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Disk RAID Calculator

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

Calculate usable RAID capacity

Calculate usable RAID capacity from drive count, drive size, RAID level, hot spares, filesystem overhead, and decimal-vs-binary units. Use the comparison table to see how RAID 0, 1, 5, 6, 10, 50, 60, and JBOD change usable space and fault tolerance.

Server-rendered example result

Example: Input: 6 drives × 12 TB, RAID 6, 0 hot spares, 0% filesystem overhead. Output: raw capacity 72 TB, usable capacity 48 TB, reserved for parity 24 TB, storage efficiency 66.7%, and fault tolerance of 2 drives.

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

Quick answer

For equal-size drives, RAID 5 uses one active drive for parity, RAID 6 uses two, RAID 10 mirrors half the drives, and RAID 0/JBOD use capacity without redundancy. Hot spares reduce active capacity before the RAID formula is applied.

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) × smallest drive size. RAID 6 = (active drives − 2) × smallest drive size. RAID 10 = floor(active drives ÷ 2) × smallest drive size. RAID 0/JBOD = active drives × drive size. 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: 6 drives × 12 TB, RAID 6, 0 hot spares, 0% filesystem overhead. Output: raw capacity 72 TB, usable capacity 48 TB, reserved for parity 24 TB, storage efficiency 66.7%, and fault tolerance of 2 drives.

Server-rendered example result

Example: Input: 6 drives × 12 TB, RAID 6, 0 hot spares, 0% filesystem overhead. Output: raw capacity 72 TB, usable capacity 48 TB, reserved for parity 24 TB, storage efficiency 66.7%, and fault tolerance of 2 drives.

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

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?

RAID 5 usable capacity = (active drives − 1) × smallest drive size. RAID 6 = (active drives − 2) × smallest drive size. RAID 10 = floor(active drives ÷ 2) × smallest drive size. RAID 0/JBOD = active drives × drive size. 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: RAID-1.0. Index status: indexable.