free tool · raid, zfs raidz and shr
RAID capacity calculator
Enter how many drives you have, how big each one is, and the RAID level. You get the usable space in TB and TiB, how many drives can fail before you lose data, and how much of the raw space goes to protection.
usable capacity
24 TB
21.83 TiB in binary units, as many dashboards count it
- Drive failures it survives
- 1
- Raw capacity
- 32 TB
- Space used for protection
- 8 TB
- Space efficiency
- 75%
RAID 5 keeps one drive's worth of parity. Any one drive can fail; a second failure before the rebuild finishes loses the array.
How the numbers are worked out
Every level uses one formula. N is the number of drives and S is the size of one drive. The classic RAID and SHR rows come from Synology's Choose a RAID Type page. The RAIDZ rows come from the OpenZFS RAIDZ documentation. Both were checked in September 2026.
| Level | Fewest drives | Usable space | Failures it survives |
|---|---|---|---|
| RAID 0 | 2 | N × S | 0 |
| RAID 1 | 2 | S | N − 1 |
| RAID 5 | 3 | (N − 1) × S | 1 |
| RAID 6 | 4 | (N − 2) × S | 2 |
| RAID 10 | 4, even | (N ÷ 2) × S | 1 for certain; up to N ÷ 2 if no two failures hit the same mirror pair |
| RAIDZ1 | 2 | about (N − 1) × S | 1 |
| RAIDZ2 | 3 | about (N − 2) × S | 2 |
| RAIDZ3 | 4 | about (N − 3) × S | 3 |
| SHR-1 | 1 (2 for protection) | S with 1 drive; (N − 1) × S with 2 or more equal drives | 0 with 1 drive; 1 with 2 or more |
| SHR-2 | 4 | (N − 2) × S with equal drives | 2 |
Three details sit behind that table.
- RAIDZ is approximate. OpenZFS says a RAIDZ group of N disks of size S with P parity disks holds approximately (N − P) × S. Real efficiency also depends on the block size and the disk sector size, and small blocks can cost as much space as a mirror. The docs give the worked examples. The minimum group is one more disk than the parity count.
- ZFS holds some space back. OpenZFS reserves "slop space" so a pool never fills completely: normally 1/32 of the pool (about 3.2%), never less than 128 MiB and never more than 128 GiB. See
spa_slop_shiftin the OpenZFS module parameters. The calculator shows the figure after that reservation for the RAIDZ levels. - Synology formats away a little more. Synology's own RAID Calculator notes that each drive keeps about 10 GB for the system partition, and that a Btrfs volume reserves 4% for metadata (2% on ext4). Expect the volume you create to be a few percent smaller than the number above.
For SHR with equal drives, the result matches RAID 1 or RAID 5 for SHR-1 and RAID 6 for SHR-2. That follows from how Synology describes SHR: it differs from classic RAID by reclaiming space that classic RAID wastes when drives are different sizes. With identical drives there is nothing to reclaim.
TB vs TiB: why your NAS shows less than the label
There are two ways to count a terabyte. Drive makers use the decimal system: 1 TB is 1,000,000,000,000 bytes. Many operating systems and NAS dashboards count in binary: 1 TiB is 240 bytes, or 1,099,511,627,776 bytes. Apple's page on how storage capacity is measured explains the two systems.
The gap is about 9%. One TB is 0.9095 TiB, so an 8 TB drive shows as about 7.28 TiB. Nothing is missing; the same bytes are counted with a bigger unit. The calculator prints both, so you can match the number your dashboard shows.
Which level for a video NAS
For an edit NAS with four to eight hard drives, RAID 6, RAIDZ2 or SHR-2 is the level we would pick. Large drives take a long time to rebuild, and a single-parity array has no protection left while it rebuilds. Two-drive protection costs one more drive and buys that margin back. RAID 10 is faster for random reads and writes, and Synology calls it the level for high I/O demand, but you keep only half the raw space.
RAID 0 has no protection. One drive failure loses everything on the array, so use it only for scratch space you can recreate.
None of these levels is a backup. Synology says so on its own SHR page, and it applies to every row in the table. A RAID array protects you against a failed drive. It does not protect you against a deleted folder, ransomware or a fire. Keep a second copy somewhere else.
Mixed drive sizes
This calculator assumes equal drives, because that is how most arrays are built. With mixed sizes, Synology's documentation says classic RAID sizes every drive down to the smallest one: in its example, a 500 GB drive in the pool makes every other drive contribute only 500 GB. Synology Hybrid RAID splits drives into smaller chunks to use the space that classic RAID wastes. The RAIDZ formula in the OpenZFS docs is written for disks of one size, so plan a RAIDZ group with equal disks.
After the array: getting it onto every editor's Mac
A RAID level decides how much space you have and what survives a failed drive. It does not decide how the NAS feels to an editor on the other end of the network. That is the job of the connection between the NAS and each Mac.
JuiceMount is the part we build for that. It is an open-source mount that puts your own NAS on each editor's Mac as a volume, streams only the parts of a file the editor plays, and keeps a local cache. It works on top of whatever array you choose here. See how it works, or read how to set it up on your NAS.
More on the storage underneath: ZFS for video editors, explained, Synology for post-production, and HDD vs SSD vs hybrid for media. If you are still deciding whether to own storage at all, the rent vs own calculator prices a NAS against the cloud mounts.
Formulas checked September 2026 against Synology's and OpenZFS's documentation. The calculator runs in your browser and sends nothing anywhere.