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RAID Calculator

Calculate usable capacity and fault tolerance.

Configuration

Usable Capacity
12 TB
Total Capacity
16 TB

Storage Efficiency

75%
🛡️
Fault Tolerance:

1 drive failure tolerance

About this tool

The RAID Calculator converts a set of drives into the numbers that actually matter: usable capacity, how many simultaneous drive failures you'd survive, and rough performance characteristics. 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 you'd see after formatting plus the fault tolerance, critical for sizing a NAS, a homelab server, or a small business file share.

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 makes the capacity vs. redundancy trade-off explicit so you can make the call with real numbers instead of vendor hand-waving.

Formula

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

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.

Storage comparisons

RAID level head-to-heads that explain the capacity-vs-redundancy trade-offs in practical terms.

RAID 5 vs RAID 6
The parity array trade-off that gets more important as drive capacities grow.
Read comparison ➜

Frequently asked questions

How much usable space do I get with 4x 4TB drives in RAID 5?
RAID 5 reserves one drive's worth of capacity for parity, so 4 × 4 TB in RAID 5 gives 12 TB usable (3 × 4 TB). 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 much safer on large arrays where rebuild times stretch past 24 hours.
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?
It depends on which drives. RAID 10 can always tolerate one drive failure. It can tolerate up to N/2 failures if each failed drive is in a different mirror pair, but loses the array if both drives in the same pair fail.
Does RAID replace backups?
No. This is the single biggest 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.
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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