RAID Calculator
Compare RAID configurations and find the perfect NAS setup
Free RAID calculator for capacity, redundancy and cost
raidsize is a free RAID calculator that shows exactly how much usable storage, fault tolerance and cost per terabyte you get from any set of drives. Add your disks, choose two RAID levels, and compare them side by side while the numbers update instantly. Where most vendor tools cover only a handful of levels, this calculator models 13 configurations, from classic RAID 0, 1, 5, 6 and 10 to Synology SHR, Unraid and ZFS RAIDZ, and it treats mixed drive sizes the way each system really does.
How the RAID calculator works
Every RAID level trades raw capacity for speed or redundancy in its own way, and the calculator applies the exact formula for each one rather than a rough estimate. For parity arrays such as RAID 5 and RAID 6, usable capacity is (N − p) × the smallest drive, where N is the number of drives and p is the parity count (one for RAID 5, two for RAID 6). Mirrored setups like RAID 1 and RAID 10 keep a full copy of your data, so usable space drops to roughly half. Flexible systems like Synology SHR and Unraid pool space across differently sized disks, which lets larger drives contribute capacity that traditional RAID would waste.
Once you enter drive prices, the result panel also converts everything into cost per usable TB, so you can weigh not just capacity but value. RAID protects against drive failure, never against deletion, ransomware or fire, so treat every result as a storage plan, not a backup plan.
RAID levels at a glance
| RAID level | Min. drives | Survives | Usable capacity | Mixed sizes |
|---|---|---|---|---|
| RAID 0 | 2 | 0 failures | All drives (no redundancy) | Inefficient |
| RAID 1 | 2 | N − 1 failures | One drive | Inefficient |
| RAID 5 | 3 | 1 failure | (N − 1) × smallest | Inefficient |
| RAID 6 | 4 | 2 failures | (N − 2) × smallest | Inefficient |
| RAID 10 | 4 | 1+ per mirror | N ÷ 2 × smallest | Inefficient |
| SHR-1 | 2 | 1 failure | Layered, uses extra space | Efficient |
| SHR-2 | 4 | 2 failures | Layered, uses extra space | Efficient |
| Unraid (1 parity) | 2 | 1 failure | Sum − largest drive | Efficient |
| Unraid (2 parity) | 3 | 2 failures | Sum − 2 largest drives | Efficient |
| ZFS RAIDZ1 | 3 | 1 failure | (N − 1) × smallest | Inefficient |
| ZFS RAIDZ2 | 4 | 2 failures | (N − 2) × smallest | Inefficient |
| ZFS RAIDZ3 | 5 | 3 failures | (N − 3) × smallest | Inefficient |
| JBOD | 1 | 0 failures | Sum of all drives | Efficient |
Which RAID level should you choose?
RAID 5 is the classic choice for a home NAS with three to five drives: it survives one failure and gives up just one drive's worth of capacity, which is why it remains the default for many builds. Once drives are large (roughly 12 TB and up) or the array grows past five disks, RAID 6 is safer, because a second parity drive covers the long rebuild window when a replacement disk is resilvering. You can see both modeled on our RAID 5 and RAID 6 pages.
RAID 10 trades capacity for speed and fast rebuilds, making it a favourite for databases and virtual machines. Synology SHR and Unraid are the best options when your drives are different sizes, since they use nearly all of the space that traditional RAID would strand. If you run TrueNAS or Proxmox, ZFS RAIDZ2 pairs strong two-drive fault tolerance with checksummed data integrity. Browse every level on the RAID levels guide, or compare two setups directly in our side-by-side comparisons.
Frequently asked questions
How much usable space does RAID 5 give with 4×4 TB drives?
(4 − 1) × 4 TB = 12 TB usable. One drive's worth of capacity is reserved for parity so the array can rebuild after a single failure.
What is the difference between RAID 5 and RAID 6?
RAID 5 uses single parity and survives one drive failure. RAID 6 uses double parity and survives two simultaneous failures, which is recommended for arrays with five or more drives or drive capacities of 12 TB and above.
Can I mix different drive sizes in RAID?
Traditional RAID (0, 1, 5, 6, 10) treats every drive as if it were the smallest one, wasting space on larger disks. Synology SHR and Unraid handle mixed drive sizes efficiently, so they are the better choice for gradual upgrades.
Is RAID a backup?
No. RAID protects against drive hardware failure only. It does not protect against accidental deletion, file corruption, ransomware, fire, theft or admin error. Always keep an independent backup, ideally off-site.
How many drives do I need for ZFS RAIDZ2?
A minimum of four drives. The recommended sweet spot is six to eight drives for a good balance of capacity, performance and fault tolerance.