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RAID 5 Calculator

Last reviewed: June 24, 2026 · Formula version: RAID5-1.0 · Formula-backed planning calculator

Estimate RAID 5 usable storage and parity

Estimate RAID 5 capacity from drive count, drive size, hot spares, and filesystem overhead. RAID 5 is space-efficient, but only tolerates one failed drive and can be risky with very large drives during rebuilds.

Server-rendered example result

Example: Input: 4 drives × 10 TB, RAID 5, 0 hot spares, 0% overhead. Output: raw capacity 40 TB, usable capacity 30 TB, parity overhead 10 TB, efficiency 75%, fault tolerance 1 drive.

The interactive result cards update this example when JavaScript runs, but this default snapshot is crawlable and printable.

Quick answer

RAID 5 reserves one active drive for parity. Four 10 TB drives provide about 30 TB usable before overhead; adding a hot spare drops active drives to three and usable capacity to about 20 TB.

Best for: comparing realistic scenarios before acting. Not for: final professional approval, emergency decisions, or replacing product labels and local requirements.

How to use the calculator

  1. Enter the measurements, quantities, costs, or target values requested above.
  2. Adjust optional assumptions such as waste, overhead, product strength, retention days, or multipliers.
  3. Read both the main result cards and any warning notes. The warnings are part of the answer, not fine print.
  4. Use the share, copy, or print/PDF controls when you want to save the scenario.

What the results mean

The first result card is the primary decision number. Supporting cards explain capacity, cost, efficiency, safety margin, input assumptions, or the next value to check. When the page returns a range, treat it as a planning envelope rather than a guaranteed price.

Formula / methodology

RAID 5 usable capacity = (active drives − 1) × drive size × (1 − filesystem overhead). Active drives = total drives − hot spares.

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.
  • High-risk medical, legal, tax, and emergency calculators are intentionally not published without expert review.

Example calculation

Input: 4 drives × 10 TB, RAID 5, 0 hot spares, 0% overhead. Output: raw capacity 40 TB, usable capacity 30 TB, parity overhead 10 TB, efficiency 75%, fault tolerance 1 drive.

Server-rendered example result

Example: Input: 4 drives × 10 TB, RAID 5, 0 hot spares, 0% overhead. Output: raw capacity 40 TB, usable capacity 30 TB, parity overhead 10 TB, efficiency 75%, fault tolerance 1 drive.

The interactive result cards update this example when JavaScript runs, but this default snapshot is crawlable and printable.

Common mistakes

  • Using RAID 5 for very large arrays without considering rebuild exposure.
  • Counting a hot spare as usable capacity.
  • Forgetting filesystem or snapshot overhead.
  • Treating parity as a backup.

FAQ

What formula does this page use?

RAID 5 usable capacity = (active drives − 1) × drive size × (1 − filesystem overhead). Active drives = total drives − hot spares.

What changes the result the most?

The most important inputs are the size, count, rate, target, or unit assumptions shown in the calculator.

Is this a final professional answer?

No. Treat it as a planning result and verify important decisions against product documentation, labels, quotes, local code, or professional 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 canonical page remains the base calculator URL.

Last reviewed

Last reviewed: June 24, 2026. Formula version: RAID5-1.0. Index status: indexable.