NAS, RAID and Backup Storage
RAID rebuild time calculator.
Rebuild time is the replaced drive size divided by the speed the array can actually sustain: a 12 TB drive at 150 MB/s takes about 22 hours, and about 44 hours if normal use takes half the speed. Enter the drive size, a rebuild speed and how much of it normal use takes away. Runs in your browser; nothing is sent or stored.
Your rebuild
Estimated rebuild time
Fill the form and press Calculate.
Worked example
A 12 TB drive at a sustained 150 MB/s with an idle array: 12 x 1,000,000 MB / 150 MB/s = 80,000 seconds = 22.2 hours. If the array keeps serving files and normal use takes 50% of the speed, the effective rate is 75 MB/s and the rebuild takes 160,000 seconds = 44.4 hours.
How it works
Rebuild time = drive size / (rebuild rate x (1 - load share)), with 1 TB counted as 1,000,000 MB. The rate is the sustained speed the array achieves while rebuilding, not the headline speed of one drive, and it often falls as the rebuild reaches the inner part of each disk.
Because a failed array is still running while it rebuilds, other drives may fail or hidden read errors may surface during that window. Check that risk with the RAID rebuild URE risk calculator and the drive failure to array probability calculator.
Estimates only. RAID is not a backup: it keeps an array running through a drive failure but does not protect against deletion, malware, fire or theft. Real arrays and drives vary, so use this for planning and check your own drive datasheets.
Sources and basis (opened 2026-10-08):
- RAID (Wikipedia): Larger-capacity drives may take hours if not days to rebuild, and the rebuild time is also limited if the entire array is still in operation at reduced capacity. The division itself is plain arithmetic.
FAQ
How long does a RAID rebuild take?
It is the drive size divided by the sustained rebuild speed. A 12 TB drive at 150 MB/s takes about 22.2 hours with an idle array, and about 44.4 hours if normal use takes half the speed.
Why is my rebuild slower than the drive speed?
The array is usually still serving files while it rebuilds, and the controller or NAS may cap the rebuild rate. Enter the share of speed lost to normal use to model this.
Does RAID 6 rebuild faster than RAID 5?
Not by itself. Rebuild time depends on drive size, speed and load. RAID 6 still reads the remaining drives, but it can survive a second failure during the rebuild.
Is it safe to use the NAS during a rebuild?
You can usually keep using it, but the rebuild will take longer and the drives work harder. Make sure a separate backup exists before you start.
Where do I find my rebuild speed?
Many NAS and RAID controllers show a rebuild or resync speed in their status screen. Use that number, or test with a smaller drive first.