 = 8 TB. Four 4 TB drives give 12 TB, five give 16 TB and eight give 28 TB. A fuller walkthrough is in how to calculate RAID 5 capacity.
RAID 6 and RAID 10 calculator examples
Six 10 TB drives in RAID 6 give (6 - 2) x 10 = 40 TB, and survive any two failures. Six 10 TB drives in RAID 10 give (6 / 2) x 10 = 30 TB, with faster small-file performance. Not sure which level fits? Read RAID explained.
Synology and QNAP compatible
Synology, QNAP and most other NAS brands use standard RAID, so these results apply directly. Synology also offers SHR, which uses mixed drive sizes more efficiently. See the Synology RAID calculator guide for SHR examples.
TB vs TiB: how the numbers work
Drive makers count terabytes in thousands of billions of bytes, while operating systems often show tebibytes. That is why a drive labelled 8 TB appears smaller on your computer. The TiB figure above shows the approximate size you will see.
Results are estimates. File system overhead and reserved space reduce real capacity slightly. Plan drive count and size with the NAS size picker, then choose drives using our NAS hard drive guide.
RAID calculator FAQ
What is a RAID calculator?
A RAID calculator estimates usable capacity, efficiency and fault tolerance from the number of drives, the drive size and the RAID level you choose.
How do I calculate RAID 5 capacity?
Usable capacity is (N - 1) x the smallest drive, where N is the number of drives. Four 8 TB drives give (4 - 1) x 8 = 24 TB.
What is the RAID 6 calculator formula?
Usable capacity is (N - 2) x the smallest drive. RAID 6 needs at least four drives and survives two drive failures.
What is the RAID 10 calculator formula?
Usable capacity is (N / 2) x the smallest drive, using an even number of drives of at least four. It survives at least one failure, and more if they fall in different mirrored pairs.
Does this RAID calculator work for Synology and QNAP?
Yes for standard RAID levels (0, 1, 5, 6, 10 and JBOD). Synology Hybrid RAID (SHR) allows mixed drive sizes and is not covered by the standard formulas. See our Synology RAID calculator guide.
Why is usable space lower than the calculator says?
File system overhead and reserved space take a little capacity, and operating systems display binary units (TiB), which look smaller than the labelled TB.
What happens if my drives are different sizes?
Standard RAID treats every drive as the smallest drive in the set, so extra capacity on larger drives is not used.