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 juli 2026

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Planning an array and trying to work out how many drives to buy? Let me make the maths simple.

Two things decide it. How much usable space you actually want, and how much redundancy you want on top.

The catch is that parity drives do not store your data. So you always buy a little more than the raw data needs.

The table above shows the live price per TB 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.

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 guesing.

Usable is less than raw

A simple worked example

Minimum drive counts

How to budget it

The bottom line

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