RAID Storage Capacity Calculator

Introduction: How RAID levels turn raw disk space into usable capacity

This RAID storage capacity calculator helps you translate a set of drives into usable space, parity overhead, and the number of failures the array can absorb. RAID is not about inventing more storage; it is about dividing raw capacity between striping, mirroring, and parity in different ways. That is why the same collection of disks can look generous in one layout and conservative in another.

The simplest layout is RAID 0, which stripes data across all drives and keeps every byte for storage. The calculator treats usable capacity as the sum of the drive sizes:

Here, n is the number of drives and s is the size of each drive. Because RAID 0 has no mirror copy and no parity block, it offers the highest usable space but survives no drive failures. One bad disk can take the whole array offline.

RAID 1 mirrors data so each pair contains the same blocks. With an even number of drives, the calculator uses half of the raw capacity as usable space:

That means the array trades capacity for redundancy. A two-drive mirror keeps one complete copy of the data, while larger mirror sets still leave only half of the total raw space available. If you enter an odd drive count, one disk does not contribute to the mirrored pair and becomes unused capacity in this simplified model.

RAID 5 stripes data and single parity across the set. One drive’s worth of space is reserved for parity, so usable capacity becomes:

RAID 5 is the common middle ground when you want more usable space than mirroring but still need to survive a single drive loss. The calculator shows that compromise directly: one disk’s capacity is reserved, and the array can rebuild after one failed drive. Large disks make the rebuild window longer, so the practical risk is not just the formula but the time the array spends degraded.

RAID 6 extends that idea with double parity. Two drives’ worth of space are set aside, which lowers the usable total but allows a second failure while the first is being rebuilt:

RAID 6 is often chosen when drive sizes are large enough that a long rebuild would make single-parity protection feel too thin. The extra parity costs capacity, but it buys a wider safety margin during repair and recovery.

RAID 10 combines mirroring and striping. The calculator treats it like mirrored pairs arranged in a striped set, so usable capacity is again half of the raw total:

That gives you the speed advantages of striping plus the redundancy of mirrors. A healthy RAID 10 array can keep working after a failure as long as both members of the same mirror pair are not lost together. Rebuilds are usually simpler here because only the affected pair has to be reconstructed.

Fault Tolerance by RAID Level

The RAID storage calculator also reports the fault tolerance implied by each layout, which is as important as the usable-capacity number.

For RAID 10, the number of tolerated failures is not about counting dead drives in the abstract; it depends on where those failures land. Two drives can fail and the array can still stay online if each loss happens in a different mirror pair. If both drives in one pair disappear, that pair is gone even if other mirrors are intact.

Practical RAID Capacity Planning Considerations

The RAID storage calculator assumes identical drive sizes, but real arrays usually need a few extra checks before you buy hardware. Mixed-capacity drives are usually limited by the smallest disk in the group, so a larger drive may have unused space if it is paired with smaller ones. Some controllers and NAS operating systems also reserve a little metadata space, which means the operating system may report slightly less than the pure parity formula suggests. The calculator is focused on raw usable capacity, not on every vendor-specific reservation.

RAID protects against a drive failure, but it does not replace a backup. Power events, controller failure, accidental deletion, filesystem corruption, and bad firmware can still cause data loss. If the array matters, keep a separate backup copy outside the RAID set.

Rebuild time matters too. A RAID 5 or RAID 6 volume with large drives can spend many hours or even days reconstructing a replacement disk, and that is the period when redundancy is reduced. RAID 10 usually rebuilds faster because only the broken mirror member has to be copied back into place.

Performance also differs by level. RAID 0 and RAID 10 usually deliver the strongest throughput because they stripe data across multiple disks. RAID 5 and RAID 6 add parity work on writes, which can reduce speed, especially on busy arrays or smaller controllers. The calculator does not attempt to model controller cache, SSD endurance, or vendor tuning; it is designed to show how much space each RAID choice leaves you to use.

When you are deciding between levels, compare the usable-capacity result with the amount of fault tolerance you actually need. The right answer is often the layout that leaves enough space for growth while still matching the rebuild risk you are willing to accept.

How to use this RAID storage capacity calculator

1. Enter Drive Size (GB) as the size of each disk in the same unit you want to use for planning.
2. Enter Number of Drives as a whole number that matches the array you are considering.
3. Choose RAID Level that matches the layout you want to compare, such as RAID 5 or RAID 10.
4. Run the calculation, then test another RAID level or drive count so you can compare usable space and failure tolerance before you buy or deploy.

Formula: how RAID capacity is calculated

This RAID storage calculator applies the rule for the layout you choose rather than a single one-size-fits-all equation. RAID 0 uses all raw space, RAID 1 and RAID 10 divide the array into mirrored pairs, RAID 5 reserves one drive for parity, and RAID 6 reserves two. That is why a change in RAID level can alter usable capacity even when the physical drives stay the same.

The result therefore depends on the drive size you enter and the drive count you choose, with the selected RAID level deciding how much of the raw total is kept for redundancy. If you are comparing options, leave the drive size and drive count fixed and switch only the RAID level so the difference is easy to see.

Worked example: comparing RAID 5 and RAID 10 on the same drives

To see the RAID storage calculator in action, imagine four drives at 1000 GB each. If you select RAID 5, the calculator shows 3000 GB of usable space, 1000 GB reserved for parity, and tolerance for one failed drive.

Now keep the same four drives and switch to RAID 10. The usable space drops to 2000 GB because the array is mirrored, but the fault tolerance rises to two drives so long as both losses are not in the same mirror pair. This side-by-side check is the quickest way to decide whether your priority is capacity, speed, or safer rebuild behavior.

Limitations and assumptions for RAID storage estimates

This RAID storage calculator is a planning aid, not a full simulator of every controller, NAS, or software-RAID quirk. It assumes equal-size drives, the standard RAID 0/1/5/6/10 patterns shown on the page, and the simple parity or mirror overhead described above.

Real systems may limit a mixed set by the smallest drive, reserve a little metadata space, or apply additional vendor rules that slightly reduce the number shown by the formula. The result also reflects raw usable capacity before any filesystem formatting or application-level storage policy is layered on top.

Use the output as a practical estimate when you are planning a build, then confirm the exact behavior in the controller or appliance documentation before you commit to hardware. That is especially important when rebuild windows, hot spares, or unusual drive mixes affect the layout you intend to deploy.