RAID Capacity Calculator
Usable capacity, fault tolerance, and the rebuild risk nobody prints.
What you provide
Result
CHECKRAID 5: 24.0 TB usable, survives 1 drive failure
Rebuilding this array means reading 24.0 TB without a single unrecoverable error. At the consumer drive specification that is roughly a 85% chance of hitting one, and with only 1 drive of redundancy an error during rebuild can mean losing the array. This is the arithmetic behind the advice to use RAID 6 rather than RAID 5 on large drives — a second parity drive costs capacity and removes this cliff.
- Usable capacity
- 24.0 TB
- Raw capacity
- 32.0 TB
- Capacity used for redundancy
- 25%
- Drive failures survived
- 1
- Rebuild readsEvery byte of the surviving drives has to be read to rebuild
- 24.0 TB
- Chance of a read error during rebuildAt the consumer specification of one unrecoverable error per 10^14 bits
- 85%
- Usable in TiBWhat the operating system will report, using binary units
- 21.8 TiB
What this cannot tell you
- Calculated in your browser. Nothing you enter is uploaded or logged.
- The rebuild risk is an order-of-magnitude estimate from the manufacturer's unrecoverable read error specification of one per 10^14 bits. Real drives usually beat their specification and controllers differ in how they handle a single bad sector, so treat it as an argument for double parity rather than a probability to bet on.
- Usable capacity is the array arithmetic only. Filesystem overhead, formatting and the difference between TB and TiB all reduce what you can actually store, and the TiB figure is shown for that reason.
- RAID is availability, not backup. It protects against a drive dying and does nothing about deletion, ransomware, a failed controller, or the building.
- Cannot account for a specific controller's behaviour, hot spares, or nested and vendor-specific levels.
Take this with you
How this calculation works
Usable capacity and fault tolerance come straight from the level: striping gives you everything and protects nothing, mirroring halves it, single parity costs one drive and double parity two. The figure worth the page is the third one. Rebuilding a parity array means reading every byte of every surviving drive without a single unrecoverable error, and consumer drives are specified at one such error per 10^14 bits — about 12.5 TB. On an array of large consumer drives the rebuild reads several times that, and the chance of hitting an error stops being theoretical. That number, not the capacity, is what decides between RAID 5 and RAID 6.
What the results mean
- Usable capacity
- What the array presents after redundancy, in decimal TB. The TiB figure below it is what the operating system will report.
- Drive failures survived
- How many drives can fail at once without losing data. RAID 10 survives one guaranteed, and up to one per mirror pair if you are lucky about which fail.
- Rebuild reads
- How much data has to be read successfully to restore redundancy after a failure. This is why rebuild time and rebuild risk both scale with array size.
- Chance of a read error during rebuild
- At the consumer specification. Above about 30% a single-parity array is a coin toss during the exact window it has no redundancy left.
Common problems and fixes
- The array reports less capacity than calculated
- Drive manufacturers use decimal TB and operating systems usually report binary TiB — about 9% smaller. Filesystem overhead takes a little more. The TiB figure above accounts for the first part.
- A rebuild failed and took the array with it
- That is the risk this calculator prints. On single parity, a read error anywhere during rebuild can end it — which is the argument for double parity on large drives and for restoring from backup rather than rebuilding a marginal array.
- Rebuilds take days
- Rebuild time scales with drive size, not array size, and a rebuild competing with normal use is slower still. Expect a large array to spend a long time with no redundancy.
Frequently asked questions
Is RAID 5 still safe with large drives?
It depends on the arithmetic above rather than on an opinion. A rebuild has to read every byte of every surviving drive, and consumer drives are specified at one unrecoverable read error per 10^14 bits — roughly 12.5 TB. Once the rebuild read approaches or exceeds that, the chance of losing the array during the rebuild becomes uncomfortable. With enterprise drives specified an order of magnitude better, or with double parity, the picture changes completely.
Does RAID replace backups?
No, and it is the most expensive misunderstanding in storage. RAID keeps a machine serving while a drive is dead. It faithfully replicates a deleted file, an encrypted-by-ransomware volume and a corrupted database to every drive in the array, instantly. Backup is a separate copy, ideally somewhere else.
What is the difference between RAID 10 and RAID 6?
Both survive two drive failures in the right circumstances, and they trade differently. RAID 10 rebuilds by copying one mirror, which is fast and low-risk, but costs half the capacity and only guarantees one failure — a second in the same pair loses everything. RAID 6 keeps more capacity and survives any two failures, at the cost of slower writes and a long, read-intensive rebuild.
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