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NAS Capacity Calculator

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

Plan NAS bays, RAID, and usable storage

Estimate whether a NAS configuration meets your target usable capacity after RAID choice, hot spares, and a planning overhead reserve. Use it before choosing a 4-bay, 6-bay, or 8-bay system.

Server-rendered example result

Example: Input: 4 bays, 12 TB drives, RAID 6, 0 hot spares, 20 TB desired. Output: about 23.3 TB usable after reserve and 3.3 TB headroom.

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

Quick answer

A 4-bay NAS with 12 TB drives in RAID 6 has 48 TB raw, about 24 TB before overhead, and roughly 23.3 TB after a 3% filesystem reserve. That is enough for a 20 TB target but leaves limited growth headroom.

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

NAS usable capacity uses the selected RAID formula, then subtracts the planning overhead reserve. Headroom = usable capacity − desired usable storage.

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 bays, 12 TB drives, RAID 6, 0 hot spares, 20 TB desired. Output: about 23.3 TB usable after reserve and 3.3 TB headroom.

Server-rendered example result

Example: Input: 4 bays, 12 TB drives, RAID 6, 0 hot spares, 20 TB desired. Output: about 23.3 TB usable after reserve and 3.3 TB headroom.

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

Common mistakes

  • Buying just enough capacity with no growth headroom.
  • Forgetting snapshots, recycle bins, and filesystem reserves.
  • Choosing RAID solely by usable TB.
  • Ignoring backup storage outside the NAS.

FAQ

What formula does this page use?

NAS usable capacity uses the selected RAID formula, then subtracts the planning overhead reserve. Headroom = usable capacity − desired usable storage.

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: NAS_CAPACITY-1.0. Index status: indexable.