How to use this RAID calculator
It's quite simple, actually.
- Pick the RAID level or ZFS layout you are considering.
- Enter how many drives you plan to use.
- If all drives are the same size, enter that capacity. If they are not, switch to Mixed and enter each drive.
- Choose the drive class. Check your drive’s spec sheet for its URE rating. Most desktop drives are 10¹⁴, and NAS drives like WD Red Plus or Seagate IronWolf are usually 10¹⁵.
- Read the results: usable capacity, how many drives can fail, and how much raw space goes to parity or mirroring.
- Scroll to Compare layouts to see the same drives in every layout side by side.
You can save a configuration or copy a link to share it. Saving and sharing both basically mean the RAID calculator is filled with the values you have selected. Data doesn't leave the browser.
How usable capacity is calculated
N is the number of drives and S is the size of the smallest drive.
| Layout | Minimum drives | Usable capacity | Drives that can fail |
|---|---|---|---|
| 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 number) | N ÷ 2 × S | 1 guaranteed, up to N ÷ 2 |
| RAIDZ1 | 3 | (N − 1) × S | 1 |
| RAIDZ2 | 4 | (N − 2) × S | 2 |
| RAIDZ3 | 5 | (N − 3) × S | 3 |
RAID 10 can survive more than one failure, but only if each failed drive comes from a different mirror pair. If both drives in the same pair die, the array is gone. That is why the calculator counts it as one guaranteed failure.
How the rebuild risk is estimated
When a drive fails, the array has to read every surviving drive in full to rebuild it. The calculator multiplies the data read by the drive’s URE rating to estimate the chance of hitting at least one unreadable sector along the way.
Take four 8 TB drives in RAID 5. A rebuild reads the three surviving drives, which is 24 TB or about 1.9 × 10¹⁴ bits. With consumer drives rated at 1 error per 10¹⁴ bits, the spec sheet says the chance of at least one error is around 85%. With NAS drives rated at 10¹⁵, it drops to around 17%.
Real drives usually beat their rating, so treat this as a worst case. It still explains why RAID 6 and RAIDZ2 are the usual advice for big drives. A second parity drive means one bad sector during a rebuild costs you nothing.
Does this work for Synology, QNAP, UGREEN or TrueNAS?
Yes, for the standard RAID levels. QNAP, UGREEN and Synology all use Linux software RAID underneath, so RAID 1, 5, 6 and 10 give the same usable capacity shown here. For TrueNAS, use the ZFS mirror and RAIDZ options.
What this RAID calculator does not cover
It shows capacity and redundancy, not speed. Real performance depends on your controller, drives and workload. The figures are also before filesystem overhead. ZFS reserves a little over 3% of a pool as slop space, and your OS shows sizes in TiB, so expect the free space you actually see to be lower.
RAID terms explained
Here is a brief explanation of the terms used on this online RAID calculator.
Striping
Data is split into chunks and written across several drives at once. It is fast because every drive works in parallel, but nothing is stored twice, so losing one drive loses everything. This is RAID 0.
Mirror
The same data is written to two or more drives. If one dies, another still has a complete copy. Simple and safe, but you only get the space of one drive per mirror. This is RAID 1.
Parity
Extra data calculated from your files, like the answer to a sum. If one number goes missing, you can work it out from the rest. Parity lets the array recreate a failed drive without keeping a full copy, so it costs less space than mirroring.
Single parity
One drive's worth of space holds parity, spread across all drives. The array survives one failed drive. RAID 5 and RAIDZ1 work this way.
Double parity
Two drives' worth of parity, so any two drives can fail. That includes a second drive dying while the first is being replaced, which is when it is most likely. RAID 6 and RAIDZ2 work this way; RAIDZ3 adds a third.
Striped mirrors
Drives are paired into mirrors, then data is striped across the pairs. You get the speed of striping and the quick recovery of mirrors, but only half the raw space. This is RAID 10.
Rebuild (resilver)
After you replace a failed drive, the array recreates its data by reading the other drives. ZFS calls this a resilver. Until it finishes, the array has less protection than usual.
ZFS vs. traditional RAID
Traditional RAID, from a hardware card or Linux's mdadm, only sees blocks of data and cannot tell if a block has gone bad. ZFS is a filesystem and RAID manager in one. It checksums every block, so it spots silent corruption and repairs it from the redundant copy. It only rebuilds the space actually in use, and a read error usually costs one file rather than the whole array. In return, it likes plenty of RAM, and growing a pool is less flexible: adding a single drive to an existing RAIDZ only arrived in OpenZFS 2.3.
URE and drive class
A URE (unrecoverable read error) is a spot on a drive that cannot be read back. Manufacturers rate how often this can happen. Consumer drives are usually rated at 1 error per 10¹⁴ bits read (about 12.5 TB). NAS and enterprise drives are rated at 1 per 10¹⁵ (about 125 TB). A rebuild reads a lot of data, so the calculator uses this rating to estimate the chance of hitting an error mid-rebuild. These are spec-sheet worst cases; real drives often do better.
How is the power draw estimated?
Each drive type uses a typical average draw for a drive that is spinning or active but not under heavy load:
- 3.5" HDD: 6.5 W
- 2.5" HDD: 2.5 W
- 2.5" SATA SSD: 2.0 W
- NVMe SSD: 4.0 W
The array total is that figure multiplied by the number of drives. The yearly figure assumes the drives run 24 hours a day, every day: watts × 8,760 hours ÷ 1,000 = kWh.
It counts the drives only. The CPU, motherboard, fans and power supply losses add more, while spinning drives down when idle lowers it. Treat it as a ballpark for comparing layouts.
Frequently Asked Questions
Here are the answers to common questions you might have:
Should I use RAID 5 or RAID 6?
RAID 6 is the safer choice for arrays built with large modern drives (8TB and above). RAID 5 tolerates only one drive failure, and rebuilding after a failure requires reading every surviving drive in full. A single unreadable sector during that window can compromise the rebuild. RAID 6 adds a second parity block, so the array survives a second failure or read error during that window. The cost is one extra drive's worth of capacity. RAID 5 remains reasonable for small arrays of small drives or non-critical data.
Is ZFS RAIDZ better than a ZFS mirror?
Neither is universally better. RAIDZ (especially RAIDZ2) gives you more usable capacity from the same drives, while mirrors give faster resilvers, better random I/O performance, and simpler expansion, since you can grow a pool by adding another mirror pair. For most home NAS builds with 4 to 10 drives where capacity matters, RAIDZ2 is the common recommendation; for performance-sensitive workloads or pools you expect to expand gradually, mirrors are often the better fit.
Is RAID a backup?
No. RAID protects against drive hardware failure and nothing else. Accidental deletion, ransomware, file corruption, controller failure, theft, and fire all affect every drive in the array simultaneously. RAID keeps your system available when a drive dies; a backup lets you recover data you have lost. You need both, and the standard advice is the 3-2-1 rule: three copies of your data, on two different types of media, with one copy off-site.
How much usable space do I get from RAID 5?
RAID 5 gives you the capacity of all drives minus one, based on the smallest drive in the array. Four 4TB drives yield 12TB usable; six 8TB drives yield 40TB usable. Note that your operating system reports capacity in TiB (binary units), so 12TB will appear as roughly 10.9TiB. The space is not missing; it is a difference in units.
Can I mix different drive sizes in RAID?
Yes, but every drive in the array is treated as if it were the size of the smallest drive, and the extra capacity on larger drives sits unused. Mixing a 4TB drive into an array of 8TB drives effectively turns them all into 4TB drives. If you must mix sizes, replacing the smallest drive is usually the cheapest way to reclaim capacity. Some systems (like Unraid or Synology SHR) handle mixed sizes more gracefully than standard RAID.
How many drives can fail in RAID 5, RAID 6 and RAID 10?
RAID 5 survives one failure and RAID 6 survives two. RAID 10 survives one for certain and more if they are in different mirror pairs. RAIDZ1, Z2 and Z3 survive one, two and three.
What is the advantage of RAID 5 over RAID 1?
Capacity. With four 4 TB drives, RAID 5 gives you 12 TB. RAID 1 mirrors always give you half or less. In exchange, RAID 1 rebuilds faster and is simpler to recover.
Can I add drives to a RAID 5 array later?
Usually, yes. mdadm can grow a RAID 5 array in place. Synology and QNAP support it from the storage manager. ZFS RAIDZ expansion arrived in OpenZFS 2.3, and space written before the expansion keeps its old parity ratio until you rewrite it.
Is RAID 0 worth it?
Only for data you can lose, like a scratch disk or a game library. One drive failure wipes out everything on every drive.
Why do I see less free space than the calculator shows?
There are two reasons. Your OS counts in TiB, so 12 TB shows up as about 10.9 TiB. Filesystems also reserve some space.
