A RAID 5 calculator answers one question: how much usable space do I get from this set of drives? You can do it by hand in seconds once you know the formula. Below is the RAID 5 capacity formula, worked examples for 3, 4, 5 and 8 drives, and how RAID 6 and RAID 10 change the result. To skip the maths, use our free RAID calculator.
The RAID 5 capacity formula
RAID 5 stores parity information equal to one drive’s worth of space, spread across all drives. That gives a simple formula:
RAID 5 examples with 4 TB drives
| Drives | Raw capacity | Usable (RAID 5) | Usable in TiB | Efficiency |
|---|---|---|---|---|
| 3 x 4 TB | 12 TB | 8 TB | about 7.3 TiB | 67% |
| 4 x 4 TB | 16 TB | 12 TB | about 10.9 TiB | 75% |
| 5 x 4 TB | 20 TB | 16 TB | about 14.6 TiB | 80% |
| 8 x 4 TB | 32 TB | 28 TB | about 25.5 TiB | 88% |
Notice the pattern. Each added drive adds its full size to the usable space, and the single parity drive becomes a smaller share of the total. That is why RAID 5 gets more space-efficient as you add drives.
There is a trade-off. More drives in one RAID 5 array means more chances of a failure, and a long rebuild leaves you exposed. For eight drives most people choose RAID 6 instead.
RAID 6 and RAID 10 for comparison
- RAID 6: usable = (N - 2) x smallest drive. It needs at least 4 drives and survives two failures.
- RAID 10: usable = (N / 2) x smallest drive. It needs an even number of drives, at least 4, and survives at least one failure.
| 8 x 4 TB drives | Formula | Usable | Failures survived |
|---|---|---|---|
| RAID 5 | (8 - 1) x 4 | 28 TB | 1 |
| RAID 6 | (8 - 2) x 4 | 24 TB | 2 |
| RAID 10 | (8 / 2) x 4 | 16 TB | At least 1 (one per mirrored pair) |
Mixed drive sizes: the smallest drive wins
Standard RAID 5 treats every drive as if it were the size of the smallest one. Suppose you install 4 TB, 4 TB, 6 TB and 8 TB drives. Raw capacity is 22 TB, but usable space is (4 - 1) x 4 = 12 TB. The extra 2 TB on the 6 TB drive and 4 TB on the 8 TB drive go unused. If you want to mix sizes efficiently, look at a system with flexible RAID, such as Synology’s SHR, covered in our Synology RAID calculator guide.
TB vs TiB: why your NAS shows less
Drive makers count 1 TB as one trillion bytes. Many operating systems count in binary, where 1 TiB is 1,099,511,627,776 bytes. So 12 TB of usable space appears as roughly 10.9 TiB. Nothing is missing. It is a difference in units, and our calculator shows both figures.
How to calculate RAID 5 space in four steps
- Count the drives (N) and find the smallest one.
- Subtract one from N.
- Multiply the result by the smallest drive size.
- Convert to TiB by multiplying by 0.909 if you want the number your system will show, then keep about 20 percent free.
Not sure RAID 5 is the right level? Read RAID explained, and check how many bays you need in our sizing guide. When you buy drives, prefer NAS-rated CMR models, as covered in choosing NAS hard drives.
When RAID 5 is a good fit
- Three to five drives of the same size and model
- Data that is mostly read, such as photos, music and video
- A separate backup already in place
- Drives of modest size, so rebuilds stay reasonably short
RAID 5 is a popular middle ground between capacity and safety. It wastes only one drive’s worth of space, which is why it often beats RAID 10 on capacity. When you add up the numbers, compare it with RAID 6 using the calculator before you decide, because the cost of one extra parity drive can be small compared with the risk it removes.
RAID 5 calculator FAQ
How do I calculate RAID 5 capacity?
Multiply the number of drives minus one by the size of the smallest drive. For example, four 4 TB drives give (4 - 1) x 4 = 12 TB.
What is the minimum number of drives for RAID 5?
Three. It uses the equivalent of one drive for parity and survives a single drive failure.
What happens if I mix drive sizes in RAID 5?
Every drive is treated as the smallest drive in the set, so the extra capacity on larger drives is not used.
Is RAID 5 safe with very large drives?
It is riskier, because a long rebuild leaves the array without protection. Many people choose RAID 6 for large drives or arrays with many disks.
