How to use the RAID capacity calculator
- Choose the intended RAID level. This calculator covers RAID 0, RAID 1, RAID 5, RAID 6 and RAID 10 as separate standard layouts.
- Enter a whole number of disks and the capacity printed for one disk. Every disk is assumed to have the same capacity.
- Choose GB or TB for decimal manufacturer units, or GiB or TiB for binary units. These labels are not interchangeable.
- Select Calculate. Editing any field clears the confirmed result and disables Copy until you calculate again.
How RAID usable capacity is calculated
Raw capacity is disk count multiplied by the capacity of one disk. RAID 0 uses all raw capacity but has no mirror or parity capacity. RAID 1 here means one copy of the data is written to every disk, so usable capacity equals one disk even when more than two disks are entered.
RAID 5 reserves the equivalent of one disk for distributed parity, so usable capacity is (N − 1) × disk size. RAID 6 reserves two disks’ worth, giving (N − 2) × disk size. RAID 10 mirrors pairs and stripes across the pairs, so half of the raw capacity is usable and the disk count must be even.
The result shows both decimal storage (GB, TB, PB) and IEC binary storage (GiB, TiB, PiB). A 1 TB drive contains 1,000,000,000,000 bytes, which is about 0.91 TiB. The number of bytes is unchanged; only the grouping and label differ.
Mirror or parity capacity is raw capacity minus usable capacity. It is a capacity accounting value, not a prediction of how many arbitrary failures an array will survive. Failure tolerance depends on RAID layout and which disks fail.
Worked examples
Four 2 TB disks in RAID 5 provide 8 TB raw, 6 TB usable and 2 TB parity capacity. The same usable bytes display as about 5.46 TiB, with 75% capacity efficiency.
Six 1 TiB disks in RAID 10 provide 6 TiB raw and 3 TiB usable. The other 3 TiB is mirror capacity. Entering five disks is rejected because standard RAID 10 requires complete mirrored pairs.
Assumptions and limits
- All disks are treated as equal size. Mixed-size arrays, vendor hybrid RAID and expansion rules are outside this calculator.
- Results estimate array-level capacity before filesystem formatting, metadata, snapshots, spare disks and product-specific reserved space.
- RAID 0 has no redundancy. RAID is not a backup, and this calculator does not estimate rebuild success, failure probability or data recovery.
- The tool does not benchmark read/write speed, choose hardware, configure a controller or operate any disk. Confirm supported RAID levels and drive limits with the actual enclosure or operating system.
- Copy saves a short text result only. No values leave the browser, and no array configuration is stored.
Frequently asked questions
Why is a 1 TB drive shown as about 0.91 TiB?
TB divides bytes by powers of 1000; TiB divides the same bytes by powers of 1024. The apparent difference is a unit conversion, not missing data.
Why does RAID 1 use only one disk of capacity?
This page uses the stated all-disk mirror assumption: every member contains the same data. Some products use different layouts under similar names, so check their documentation.
Is parity capacity the same as a dedicated parity disk?
Not necessarily. RAID 5 and RAID 6 normally distribute parity blocks, but their total capacity overhead is equivalent to one or two disks.
Does the result include a hot spare?
No. Enter only active array members. A spare that is not part of the active array must be accounted for separately.
Can this tell me which RAID level is safest?
No. It compares capacity arithmetic only. Reliability also depends on failure combinations, backups, rebuild time, drive health and the implementation.