Computer Science · Digital & Computing
RAID Storage Calculator
Calculates usable storage capacity, overhead, and fault tolerance for RAID 0, 1, 5, 6, and 10 configurations.
Calculator
Formula
n = number of drives; d = capacity of each drive (TB). RAID 0 stripes data across all drives with no redundancy. RAID 1 mirrors all data, leaving only one drive worth of usable space. RAID 5 dedicates one drive equivalent to parity. RAID 6 dedicates two drive equivalents to double parity. RAID 10 mirrors pairs then stripes across them, using half the total capacity.
Source: SNIA (Storage Networking Industry Association) RAID Level Definitions, 2023.
How it works
RAID (Redundant Array of Independent Disks) is a data storage virtualization technology that combines multiple physical drives into one logical unit to improve performance, redundancy, or both. The usable capacity of a RAID array depends entirely on the RAID level chosen and the number and size of drives installed. Understanding the tradeoff between raw capacity and fault tolerance is essential for any storage planning exercise.
Each RAID level uses a different mathematical relationship between drive count and usable space. In RAID 0, all drives contribute fully — n drives of d TB each yield n × d TB usable. In RAID 1, every byte is mirrored, so regardless of drive count, only d TB is usable. RAID 5 dedicates the equivalent of one drive to distributed parity, giving (n − 1) × d. RAID 6 uses double parity for two-drive fault tolerance, yielding (n − 2) × d. RAID 10 mirrors drives in pairs then stripes across those pairs, always consuming exactly half the raw capacity regardless of array size. Storage efficiency is calculated as usable ÷ raw capacity, expressed as a percentage.
These calculations apply to arrays of equal-sized drives, which is standard practice for production deployments. Real-world usable space is typically slightly less than these theoretical values due to drive formatting overhead, file system metadata, and controller reservation. Nevertheless, these formulas are the industry standard for initial capacity planning and procurement decisions.
Worked example
Suppose you are building a NAS server with 8 drives of 4 TB each and you need to choose between RAID 5 and RAID 6.
Total raw capacity: 8 × 4 TB = 32 TB
RAID 5 usable capacity: (8 − 1) × 4 = 28 TB with 1 drive fault tolerance. Storage efficiency = 7/8 = 87.5%. Parity overhead = 4 TB.
RAID 6 usable capacity: (8 − 2) × 4 = 24 TB with 2 drive fault tolerance. Storage efficiency = 6/8 = 75.0%. Parity overhead = 8 TB.
The RAID 6 configuration sacrifices 4 TB of usable space compared to RAID 5 but survives simultaneous failure of any two drives — a meaningful advantage for arrays where a rebuild after a single failure could stress remaining drives enough to trigger a second failure. For an 8-drive array storing critical data, RAID 6 is typically recommended by storage engineers.
Limitations & notes
This calculator assumes all drives are of identical capacity. In a mixed-drive array, the RAID controller uses the smallest drive's capacity for all members, wasting any additional space on larger drives — a scenario this tool does not model. RAID 10 requires an even number of drives (minimum 4); odd counts are invalid. The fault tolerance figure for RAID 1 and RAID 10 represents the maximum number of drives that can fail in the best-case scenario (no two mirrors losing both members simultaneously); in the worst case, RAID 10 can fail after losing just one drive if it is the sole surviving mirror of a pair. Additionally, these figures represent raw logical capacity — actual formatted capacity will be roughly 7–10% lower due to file system overhead. SSDs may also reserve additional space for wear leveling. Software RAID implementations may carry small additional performance and capacity penalties not reflected here.
Frequently asked questions
What is the minimum number of drives for each RAID level?
RAID 0 and RAID 1 require a minimum of 2 drives. RAID 5 requires at least 3 drives. RAID 6 requires at least 4 drives. RAID 10 requires a minimum of 4 drives and always an even number.
Which RAID level gives the most usable storage?
RAID 0 provides 100% storage efficiency since there is no redundancy overhead. However, it offers zero fault tolerance — a single drive failure destroys all data. For arrays requiring redundancy, RAID 5 offers the best balance of usable space and protection for most workloads.
How does RAID 5 differ from RAID 6 in terms of fault tolerance?
RAID 5 can tolerate the failure of exactly one drive. RAID 6 uses dual distributed parity and can survive two simultaneous drive failures without data loss. RAID 6 is preferred for larger arrays (6+ drives) where the probability of a second drive failing during a rebuild operation is statistically significant.
Does RAID replace backups?
No. RAID provides fault tolerance against hardware failure but does not protect against accidental deletion, ransomware, software corruption, or physical disasters like fire or flooding. RAID should always be used alongside a proper backup strategy following the 3-2-1 rule: three copies, two media types, one offsite.
Why is RAID 10 storage efficiency always 50%?
RAID 10 mirrors every drive in a pair, then stripes data across those mirror pairs. Because mirroring always consumes exactly half of each pair's raw capacity regardless of the number of drives, the overall efficiency is always 50% for any valid RAID 10 array size.
Last updated: 2025-01-15 · Formula verified against primary sources.