RAID Rebuild Time Calculator
Estimate how long a RAID array takes to rebuild after a drive failure from the disk size and rebuild throughput — plus the window your array runs degraded.
How it works
Rebuild time = drive bytes ÷ effective speed · Effective speed = throughput × (1 − load penalty)
A rebuild has to read or reconstruct every byte of the replacement drive, so time is simply the drive size divided by how fast the array can write the recovered data. Real arrays rarely rebuild at full speed because they keep serving live I/O, so a load penalty stretches the window considerably. That window matters: while degraded, the array has no (or reduced) redundancy, and larger modern drives take so long that a second failure or an unrecoverable read error during rebuild is a real risk.
Worked example
A 4 TB drive (4,000,000,000,000 bytes) rebuilt at 100 MB/s would finish in about 11 h 6 min if the array were idle. Give up 40% of that speed to live traffic and the effective rate is 60 MB/s, pushing the rebuild to roughly 18 h 31 min of degraded operation.
Frequently asked questions
Why do big drives make RAID riskier?
Rebuild time scales with capacity, so an 8–16 TB drive can take a day or more to rebuild. The longer the array runs degraded, the higher the chance a second drive fails or an unrecoverable read error aborts the rebuild — the reason RAID 6 and mirroring are preferred at large capacities.
Does normal use slow a rebuild?
Yes. Rebuilds compete with production I/O for disk bandwidth, so an array under heavy load rebuilds far slower than the raw drive speed suggests. This calculator’s load penalty models that trade-off between rebuild speed and serving users.
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This calculator is for educational purposes. Double-check important results before acting on them.