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What is the formula for working out the final raid size of a raid 5 array knowing the number of disk and the size of each disk?

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4 Answers 4

Ripped from Wikipedia

Therefore, the usable capacity of a RAID 5 array is (N-1) x S(min) , where N is the total number of drives in the array and S(min) is the capacity of the smallest drive in the array.

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Bear in mind that although RAID 5 gives good fault tolerance at a (relatively) low price, other RAID levels can often outperform it, and in the right circumstances provide better fault tolerance. A RAID 5 set can tolerate a single disk failure (assuming no auto-rebuild on a hot spare is implemented), but a 0+1 stripe-mirror set can suffer multiple disk failures before the whole set dies.*

  • this is dependent upon which actual members of the array die. If two disks holding the same mirrored data die, then you have lost the set, but if 2 disks holding different data die, the set as a whole may continue.
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Add the number of hard drives minus 1. Either all the drives have to be the same size, or if you use different sizes, the RAID will treat all the drives the same as it would the smaller one.

10 1TB hard drives equal 9TB of storage.

9 1TB drives and one 500GB drives is 4.5TB of storage (each 1TB is treated as a 500GB)

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In addition to the other answers, Synology has a nice RAID calculator.

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