How many drives do I need for a RAID array?

It depends on your target usable space plus the redundancy you want. RAIDZ1 or RAID5 loses one drive's worth of capacity to parity, RAIDZ2 or RAID6 loses two. So for usable space, add one or two drives on top of what the data alone needs.

How much usable space do I get from RAID?

Less than the raw total, because parity drives do not store your data. In a RAIDZ2 pool of six drives, two are parity, so you get roughly four drives worth of usable space. The live table helps you cost the extra drives.

Is a minimum number of drives needed for RAID?

Yes. A mirror needs two, RAID5 or RAIDZ1 needs at least three, and RAID6 or RAIDZ2 needs at least four. More drives spreads the parity cost, so bigger arrays lose a smaller percentage of capacity to redundancy.

How Many Drives Do You Need for a RAID Array?

Published: 29 de julho de 2026

Live answer · Amazon.com

Pulling live prices…

📊 As of August 15, 2026, the typical HDD (12TB+) on Amazon.com is $41.01/TB or $0.041/GB, widest choice around $27.48/TB, with some as cheap as $21.59/TB across 722 live listings.

Sourced from real discussions on r/homelab, r/zfs and r/DataHoarder, cross-checked against live PricePerGig listings.

2 Drives are needed for a RAID as a minimum, but as always, it’s a little more complicated than that, but not much. A RAID provides a mechanism to recover or not even notice when a drive fails for whatever reason. To figure out actually how many drives you need, you’ll want to add up the number of drives you want in total for storage, then ADD ON at least one or two other drives for redundancy.

Here is a full example. Say you want 48TB of usable space, and 12TB drives are the best value on the table above. That is four drives of data. Now add the redundancy: one extra drive gives you RAIDZ1, so five drives in total and you survive one failure. Two extra gives you RAIDZ2, so six drives in total and you survive any two. Buy six, get 48TB usable, and lose any two drives without losing a thing. The ZFS calculator does this exact sum for you.

The table above shows the live price per TB for hard drives of 12TB and up on your marketplace, so once you know your drive count, you know your budget.

Usable is less than raw

This is the number one thing people get wrong, so let me nail it.

In a redundant array, some of your drives hold parity, not your files. That parity is what lets a dead drive be rebuilt. But it means your usable space is less than the raw total.

RAIDZ1 or RAID5: you lose one drive’s worth of capacity to parity.
RAIDZ2 or RAID6: you lose two drives’ worth.
RAIDZ3: you lose three.

So if you want a certain amount of usable space, you buy that much in data drives, then add the parity drives on top.

A simple worked example

Numbers make this click.

Say you want roughly four big drives’ worth of usable space, and you want to survive two failures.

That is RAIDZ2. Four data drives plus two parity drives equals a six drive array. Two of the six are redundancy, so you get about four drives of usable space and can lose any two without tears.

Want the same usable space but only single failure protection? RAIDZ1 with five drives does it. One less drive, one less safety margin.

Minimum drive counts

You cannot build every level with any number of drives.

Mirror: two drives minimum.
RAID5 or RAIDZ1: three minimum.
RAID6 or RAIDZ2: four minimum.

And here is a nice efficiency point. The more drives in the array, the smaller the percentage lost to parity. Two parity drives out of six is a third. Two out of twelve is a sixth. Bigger arrays use redundancy more efficiently.

One more reason to get the count right at the start: changing an array afterwards is awkward. Traditional RAID levels generally mean rebuilding the whole thing. ZFS is the exception now, since OpenZFS 2.3 added RAIDZ expansion so a pool can grow by a disk, but it is still far easier to buy the drives you need up front than to grow into them later.

How to budget it

Putting it together.

Decide your target usable space.
Pick your redundancy, RAIDZ2 is the home sweet spot.
Add one or two parity drives to your data drive count.
Multiply the total drive count by the price of your chosen capacity on the live table above.

That gives you a real budget, parity included, with no nasty surprises.

The bottom line

How many drives you need is your usable target plus the parity for your chosen redundancy. RAIDZ1 adds one drive, RAIDZ2 adds two, and bigger arrays lose a smaller share to parity.

Work out your usable goal, pick your safety level, add the parity drives, and cost it all using the live price per TB above. Then you build with the right number of drives the first time, no guessing.

So the total needed is ‘Data Drives’ + ‘Redundancy Drives’, simples. And the chances are you’ll be wanting another couple of drives, because once you’ve setup the RAID array it’s really quite challenging if not impossible to change it, so getting it right first time is a must.

Usable is less than raw

A simple worked example

Minimum drive counts

How to budget it

The bottom line

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