RAID Calculator
Estimate usable capacity, efficiency, and fault tolerance for common RAID levels.
Equal-disk model used by most first-pass RAID calculators. Controllers, metadata, and filesystems reduce free space. RAID is not a backup.
Free RAID calculator for capacity, efficiency, and fault tolerance
Use this RAID calculator to estimate usable storage before you buy disks or quote a server. Choose RAID 0, 1, 1E, 5, 6, 10, 50, or 60, set disk count, disk size, optional hot spares, groups for RAID 50/60, approximate IOPS per disk, and URE guidance. Results update live with usable capacity, raw capacity, efficiency, redundancy space, fault tolerance, relative read/write performance, and a rebuild-risk note for parity arrays.
This tool is designed for IT managers, MSPs, and business owners comparing NAS/SAN layouts — the same job classic tools like raid-calculator.com and ServeTheHome-style calculators target, with a modern interface and clearer planning outputs.
RAID formulas used (equal-sized disks)
Let n = data disks (excluding hot spares) and S = size of each disk.
| Level | Min disks | Usable capacity | Typical fault tolerance |
|---|---|---|---|
| RAID 0 | 2 | n × S | None |
| RAID 1 | 2 | S | n − 1 (full mirror set) |
| RAID 1E | 3 | floor(n/2) × S | Usually 1 (layout-dependent) |
| RAID 5 | 3 | (n − 1) × S | 1 disk |
| RAID 6 | 4 | (n − 2) × S | 2 disks |
| RAID 10 | 4 (even) | (n / 2) × S | Up to one disk per mirror pair |
| RAID 50 | 6 | (n − groups) × S | 1 disk per RAID-5 group |
| RAID 60 | 8 | (n − 2×groups) × S | 2 disks per RAID-6 group |
Hot spares are counted toward total drives ordered but not toward usable capacity until they replace a failed disk.
Performance notes (approximate)
Read and write multipliers are educational first-pass estimates:
- RAID 0 / 10 scale reads well; writes scale with stripe width (half for pure mirrors in RAID 10).
- RAID 5 / 6 pay a parity write penalty (classic models often use ~4× and ~6× operation costs).
- Real IOPS depend on disk type, queue depth, controller cache, and workload mix. Enter your disk IOPS to scale the estimate.
Rebuild risk on large disks
Parity RAID rebuilds read almost the entire remaining array. As multi-terabyte disks became common, long rebuild windows increased the chance of a second failure or an unrecoverable read error (URE). That is why many designs prefer RAID 6 or RAID 10 for production data, plus tested backups. The rebuild-risk note uses your URE exponent as a teaching signal — not a guarantee from any single drive vendor.
RAID is not backup
An array can survive a disk failure and still lose everything to ransomware, accidental deletion, fire, or a failed controller write hole. Pair RAID with:
- Versioned, immutable or offline backups
- Restore tests on a schedule
- Monitoring for degraded arrays
- Documented RTO/RPO targets
See backup and disaster recovery and managed infrastructure. Camera systems should also use the surveillance storage calculator.
Worked example
Six 4 TB disks in RAID 6, one hot spare
- Data disks n = 6, S = 4 TB
- Usable = (6 − 2) × 4 = 16 TB
- Raw data pool = 24 TB · efficiency ≈ 67%
- Total drives to buy = 7 (6 + spare)
When to call Secure Techies
If you are designing SAN/NAS for VMs, SQL, file shares, or backup repositories, we can size the array, monitoring, and recovery plan together. Contact us for a free assessment.
Frequently asked questions
What does a RAID calculator show?
A RAID calculator estimates usable capacity, storage efficiency, and how many drives can fail before data is lost. It uses the number of disks, single-disk size, and RAID level (0, 1, 5, 6, or 10).
Is RAID a backup?
No. RAID protects against some disk failures, not ransomware, accidental deletion, or site disasters. You still need tested backups and a recovery plan.
Which RAID level should I use?
RAID 10 is popular for performance and rebuild speed. RAID 6 is common for bulk capacity with dual parity. RAID 5 is widely used but rebuild risk grows on large disks. RAID 0 is for speed only and is not fault tolerant.
Why is usable capacity lower than total raw capacity?
Parity or mirroring reserves space for redundancy. RAID 5 uses one disk worth of parity. RAID 6 uses two. RAID 1 and 10 mirror data, so roughly half the raw space is usable.
Do different drive sizes work in one array?
Most traditional RAID arrays are limited by the smallest disk. Mixing sizes usually wastes capacity unless you use a vendor hybrid RAID mode.
Need help designing resilient storage?
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