RAID capacity calculator
Compare raw and usable capacity for equal-size drives in common RAID layouts.
Equal-size layout arithmetic only; excludes filesystem overhead, hot spares, mixed drives, rebuild time, reliability and performance. RAID is not a backup.
Included
- Every visible input and selected unit
- Reconciled intermediate values and result breakdown
- Reproducible URL state
Not included
- Equal-size layout arithmetic only; excludes filesystem overhead, hot spares, mixed drives, rebuild time, reliability and performance. RAID is not a backup.
What this means
Compare raw and usable capacity for equal-size drives in common RAID layouts.
This is ideal RAID layout capacity: the conventional redundancy arithmetic before filesystem, platform, spare and application overhead. It is not a measured free-space result.
There is no universally standard RAID level. RAID 5, RAID 6 and RAID 10 are investigated for different platforms, workloads, recovery requirements and capacity trade-offs.
Equal-size layout arithmetic only; excludes filesystem overhead, hot spares, mixed drives, rebuild time, reliability and performance. RAID is not a backup.
Intermediate values are not rounded; only the localized display formats the result.
How RAID works — and where this calculator stops
RAID combines drives; it does not create a universal best choice
A RAID set is a group of drives arranged as one logical storage unit. Striping spreads data across members; mirroring keeps another copy; distributed parity stores recovery information alongside the data. RAID 0 uses striping only, RAID 1 mirrors, RAID 5 and RAID 6 use parity, and conventional RAID 10 stripes across mirror pairs.
No single RAID level is the modern default everywhere. A NAS, operating system, controller or enterprise array can support different layouts and make different trade-offs. Start with the exact platform, workload, acceptable downtime and recovery plan—not a generic ranking.
Raw capacity, ideal layout capacity and filesystem space are different layers
Raw capacity is every selected drive added together. Ideal RAID layout capacity is what remains after the conventional mirror or parity allocation in this equal-size model. The result is useful for comparing layouts, but it is not a promise of filesystem free space.
A real system can reserve or consume capacity for metadata, formatting, hot spares, snapshots, thin provisioning, checksums, compression, deduplication and application rules. Equal-sized drives keep this teaching model clear; mixed-drive and proprietary layouts need their own platform documentation.
Failure tolerance describes a defined member-failure pattern
RAID 5 can reconstruct after one participating drive failure, and conventional RAID 6 after two. That says nothing about controller, enclosure, power, software, metadata, corruption or site-wide failures. It also does not guarantee that every surviving drive or the recovered data is healthy.
RAID 10 deserves special care. Any one drive can fail; several can fail only when they are in different mirror pairs. Losing both drives in any one pair exceeds that pair’s redundancy, so “RAID 10 survives two failures” is incomplete advice.
TB and TiB name the same bytes with different rulers
Drive makers commonly use decimal TB: 1 TB is 1,000,000,000,000 bytes. Operating systems may show binary TiB: 1 TiB is 1,099,511,627,776 bytes. A nominal 4 TB drive is about 3.64 TiB; no capacity disappeared.
This calculator converts the selected unit to bytes for the calculation, then formats the result for your locale. The RAID formula does not change when the unit label changes.
RAID is not a backup
RAID helps a defined drive failure not interrupt access to the current data. It does not inherently preserve an earlier version after accidental deletion, ransomware, corruption or a destructive administration change. A shared controller, enclosure, power source or site can also defeat every member together.
Backups, snapshots and replication solve different problems. A useful recovery plan considers independent copies, retention, isolation or immutability, recovery point and recovery time objectives, and a complete restore test. Treat RAID as one part of availability—not as the only copy of important data.
The conventional layouts in this calculator
These are equal-size, conventional-layout formulae. They describe capacity and a member-failure boundary, not a performance benchmark or a production recommendation.
| Layout | Mechanism | Ideal capacity | Member-failure boundary | Main caution |
|---|---|---|---|---|
| RAID 0 | Striping | N × S | None | Any member failure can make striped data unavailable. |
| RAID 1 | Full mirror | S | A valid copy must remain | A mirror copies current data; it is not version history. |
| RAID 5 | Single distributed parity | (N − 1) × S | One member | No remaining member-failure margin after one failure. |
| RAID 6 | Dual distributed parity | (N − 2) × S | Two members | Two drive-equivalents are allocated to parity; behaviour is platform-specific. |
| RAID 10 | Stripe across mirror pairs | (N ÷ 2) × S | One per mirror pair | Pair topology decides whether multiple failures survive. |
How to investigate RAID 5, RAID 6 or RAID 10
Use these as questions to take to your platform documentation or administrator, not as a one-size-fits-all recommendation.
RAID 5
Investigate when one-parity capacity efficiency matters and the named platform supports it for the expected workload. Check rebuild behaviour and the consequence of having no second-drive margin.
RAID 6
Investigate when two-parity protection is a stated requirement and the system has at least four members. Check the platform’s recovery, spare and write guidance rather than assuming RAID 6 is the default.
RAID 10
Investigate when mirror-based recovery or latency-sensitive, write-heavy work justifies 50% ideal capacity efficiency. Confirm actual pair mapping, supported member counts and expansion rules.
Stop relying on generic guidance when the product, controller, filesystem, drive media, workload, fault domains, monitoring, replacement plan, backup independence or restore test are unknown.
Quick glossary
- Member
- A drive or logical device participating in one RAID set.
- Stripe
- Data blocks, and sometimes parity, distributed across members.
- Parity
- Recovery information calculated from data blocks; it is not a separate copy of every file.
- Degraded
- A state with reduced redundancy after a member failure or removal.
- Rebuild
- Reconstructing missing data or redundancy onto replacement or spare capacity.
- Hot spare
- Unused or reserved capacity available to help recovery after a failure.
- Snapshot
- A point-in-time data view whose independence depends on the platform.
- Restore test
- A controlled proof that required data or systems can actually be recovered.
Formula & worked example
ideal RAID layout capacity = usable drive count × equal drive size
Four equal 4 TB drives in RAID 5
- Inputs
- RAID 5 uses one drive-equivalent for parity
- Calculation
- (4 − 1) × 4 TB = 12 TB
Ideal RAID layout capacity is 12 TB from 16 TB raw, with one-drive failure tolerance before filesystem and platform overhead.
How this calculation works
The calculation follows the formulas, definitions and assumptions explained on this page. The references below support the method and any stated boundaries.
Official sources
- SNIA — Usable capacity — Storage-capacity terminology and technical semantics used by this calculation
- Red Hat — Configuring RAID logical volumes — Conventional RAID layout behaviour and implementation-specific caveats
- NIST — Binary prefixes — Decimal TB and binary TiB definitions
- Australian Cyber Security Centre — Regular backups — Independent backups and restore testing