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RAID Calculator

Calculate usable capacity, fault tolerance, and read/write scaling for different RAID levels.

Configuration

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 capacity for fault tolerance and performance 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.

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.

Popular RAID scenarios

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

RAID 5 with 4×4 TB
12 TB usable, single-parity workhorse
View details →
RAID 1 with 2×4 TB
4 TB mirrored, simple redundancy
View details →
RAID 0 with 2×2 TB
4 TB striped, max speed no safety
View details →

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 →
RAID 0 vs RAID 1
The most opposite pair in storage: all performance versus all protection.
Read comparison →
RAID 5 vs RAID 10
The classic NAS fork: one more drive of space, or faster and safer rebuilds.
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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