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RAID Calculator

Calculate usable capacity and fault tolerance for different RAID levels, or plan a pool from drive sets of different sizes.

Configuration

RAID Level

Usable Capacity
12 TB
Total Capacity
16 TB

Storage Efficiency

75%
Fault Tolerance:

Survives 1 drive failure

How to use this tool

  1. Enter the drive count and size

    Set the number of drives in the array and the capacity of each drive in TB. All drives should be the same size for accurate results, since the calculator uses the smallest drive's capacity.

  2. Pick a RAID level

    Choose RAID 0 (striping, no redundancy), RAID 1 (mirroring), RAID 5 (single parity), RAID 6 (double parity), or RAID 10 (mirrored stripes). Each level trades usable capacity for fault tolerance differently.

  3. Read the usable capacity

    The calculator applies the RAID level's formula: RAID 5 usable is (N - 1) x drive size, RAID 6 is (N - 2) x drive size, RAID 10 is (N / 2) x drive size. The result is the usable TB after the parity or mirror overhead.

  4. Check the fault tolerance

    The calculator shows how many simultaneous drive failures the array can survive. RAID 5 tolerates one, RAID 6 tolerates two, RAID 10 tolerates one guaranteed and up to N/2 if failures are in different mirror pairs.

  5. Plan a pool for mixed drive sizes

    Switch to Pool of Sets when your drives aren't all the same size. Add one set per drive size, each with its own count and layout. The calculator adds up raw, usable and reserved space for the pool and explains any set or layout mix that can't be built.

About this tool

The RAID Calculator turns a set of drives into usable capacity and the number of simultaneous drive failures you'd survive. Enter the number of drives and their size, pick an array level (RAID 0, 1, 5, 6, or 10), and the tool returns the usable TB after parity or mirror overhead plus the fault tolerance.

That helps when sizing a NAS, a homelab server, or a small business file share. It does not benchmark read or write speed.

Use it when you're spec'ing a new NAS, comparing whether it's cheaper to run RAID 6 with 8 drives or RAID 10 with 6 larger drives, or deciding if your current array has enough safety margin for a capacity upgrade. The calculator shows the trade-off between capacity and redundancy, so you can decide with real numbers.

Formula

N is the drive count and S is the smallest drive's capacity.

  • RAID 0 usable = N × S (no redundancy)
  • RAID 1 usable = S (1 drive, N copies)
  • RAID 5 usable = (N − 1) × S, tolerates 1 failure
  • RAID 6 usable = (N − 2) × S, tolerates 2 failures
  • RAID 10 usable = (N ÷ 2) × S, tolerates up to N/2 failures if they're in different mirror pairs

Pools of mixed drive sets

Pool of Sets mode treats each set as its own array of same-size drives and stripes data across the sets, like ZFS vdevs in one pool or mdadm arrays joined with LVM. Usable space is the sum of the sets, and the pool survives as many failures as its weakest set.

RAID-Z sets can't share a pool with RAID 5, 6 or 10 sets, since they come from different systems. Vendor schemes like Synology SHR and Unraid use their own rules and aren't modeled.

When to use it

Reach for this when planning a NAS or server build, deciding between more small drives or fewer large ones, or sanity-checking an array's redundancy before adding critical data.

Pair with the Data Transfer Calculator to estimate how long the initial seed copy will take, and the Power Cost Estimator to budget for drives that spin 24/7.

Popular RAID scenarios

Pre-computed capacity and fault tolerance for common RAID levels and drive counts.

RAID 5 with 4×4 TB
12 TB usable, single parity
View details
RAID 1 with 2×4 TB
4 TB mirrored, simple redundancy
View details
RAID 0 with 2×2 TB
4 TB striped, no redundancy
View details

Storage comparisons

RAID level comparisons that show the trade-off between capacity and redundancy.

RAID 5 vs RAID 6
RAID 6 gives up 8 TB of space, 48 TB against 56 TB usable, to survive 2 drive failures instead of 1.
Read comparison
RAID 0 vs RAID 1
RAID 0 keeps 8 TB usable and survives 0 drive failures, and RAID 1 keeps 4 TB and survives 1.
Read comparison
RAID 5 vs RAID 10
Four drives give 24 TB usable in RAID 5 and 16 TB in RAID 10.
Read comparison

Frequently asked questions

How much usable space do I get with 4x 4TB drives in RAID 5?
4 × 4 TB in RAID 5 gives 12 TB usable (3 × 4 TB), because RAID 5 reserves one drive's worth of capacity for parity. You can survive one drive failure without data loss, but lose a second drive during rebuild and everything is gone.
What's the difference between RAID 5 and RAID 6?
RAID 5 uses single parity and tolerates one failed drive. RAID 6 uses double parity and tolerates two. RAID 6 costs one more drive's worth of capacity but is safer on large arrays, since big drives can take a day or more to rebuild under load.
Is RAID 10 better than RAID 5 for performance?
Yes, for most workloads. RAID 10 mirrors then stripes, so writes don't pay a parity penalty. It's roughly twice as fast as RAID 5 on random writes. The trade-off is capacity: RAID 10 always gives only 50% of raw capacity as usable space.
How many drives can fail in RAID 10?
One for certain, and up to N/2 if the failures land in different mirror pairs. RAID 10 always survives one drive failure, but it loses the array if both drives in the same pair fail.
Does RAID replace backups?
No. It's a common RAID misconception. RAID protects against drive hardware failure, not against ransomware, accidental deletion, filesystem corruption, or fire. Always run an independent backup in addition to whatever RAID level you choose. When that backup means copying terabytes, the file transfer time calculator shows how long a full array copy will take over your link.
Can I use drives of different sizes?
Yes, but where they go matters. Inside one RAID group every drive counts as the smallest one, so a 12 TB drive in a group of 8 TB drives only adds 8 TB until the rest are upgraded. Grouping same-size drives into separate sets avoids that. Switch the calculator to Pool of Sets, add a set for each drive size, and it shows each set's usable and reserved space, the pool total, and what the same drives would leave unavailable as one group.
How does a pool of drive sets handle failures?
The pool spreads data across all its sets, so losing any whole set loses the pool. That means the pool is only as safe as its weakest set. A pool of two RAID 6 sets survives two failures anywhere, and more only if they're split between the sets. Mixing RAID-Z with RAID 5, 6 or 10 isn't allowed in one pool, since RAID-Z only exists inside ZFS.
What's RAID 0 good for?
Scratch space, video editing caches, and anything you can afford to lose. RAID 0 stripes data across drives with no redundancy, so capacity and read speed scale with drive count, but a single drive failure destroys the whole array. Never use it for irreplaceable data.

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