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Storage · Interactive calculator

NAS Capacity Calculator

Plan NAS bays, drive sizes, RAID mode, spares, and usable headroom before buying disks.

Interactive calculatorFormula shownVisible assumptions
Updated 2026-06-24
Calculation method NAS_CAPACITY-1.0
Page type Interactive calculator
Tool type Formula shown

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.

Primary result

Estimated usable 23.28 TB
Target usable 20 TB
Headroom 3.28 TB
Raw capacity 48 TB
Efficiency 48.5%
Recommendation Meets target

Use the headroom result to compare 4-bay, 6-bay, and 8-bay NAS options before buying drives.

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 guidance, emergency decisions, or replacing product labels and local requirements.

How to use the calculator

  1. Enter the labeled inputs for this nas capacity scenario.
  2. Adjust the optional assumptions shown in the form instead of relying on defaults blindly.
  3. Read the primary result, supporting result cards, and any warning notes together.
  4. 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.

Sources and assumptions

Use the inputs, units, and assumptions shown on the page, then compare important results with product documentation, vendor guidance, local code, or a qualified professional when the decision has safety or cost consequences.

Method version: NAS_CAPACITY-1.0. Send corrections through the contact page.

What to do next

Compare a conservative scenario with your real measurements before acting.

Plan NAS bays, headroom, and drive purchases

This NAS capacity calculator is for purchase planning: how many bays you need, how much usable storage remains after RAID, and how much free-space headroom is left for growth. It helps separate raw advertised disk capacity from practical usable capacity.

  • Inputs: bay count, drive size, RAID mode, hot spares, filesystem overhead, and free-space headroom target.
  • Outputs: raw capacity, usable capacity, recommended headroom, effective storage after reserve, and bay-use notes.
  • Related: compare the same disks in the Disk RAID calculator or check transfer time with the download time calculator.

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.
  • Health, legal, tax, emergency, structural, chemical, and other high-risk decisions need qualified guidance beyond a web calculator.

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.

The worked example uses the same default scenario so the formula can be followed without running JavaScript.

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?

Bay count, drive size, RAID/resiliency mode, hot spares, and desired headroom drive whether a NAS plan meets the target.

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.

Method details

Updated: June 24, 2026. Calculation method: NAS_CAPACITY-1.0. Availability: interactive calculator.