# RAID Rebuild Time Calculator

> Estimate how long a RAID array takes to rebuild onto a replacement drive from its capacity, the rebuild rate in MB/s and how much of it must be rewritten.

Category: IT, networking & security · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/raid-rebuild-time-calculator

A rebuild rewrites the replacement drive from the survivors, so its length is the data to rebuild divided by the sustained rebuild rate. Long rebuilds are the window in which a second failure hurts most.

Also called: RAID resilver time calculator, Array rebuild calculator.

## Inputs
- Drive capacity (default 12)
- Size unit (options: TB, GB; default TB)
- Sustained rebuild rate (unit MB/s; default 150)
- Share of the drive to rebuild (unit %; default 100)

## Results
- Rebuild time
- Rebuild time
- Rebuild time
- Data rewritten

## Formula

```
Time (s) = Drive capacity (bytes) × Share ÷ (Rate × 10⁶ bytes/s)
```

Where:
- Drive capacity: decimal: 1 TB = 10¹² bytes
- Rate: megabytes (10⁶ bytes) per second

## Worked example
1. 12 TB = 12 × 10¹² bytes.
2. 12 × 10¹² ÷ 150 × 10⁶ bytes/s = 80,000 s.
3. 80,000 ÷ 3,600 = 22.22 hours.

**Result:** Rebuilding a 12 TB drive at 150 MB/s takes about 22 hours.

## What the result means

If the rebuild takes days, consider RAID 6 or triple parity so a second failure during the rebuild is survivable, and keep a hot spare so the rebuild starts at once.

## Assumptions and limits
- The rebuild rate stays constant from start to finish.
- Production I/O, controller rebuild-priority settings and SMR drives can make real rebuilds several times slower.

## Questions

### Why is my rebuild slower than the drive’s rated speed?

Controllers throttle rebuilds to keep the array responsive, and every surviving drive must be read in parallel. Random-access workloads during the rebuild slow it further.

### Does a RAID 10 rebuild faster than RAID 5?

Usually: RAID 10 copies from one mirror partner, while RAID 5 and 6 read all surviving drives and compute parity.

## Related calculators
- [RAID Capacity Calculator](https://calculators.geekmidst.com/calculators/raid-capacity-calculator)
- [Database Backup Window Calculator](https://calculators.geekmidst.com/calculators/database-backup-window-calculator)
- [Download Time Calculator](https://calculators.geekmidst.com/calculators/download-time-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/raid-rebuild-time-calculator