Usable Capacity
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RAID Calculator
Estimate usable storage capacity for common RAID levels.
Comprehensive Guide
RAID stands for Redundant Array of Independent Disks. The big idea is simple: combine multiple drives into one storage system that trades some raw capacity for performance, redundancy, or both. A RAID calculator helps you estimate the usable space after parity or mirroring is taken into account.
That matters because raw drive capacity and usable capacity are not the same thing. A set of four 4 TB drives may not give you 16 TB of usable space once redundancy is factored in. Storage teams, home lab builders, and IT buyers use RAID calculators to avoid buying the wrong number of drives or underestimating how much space the array will actually provide.
Real-World Examples
With RAID 5, one drive’s worth of capacity is reserved for parity, so a four-drive array of 4 TB drives yields about 12 TB usable. That is a very different number from the raw 16 TB many people expect at first glance. The calculator makes the difference visible before you build the array.
With RAID 10, drives are mirrored in pairs, which means half of the total raw capacity is effectively reserved for redundancy. Four 4 TB drives again produce about 8 TB usable. The tradeoff is that the array can offer better performance and fault tolerance at the cost of more space.
Reference Table
| RAID Level | Minimum Drives | Usable Capacity Rule |
|---|---|---|
| RAID 0 | 2 | All drives usable |
| RAID 1 | 2 | One drive’s capacity |
| RAID 5 | 3 | (n-1) × drive size |
| RAID 6 | 4 | (n-2) × drive size |
| RAID 10 | 4 | Half of raw capacity |
Frequently Asked Questions
Why is usable capacity smaller than raw capacity?
Because RAID often reserves part of the space for parity or mirrors. That reserved space helps the array survive drive failures or maintain performance. The more redundancy you want, the less usable capacity remains.
What does RAID 0 do?
RAID 0 stripes data across drives for speed and uses all of the raw space, but it offers no redundancy. If one drive fails, the whole array is lost. It is fast, but it is not the safest choice.
Why choose RAID 10?
RAID 10 balances speed and redundancy. It is a popular choice when performance matters but losing data would be expensive. You give up half the raw capacity to get mirroring benefits.
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