How RAID capacity works
RAID combines several drives into one volume. Some levels split data across drives for speed, some copy it for safety, and some store parity: extra data that lets the array rebuild a failed drive. That protection costs space, so usable capacity is always less than the total of the drives (except RAID 0, which has no protection at all).
The formulas (n drives of size s)
RAID 1: s
RAID 5: (n − 1) × s
RAID 6: (n − 2) × s
RAID 10: (n ÷ 2) × s
Example: four 8 TB drives in RAID 5
| Step | Result |
|---|---|
| Raw: 4 × 8 TB | 32 TB |
| Usable: (4 − 1) × 8 TB | 24 TB |
| Shown in the OS: 24 × 10¹² ÷ 1,024⁴ | 21.83 TB |
| Efficiency | 75% |
| Survives | 1 drive failure |
Choosing a RAID level
| Level | Min drives | Best for |
|---|---|---|
| RAID 0 | 2 | Scratch space and speed where losing the data doesn't matter |
| RAID 1 | 2 | Simple two-drive NAS or boot drives |
| RAID 5 | 3 | Home NAS with smaller drives; good space efficiency |
| RAID 6 | 4 | Large drives and bigger arrays where rebuilds take a long time |
| RAID 10 | 4 (even) | Databases and VMs that need fast writes plus redundancy |
Frequently asked questions
How much usable space does RAID 5 give?
RAID 5 uses one drive's worth of space for parity, so usable capacity is (number of drives − 1) × the smallest drive size. Four 8 TB drives give 24 TB usable, and the array survives one drive failure.
What's the difference between RAID 5 and RAID 6?
RAID 6 stores two sets of parity, using two drives' worth of space, and survives any two drive failures. With large drives, rebuilding a failed disk takes many hours, so RAID 6 protects you if a second drive fails during the rebuild.
Is RAID a backup?
No. RAID protects against a drive failing, not against deleting files, ransomware, theft, fire or the controller corrupting the array. Keep a separate backup, ideally following the 3-2-1 rule: three copies, two kinds of media, one offsite.
What happens if my drives are different sizes?
Standard RAID treats every drive as the size of the smallest one, wasting the extra space on larger drives. Some NAS systems (Synology SHR, Unraid) can use mixed sizes more efficiently; this calculator shows standard RAID.
Why does my NAS show less space than this?
Drive makers use decimal terabytes; operating systems often show binary tebibytes, about 9% smaller. File system overhead and reserved space take a little more. The 'shown as' figure accounts for the TB to TiB difference.
Rates and figures last checked October 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.