RAID combines several disks into one storage system, but the usable capacity is rarely the simple sum of the disks, because different RAID levels trade space for redundancy in different ways. RAID 0 stripes data for speed with no redundancy, RAID 1 mirrors it for safety, RAID 5 spreads parity so one disk can fail, RAID 6 uses double parity so two can fail, and RAID 10 combines mirroring and striping for both speed and safety. This calculator takes the disk count, the size of each disk, and the RAID level, and reports the usable capacity and how many disk failures the array can survive. Those two numbers are the core of any storage plan, and getting them wrong means either running out of space or discovering too late that a failure was not survivable.
Quick Answer: Enter the number of disks, the size of each disk, and the RAID level, then click Calculate RAID Capacity for usable space and fault tolerance.
Free Online Tool
RAID Capacity Calculator
Usable capacity and survivable failures for RAID 0, 1, 5, 6 and 10.
How to Use the RAID Capacity Calculator
- Count the disks you plan to use in the array, using identical sizes for a straightforward calculation.
- Enter the size of each disk in terabytes, using the same unit for every disk.
- Choose the RAID level that matches your priority: speed, safety, or a balance of both.
- Click Calculate RAID Capacity to see the usable capacity and the number of disk failures tolerated.
- Compare levels before committing, since changing RAID level later usually means rebuilding the array.
Key Features
- Usable capacity for RAID 0, 1, 5, 6 and 10 from disk count and disk size.
- Fault tolerance reporting in plain language, stating how many disks can fail safely.
- Minimum disk validation, so levels that need a certain disk count refuse impossible configurations.
- Terabyte based results that match how storage is usually planned and purchased.
- Instant comparison support: change only the level to see how capacity and safety trade off.
Best Practices and Tips
- Use identical disks in an array. Mixed sizes waste the extra capacity of larger disks, which the array cannot use.
- Remember that RAID is not a backup. It protects against disk failure, but not against deletion, corruption or ransomware, which a backup must cover.
- Rebuild time grows with disk size, so very large disks in RAID 5 carry a longer vulnerable window during a rebuild.
- For small arrays where safety matters most, RAID 1 or RAID 10 are simpler and more predictable than parity levels.
Common Mistakes to Avoid
- Adding up all disk sizes for a redundant level and expecting that total to be usable, which overstates capacity for every level except RAID 0.
- Choosing RAID 0 for important data because it is fastest, forgetting that one disk failure destroys the whole array.
- Assuming RAID 5 survives any two failures, when it survives exactly one, and a second failure during rebuild is fatal.
Frequently Asked Questions
How is usable capacity calculated for each level?
RAID 0 uses all disks, so capacity is disks times size. RAID 1 mirrors, so capacity is one disk. RAID 5 reserves one disk worth of parity, so capacity is disks minus one times size. RAID 6 reserves two disks worth, so capacity is disks minus two times size. RAID 10 mirrors then stripes, so capacity is half the disks times size.
How many disks does each level need?
RAID 0 needs at least two, RAID 1 needs exactly two for a simple mirror, RAID 5 needs at least three, RAID 6 needs at least four, and RAID 10 needs at least four in even numbers. This calculator flags configurations below the minimum rather than producing a misleading capacity.
Is RAID 5 still safe with large disks?
It is less safe than it once was, because rebuilding a multi terabyte array takes many hours, during which another failure or an unreadable sector can destroy the array. For large disks, RAID 6 or RAID 10 are commonly preferred for exactly this reason.
Does RAID replace backups?
No. RAID protects availability when a disk fails, but it does not protect against accidental deletion, file corruption, theft or malware, all of which affect the whole array at once. A separate backup, ideally off site, remains essential.