Quick answer
RAID 6 reserves two active drives for parity. Eight 8 TB drives provide about 48 TB usable before overhead, compared with 56 TB for RAID 5 but with stronger failure tolerance.
Best for: comparing realistic scenarios before acting. Not for: final professional guidance, emergency decisions, or replacing product labels and local requirements.
How to use the calculator
- Enter the labeled inputs for this raid 6 scenario.
- Adjust the optional assumptions shown in the form instead of relying on defaults blindly.
- Read the primary result, supporting result cards, and any warning notes together.
- Use share, copy, or print/PDF to save the exact scenario for later comparison.
What the results mean
The primary result is the main number to use first. 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
Compare a conservative scenario with your real measurements before acting.
Save this scenario
Use Share, Copy result, or Print / PDF to keep the exact inputs with the result. If something looks wrong, report an issue with the RAID 6 Calculator.
Use RAID 6 when dual parity matters
RAID 6 reserves two active drives for parity, so it trades usable space for tolerance of two drive failures. This page makes that tradeoff visible before you buy disks or choose a NAS layout.
Formula / methodology
RAID 6 usable capacity = (active drives − 2) × drive size × (1 − filesystem overhead). Minimum active drives: 4.
When RAID 6 may be worth the extra parity
For arrays with large drives, long rebuild windows, or important uptime requirements, compare this result with the RAID 5 vs RAID 6 guide and the RAID 10 calculator.
Assumptions and limitations
- Inputs are assumed to be measured accurately and entered in the units shown.
- Rounding is intentional so the result is easier to use in real decisions.
- Vendor-specific behavior, local code, product labels, and regional pricing can override a generic calculator.
- Health, legal, tax, emergency, structural, chemical, and other high-risk decisions need qualified guidance beyond a web calculator.
Example calculation
Input: 8 drives × 8 TB, RAID 6, no hot spare, 0% overhead. Output: raw capacity 64 TB, usable capacity 48 TB, dual-parity overhead 16 TB, efficiency 75%, fault tolerance 2 drives.
Common mistakes
- Comparing only usable TB and ignoring rebuild risk.
- Using fewer than four active drives.
- Ignoring hot spares and filesystem reserves.
- Assuming dual parity replaces off-array backups.
FAQ
What formula does this page use?
RAID 6 usable capacity = (active drives − 2) × drive size × (1 − filesystem overhead). Minimum active drives: 4.
What changes the result the most?
Drive count and drive size dominate the result because RAID 6 reserves two active-drive equivalents for dual parity.
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 6 capacity equals active drives minus two parity drives, with optional overhead adjustment.
- Does not model vendor reserves, rebuild windows, snapshots, compression, dedupe, or backup policy.
- Method note date: June 24, 2026.
- Editorial scope: CalculatorShelf editorial math check.
Method details
Updated: June 24, 2026. Calculation method: RAID6-1.0. Availability: interactive calculator.