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All calculatorsMethodology

NAS Capacity Calculator

Compare usable NAS capacity with your target using equal drives, RAID, spares and an entered storage allowance. Invalid active-drive layouts are rejected.

Interactive calculator · Method NAS-2.1 · Last reviewed: 2026-11-04

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.

Your inputs

Try an example

Default worked example. Edit the inputs to calculate your scenario.

Default worked example

Estimated usable capacity 23.28 TB
Entered target 20 TB
Usable headroom 3.28 TB
Raw installed capacity 48 TB
Entered filesystem allowance 3%
Target comparison Meets entered target
Layout failure boundary 2 drives

Apply the selected RAID reserve and 3% entered allowance, then subtract 20 TB target from 23.28 TB usable capacity. Every listed drive is installed; empty enclosure bays add no capacity.

How to use the calculator

Enter installed drives, including any dedicated hot spares, equal decimal-TB capacity, a RAID layout and an explicit filesystem/storage allowance. Enter the usable-capacity target separately. Empty enclosure bays add no storage.

What the result means

The entered population and four-, six- and eight-drive populations use the same drive size, layout, spares, allowance and target. Unsupported populations show N/A. Zero headroom meets the target exactly. The table compares installed-drive counts, rather than guaranteeing enclosure compatibility.

Formula and units

Active drives = installed drives − spares. Apply the selected parity or mirror reserve, then multiply by (1 − entered allowance/100). Headroom = usable − target. RAID 50 and 60 use two equal groups; an odd active RAID 10 position is unused.

Worked examples

Installed drives in NAS: 4 installed drives; Drive size (TB): 12 decimal TB: Installed drives in NAS: 4 installed drives; Drive size (TB): 12 decimal TB; Desired usable TB: 20 decimal TB; Hot spare count: 0 drives; Filesystem/storage allowance: 3 %; RAID preference: 6.
Example result: Estimated usable capacity: 23.28 TB; Entered target: 20 TB; Usable headroom: 3.28 TB; Raw installed capacity: 48 TB; Entered filesystem allowance: 3%; Target comparison: Meets entered target; Layout failure boundary: 2 drives.

Installed drives in NAS: 6 installed drives; Drive size (TB): 18 decimal TB: Installed drives in NAS: 6 installed drives; Drive size (TB): 18 decimal TB; Desired usable TB: 30 decimal TB; Hot spare count: 0 drives; Filesystem/storage allowance: 3 %; RAID preference: 10.
Example result: Estimated usable capacity: 52.38 TB; Entered target: 30 TB; Usable headroom: 22.38 TB; Raw installed capacity: 108 TB; Entered filesystem allowance: 3%; Target comparison: Meets entered target; Layout failure boundary: One drive in each mirror pair.

Limitations and common mistakes

Vendor pools, filesystem use, snapshots, expansion and supported RAID levels vary. Enter an allowance with a known basis; the illustrative default is not a universal reserve. No rebuild-time, reliability or recommended growth margin is inferred. Keep a separate backup.

Count populated drives rather than empty bays, and compare a usable target with usable capacity. Confirm the layout and extra reserves in the chosen NAS documentation.

Sources and assumptions

Capacity arithmetic is mathematical; vendor implementation, snapshots, and filesystem reserves need the NAS documentation.

Method NAS-2.1; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.

NIST byte and binary-prefix definitions support the unit conversions. Western Digital generic RAID capacity definitions describe parity, mirrors, smallest-drive use and JBOD aggregate capacity.