RAID Capacity Calculator
Enter your drive sizes and pick a RAID level to see how much storage you actually get. This is the number to plan against: a 4-drive array of 4 TB disks holds 16 TB raw, but considerably less once redundancy is accounted for.
Comma or space separated, one entry per drive. Decimals allowed, so
3.5, 3.5 is a valid pair of 4 TB-class drives.
Usable capacity
4 TB
Raw capacity
8 TB
Lost to redundancy
4 TB
Drives that can fail
1
Every drive is a full copy of the data. Usable capacity equals the smallest drive.
All RAID levels for this configuration
| Level | Usable | Overhead | Failures survived |
|---|---|---|---|
| RAID 0 (striping) | 8 TB | 0 TB | 0 |
| RAID 1 (mirroring) | 4 TB | 4 TB | 1 |
| RAID 5 (single parity) | 4 TB | 4 TB | 1 |
| RAID 6 (double parity) | 0 TB | 8 TB | 2 |
| RAID 10 (mirrored pairs) | 4 TB | 4 TB | 1 |
| SHR-1 (Synology Hybrid RAID) | 4 TB | 4 TB | 1 |
| SHR-2 (Synology Hybrid RAID) | 0 TB | 8 TB | 2 |
Levels that need more drives than you entered are marked as not applicable instead of showing a misleading number.
How this is calculated
Parity-based levels store one parity stripe per drive group for RAID 5 and SHR-1, and two for RAID 6 and SHR-2, so usable capacity is the drive count minus the parity count, multiplied by the smallest drive in the array. RAID 1 mirrors every drive, so only one drive's worth survives. RAID 10 mirrors in pairs, giving half the count at the smallest drive size. RAID 0 stripes with no redundancy, so usable capacity equals the sum of all drives.
Planning the hardware? The NAS buying guide covers which models to pair with these configurations, and the Terraform modules cover the backup target once the array is built.
Frequently Asked Questions
Why is my usable capacity smaller than the raw capacity?
Parity-based levels (RAID 5, RAID 6, SHR-1, SHR-2) store distributed parity so the array can survive a drive failure. RAID 1 and RAID 10 keep a full or half copy of the data instead. RAID 0 has no redundancy at all, so it is the only level where usable capacity equals raw capacity.
How do mixed drive sizes change the result?
Every parity-based level is limited by the smallest drive in the array. A 4 TB + 2 TB pair in SHR-1 gives 2 TB usable, not 6 TB, because the 4 TB drive cannot hold parity for a capacity the 2 TB drive cannot match. SHR and RAID 10 let you start small and expand later; classic RAID 5 and 6 generally want matching drives.
Which RAID level should a home NAS use?
Two drives in a mirror (RAID 1 or SHR-1) is the usual starting point: it survives a single failure and keeps rebuilds short. SHR is the more flexible choice if you plan to add drives of a different size later. Avoid RAID 0 for anything you would miss, since a single failure loses the entire array.
Does usable capacity match what the NAS shows after formatting?
Not exactly. Beyond parity, the filesystem reserves space for metadata and journal, and vendors usually report capacity in decimal terabytes while the operating system may display binary tebibytes. Expect a small additional difference on top of the parity overhead calculated here.