THE PRACTICAL GUIDE · NORMANDIN SYSTEMS
Get more from RAID Calculator
RAID Calculator: Calculate storage capacity, fault tolerance and space efficiency of an array based on the number of disks, size, and RAID type. RAID capacity calculations illustrate how a layout trades raw disk space for redundancy or distribution across drives. Storage calculations depend on units, layout, and assumptions. Manufacturers commonly describe capacity in decimal units, while software may display binary units or reserve space for metadata and system use. Redundancy can reduce usable capacity, but it does not replace backups or protect against every cause of data loss. Treat the result as a planning estimate and compare it with the actual controller, filesystem, or storage platform’s documented behavior. Record disk sizes and layout choices so that another person can reproduce the estimate and understand what overhead is included.
Guide updated October 13, 2026
How to use this tool
- Begin with a representative input. For a practical test, use: Four equal 4 TB disks in RAID 5. Replace demonstration values only after you understand the output.
- Enter the storage sizes, quantities, or raw values requested by the tool and confirm whether the selected units are decimal or binary.
- Review the calculated result together with its assumptions, especially redundancy, reserved capacity, and the meaning of each displayed metric.
- Compare the estimate with the actual storage platform’s reporting or documentation before purchasing hardware or planning a migration.
SEE IT IN PRACTICE
A worked example
Four equal 4 TB disks in RAID 5
About 12 TB of nominal data capacity before additional overhead
Use the effective size of the smallest participating disk when the layout treats unequal drives uniformly.
Understand what the layout groups together
RAID and ZFS layouts distribute data and redundancy across particular groups of devices. Nominal parity counts do not fully describe every failure combination, especially when a pool contains multiple vdevs. Device size mismatches can also reduce effective capacity. Record the exact group structure rather than describing the system only by total disk count and a broad RAID label.
Reliability estimates depend on assumptions
An MTTDL model can illustrate how repair time and failure-rate assumptions affect a design, but real failures may be correlated and operational recovery may be slower than planned. Treat the number as a comparison under stated assumptions, not a promise of years without loss. Maintain backups, test restoration, and leave capacity headroom appropriate to the storage platform and workload.
When this tool is useful
- Compare array layouts
- Estimate nominal usable capacity
- Plan a storage upgrade
Tips for a better result
- RAID 5’s single-drive fault tolerance does not eliminate rebuild risk, backup needs, or controller-specific limitations.
- Keep GB and GiB distinct in planning notes; a change of units can explain an apparent capacity discrepancy.
- Redundancy improves availability for some failures, while separate backups address recovery needs that an array layout alone cannot cover.
Frequently asked questions
Why is the highest usable capacity not always the best storage layout?
Capacity is one tradeoff among fault tolerance, rebuild behavior, performance, expansion options, and recovery needs. A layout decision should consider the complete workload and failure model.
Does fault tolerance replace a backup?
No. An array can preserve availability through certain hardware failures while remaining vulnerable to deletion, corruption, theft, or other incidents. Backup and recovery planning address different failure scenarios.
How should I document a calculation?
Record the number and size of devices, selected units, layout, and any overhead assumptions. Treat reliability figures as model outputs rather than guarantees, and compare them with real operational constraints.
What happens to the information I enter?
The utility processes your input in this browser. Some tools retain field values in browser storage or the URL; avoid entering secrets in shareable fields. Page and asset requests still go to the hosting provider. Read the privacy and offline-use notes for details.