How to use the calculator
- Enter drive count, drive size, RAID level, hot spares, and overhead.
- Compare usable capacity, reserved space, efficiency, and fault tolerance.
- Use the RAID comparison table to see how the same drives behave under other levels.
- Remember that RAID is not backup; keep separate backups for important data.
What the results mean
Start with the displayed Disk RAID Calculator values, then read its assumptions and limits before acting. Supporting cards show the inputs, ranges, assumptions, or follow-up checks that explain how to interpret it.
Decision notes for this calculator
How to interpret this calculator
The labeled inputs and visible assumptions drive the result.
What to do next
Check the result against your NAS or controller documentation, confirm every drive and bay is supported, and plan a separate backup before building the array.
Save this scenario
Use Share, Copy result, or Print / Save as PDF to keep the exact inputs with the result. If something looks wrong, report an issue with the Disk RAID Calculator.
Compare RAID levels before choosing hardware
Use the disk RAID calculator when you need one page to compare usable capacity across RAID 0, RAID 1, RAID 5, RAID 6, RAID 10, RAID 50, RAID 60, and JBOD. The same drive count and drive size can produce very different usable capacity once parity, mirrors, hot spares, and overhead are included.
- Inputs: drive count, drive size, RAID level, hot spares, overhead, and TB/TiB display preference.
- Outputs: raw capacity, usable capacity, efficiency, parity or mirror reserve, and warning notes.
- Deep dives: use RAID 5, RAID 6, RAID 10, and RAID 5 vs RAID 6 pages for layout-specific tradeoffs.
Formula / methodology
Active drives = total drives − hot spares. RAID 0/JBOD usable capacity = active drives × drive size. RAID 1 = one drive size for a full n-way mirror. RAID 5 = (active drives − 1) × drive size. RAID 6 = (active drives − 2) × drive size. RAID 10 = floor(active drives ÷ 2) × drive size. RAID 50 = (active drives − 2) × drive size under this calculator’s fixed assumption of two equal RAID 5 groups. RAID 60 = (active drives − 4) × drive size under the fixed assumption of two equal RAID 6 groups. RAID 50/60 require an even active drive count; controller layouts can differ.
Choosing between RAID 5, RAID 6, and RAID 10
If your main question is which parity level fits the risk, compare the RAID 5 vs RAID 6 guide, then run the RAID 5, RAID 6, and RAID 10 calculators with the same drive count.
Assumptions and limitations
- Capacity math assumes the active drives contribute the selected drive size; many controllers limit mixed-size arrays to the smallest active drive.
- Displayed overhead covers the percentage entered in the form. Filesystem metadata, controller reservations, spare sectors, snapshots, and thin-provisioning behavior can reduce usable space further.
- Fault-tolerance figures count disk failures the RAID layout can survive under ideal conditions. They do not model rebuild time, unrecoverable read errors, correlated failures, controller failure, or operator mistakes.
- RAID 50 and RAID 60 layout details vary by controller. Verify stripe-group requirements and supported drive counts in the hardware documentation.
- Redundancy is not backup. Keep important data in a separate, tested backup system.
Example calculation
Default form scenario: Input: 6 drives × 12 TB, RAID 6, 0 hot spares, 0% filesystem overhead, decimal TB display. Output: raw capacity 72 TB, usable capacity 48 TB, reserved for parity 24 TB, storage efficiency 66.7%, and fault tolerance of 2 drives.
Common mistakes
- Comparing TB and TiB without realizing they are different displayed units.
- Forgetting that hot spares reduce active capacity.
- Assuming RAID is a backup; it is not a replacement for separate backups.
- Mixing drive sizes and expecting every terabyte to be usable.
FAQ
What formula does this page use?
Active drives = total drives − hot spares. RAID 0/JBOD usable capacity = active drives × drive size. RAID 1 = one drive size for a full n-way mirror. RAID 5 = (active drives − 1) × drive size. RAID 6 = (active drives − 2) × drive size. RAID 10 = floor(active drives ÷ 2) × drive size. RAID 50 = (active drives − 2) × drive size under this calculator’s fixed assumption of two equal RAID 5 groups. RAID 60 = (active drives − 4) × drive size under the fixed assumption of two equal RAID 6 groups. RAID 50/60 require an even active drive count; controller layouts can differ.
What changes the result the most?
Drive count, drive size, RAID level, hot spares, and overhead move usable capacity most; changing parity level often matters more than small overhead tweaks.
How should I use this result?
Use it as a planning estimate and compare important decisions with product documentation, labels, quotes, local code, or qualified guidance.
Why does this page show warnings?
Warnings call out assumptions that can materially change the result or create safety, cost, or reliability problems.
Can I share this scenario?
Yes. The share button copies a URL with the current inputs, while the main page remains the base calculator URL.
Sources and assumptions
References used for this calculator:
- Formula basis: equal-drive RAID capacity rules by RAID level, hot spare adjustment, optional filesystem overhead, and TB/TiB display conversion.
- Does not cover vendor-specific reserved space, controller behavior, rebuild risk, snapshots, compression, dedupe, or backup policy.
- Method note date: June 24, 2026.
Method details
Updated: June 24, 2026. Calculation method: RAID-2.2. Availability: interactive calculator.
