Raid Usable Capacity
Calculator
Results
- Usable capacity (TB, 1 TB = 1000 GB)
- 20
- Raw capacity (TB)
- 24
- Usable capacity (TiB, 1 TiB = 1024 GiB)
- 18.189894
- Overhead (%)
- 16.666666
Computing results
| Usable capacity (TB, 1 TB = 1000 GB) | 20 |
| Raw capacity (TB) | 24 |
| Usable capacity (TiB, 1 TiB = 1024 GiB) | 18.189894 |
| Overhead (%) | 16.666666 |
formula-map diagram
- Usable capacity (TB, 1 TB = 1000 GB)
- 20
- Raw capacity (TB)
- 24
- Usable capacity (TiB, 1 TiB = 1024 GiB)
- 18.189894
- Overhead (%)
- 16.666666
Computing relationship
Formula
usable = (disks − parity) × disk_size= 20
Note
This is a simplified model: it applies the standard computing formula to the numbers you entered and ignores protocol overhead, compression variability, retries, contention and other real-world effects. Size your systems with measured data.
More in Technology and computing
See all →Frequently asked questions
What does 'usable capacity' mean in a RAID array?+
Usable capacity is the amount of storage actually available for your data after the RAID level's redundancy or striping overhead is subtracted from the raw combined capacity of all drives. It's always less than or equal to the sum of the drives' raw sizes.
Why isn't usable capacity just the sum of all my drives?+
Most RAID levels reserve some capacity for redundancy so the array can survive a drive failure without losing data — for example, RAID 5 reserves the equivalent of one drive, and RAID 1 reserves half of total capacity by mirroring every drive. Only RAID 0 uses 100% of raw capacity, at the cost of having no redundancy at all.
Does using drives of different sizes affect the calculation?+
Yes, in most RAID levels the array is limited by the smallest drive's capacity, since parity or mirroring needs matching-sized chunks across all drives. Mixing drive sizes typically wastes the extra capacity on the larger drives, so using identically sized drives is recommended.
Why choose a RAID level with lower usable capacity, like RAID 1 or RAID 6?+
Lower usable capacity buys more fault tolerance: RAID 1 can survive a single drive failure with a straightforward rebuild, and RAID 6 can survive two simultaneous drive failures. The right trade-off between capacity and safety depends on how critical the data is and how quickly you could restore from a backup instead.
Is usable capacity the same as what my operating system will show?+
It's very close, but the operating system will typically show a slightly smaller number due to filesystem overhead and the difference between decimal (manufacturer) and binary (operating system) definitions of a gigabyte or terabyte.