Raid Array Calculator
Raid array calculator works out Usable capacity and Raw capacity from RAID level, Number of disks and Size of each disk. The default figures on this page give usable capacity of 12 TB; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
3 steps
- RAID 5 (single parity) with 4 disks of 4 TB each.
- Redundancy consumes the equivalent of 1 disk(s); 3 disks' worth carries data.
- Usable = 3 x 4 TB = 12 TB
Its 6 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the raid array calculator calculates
Computes usable capacity, redundancy overhead, fault tolerance and rebuild time for a RAID array of a given level and disk count.
How to use this raid array calculator
Enter RAID level, Number of disks, Size of each disk and Disk size unit. Usable capacity updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Usable capacity, Raw capacity and Lost to redundancy.
The method follows SNIA Common RAID Disk Data Format Specification, Patterson, Gibson & Katz, A Case for Redundant Arrays of Inexpensive Disks (RAID), SIGMOD 1988 and Oracle ZFS Administration Guide, RAID-Z configuration.
RAID 5 with four 4 TB disks example
With RAID level: 5, Number of disks: 4, Size of each disk: 4, Disk size unit: TB, the calculator returns Usable capacity: 12, Raw capacity: 16, Disks that may fail: 1. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Usable capacity, Raw capacity and Lost to redundancy. The inputs you can vary are RAID level, Number of disks, Size of each disk, Disk size unit, Number of RAID groups and Hot spare disks.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
Are RAID arrays still used?
Computes usable capacity, redundancy overhead, fault tolerance and rebuild time for a RAID array of a given level and disk count. On the figures this page opens with — size of each disk 4 — the usable capacity is 12 TB. Put your own figures in their place and the working updates beside the result.
Are RAID calculators free to use?
The calculation is free and does not require an account. An account is only needed for optional features such as saving, comparing, exporting, or sharing supported scenarios.
Is it possible to run 4 8TB drives in RAID 5?
It needs RAID level, Number of disks, Size of each disk, Disk size unit, Number of RAID groups and Hot spare disks. Nothing else is required, and the figures stay in your browser.
How many disks are needed for RAID 50?
On the figures this page opens with — size of each disk 4 — the usable capacity is 12 TB. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
What information do I need for the raid array calculator?
You need RAID level, Number of disks, Size of each disk, Disk size unit, Number of RAID groups, Hot spare disks, Filesystem overhead and Rebuild throughput. The page opens with default figures that give usable capacity of 12 TB, so you can see the scale before entering your own.
What does this raid array calculator show?
It shows Usable capacity, Raw capacity, Lost to redundancy, Storage efficiency, Disks that may fail, Minimum disks for this level, Usable capacity in TiB and Read throughput multiplier, with the working and any warnings for your entries.