RAID Calculator
Pick a RAID level, enter how many disks you have and how big each one is, and this calculator shows the usable capacity, the total raw capacity, the storage efficiency and how many disk failures the array can survive - the exact numbers A+ storage questions ask for.
| Usable capacity | |
|---|---|
| Usable capacity (other unit) | |
| Total raw capacity | |
| Storage efficiency | |
| Fault tolerance |
The formulas the exam expects
- RAID 0: disks x size. All capacity, no protection.
- RAID 1: just the size of one disk - the rest are mirrors.
- RAID 5: (disks - 1) x size. One disk worth of parity, survives one failure.
- RAID 6: (disks - 2) x size. Two disks worth of parity, survives two failures.
- RAID 10: (disks / 2) x size. Half the disks are mirrors, so efficiency is always 50%.
And remember the A+ favourite: RAID is not a backup. It keeps a server running through a disk failure; it does nothing against deletion, ransomware or fire.
Storage is core A+ material - practice free
Try the free 30-question samples for A+ Core 1 (220-1201) and Core 2 (220-1202) and see where you stand.
Free A+ Core 1 SampleFree A+ Core 2 SampleFAQ
Which RAID level should I choose for a small server?
For most small servers RAID 10 is the safe default: it mirrors and stripes, so it is fast and survives a disk failure, at 50% capacity efficiency. RAID 5 is cheaper on disks but rebuilds slowly and stresses the remaining disks; RAID 6 adds a second parity disk for larger arrays.
Why does RAID 5 lose one disk of capacity?
RAID 5 stores parity information spread across all the disks, and the parity for each stripe takes the space of one disk in total. Four 2 TB disks therefore give (4 - 1) x 2 TB = 6 TB usable, no matter which physical disk fails.
Is RAID a backup?
No. RAID protects against a disk hardware failure so the system keeps running; it does not protect against deletion, ransomware, fire or theft, because those hit every disk at once. You still need separate backups - the classic rule is 3-2-1: three copies, two media types, one off-site.
Why does RAID 10 need an even number of disks?
RAID 10 is built from mirror pairs that are then striped. Every disk needs a mirror partner, so the count must be even - and at least 4 - giving a usable capacity of (disks / 2) x disk size.