Size it on what you hold today, what a year of growth adds, and what a redundancy level gives up, then leave room to work in. The bay count you choose matters as much as the capacity, because it decides how the array can grow later.
Four numbers feed the answer, and only the first is the one people start with.
Start with what you actually hold
Add up the collections that will move onto the machine: photographs, video, music, documents, machine backups, and anything currently spread across external drives. Measure it rather than estimating, because backups of several machines are usually much larger than expected and media libraries usually smaller.
Write the figure down in whichever unit your machine displays, and keep that unit for the rest of the exercise.
Then add the growth
Look at how much the same collections grew over the last year and assume something similar. This is the number that decides whether you are sizing for two years or for five, and it is the one worth being generous about, because the cost of being wrong is a migration rather than a purchase.
Subtract what the redundancy level takes
This is where the plan usually changes shape. A mirrored pair presents the capacity of one member. A single-parity array presents all members but one, at the size of the smallest. A double-parity array gives up two.
So the raw capacity to buy is the usable figure you need, scaled up by whatever the chosen level gives away — and then read in the maker’s decimal unit, since that is how drives are sold and it is not the unit the enclosure will display.
Leave working room
An array that is nearly full is unpleasant in several ways at once. Filesystems slow as they fill, snapshots and versioning need somewhere to live, and a large incoming transfer needs space to land in before anything is tidied.
More practically, a full array is one whose only remedies are expensive: larger drives, a bigger chassis, or a hurried decision about what to delete. Leaving headroom is buying time to make that decision calmly.
Bay count is a separate decision from capacity
Two drives of a given size and four drives of half that size reach a similar raw figure and behave nothing alike. The bay count decides which levels are available, how the array can grow, how much noise and heat the chassis makes, and what a power supply and a UPS have to carry.
Two further published lines belong here. Enclosure makers publish a compatibility list of qualified drives and often a maximum capacity per bay, and both are specific to the model. Read them before choosing drive sizes, not after.
And the copy that lives somewhere else
Whatever the array ends up holding, it is one copy in one box. A redundancy level keeps that copy readable through the failure of a drive and does nothing else — not a deletion, not hostile encryption, not a theft, not a fire.
So the sizing exercise has a second half: where the second copy goes, on what, and how often it is refreshed. That is a separate purchase and a separate capacity figure, and leaving it out of the plan is how a well-specified array ends up being the only place something exists.
The questions that come up before an order
Is it cheaper to buy larger drives or more of them?
Larger drives use fewer bays, draw less power and leave room to expand; more drives spread parity across more members and let a level be chosen more freely. Enclosure makers publish a maximum capacity per bay for each model, and that published limit is what decides whether the largest drives are an option at all.
Should the backup copy live on the same machine?
No. A second copy on the same enclosure shares everything that can go wrong with it — a deletion, hostile encryption, a theft, a fire — and a redundancy level protects against none of those. A copy that counts is one on separate hardware, and ideally in a separate place.
Last reviewed 10 September 2026