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All calculatorsMethodology

Mixed Drive RAID Calculator

See how mixed drive sizes can waste capacity when an array is limited by the smallest disk.

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

Calculate mixed-drive RAID waste

Estimate what happens when a RAID array contains different drive sizes. Generic RAID usually treats each disk as if it were the size of the smallest disk, leaving extra space unused.

Your inputs

Try an example

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

Default worked example

Usable capacity 24 TB
Installed raw 38 TB
Wasted by smallest-drive rule 6 TB
Smallest drive 8 TB
Efficiency vs installed 63.2%
Fault tolerance 1 drive
Parity / mirror reserve 8 TB
Unpaired active capacity 0 TB

The generic RAID 5 estimate applies the 8 TB smallest-drive capacity to each of 4 active positions before reserving parity or mirror copies.

How to use the calculator

List positive decimal-TB capacities for the installed active drives, separated by spaces, commas or newlines. Choose a RAID layout or JBOD aggregate. Up to 120 drives and 4,000 text characters are supported; every number must be complete.

What the result means

The breakdown accounts for usable space, capacity stranded by the smallest-drive rule, parity/mirror copies and any unpaired member. For 8, 8, 10 and 12 TB in RAID 5, 38 TB installed splits into 24 TB usable, 6 TB smallest-drive loss and 8 TB parity. JBOD totals 38 TB across the individual drives.

Formula and units

RAID uses the smallest listed capacity for each position, then reserves parity or mirror copies. Smallest-drive loss = installed total − drive count × smallest capacity. An unpaired RAID 10 position is excluded separately. JBOD aggregate sums all individual capacities with no smallest-drive loss or redundancy.

Worked examples

Individual drive sizes in TB: 8 8 10 12 positive decimal TB values; up to 120 active drives: RAID level: 5; Individual drive sizes in TB: 8 8 10 12 positive decimal TB values; up to 120 active drives.
Example result: Usable capacity: 24 TB; Installed raw: 38 TB; Wasted by smallest-drive rule: 6 TB; Smallest drive: 8 TB; Efficiency vs installed: 63.2%; Fault tolerance: 1 drive; Parity / mirror reserve: 8 TB; Unpaired active capacity: 0 TB.

Individual drive sizes in TB: 2 2 4 6 positive decimal TB values; up to 120 active drives: RAID level: 6; Individual drive sizes in TB: 2 2 4 6 positive decimal TB values; up to 120 active drives.
Example result: Usable capacity: 4 TB; Installed raw: 14 TB; Wasted by smallest-drive rule: 6 TB; Smallest drive: 2 TB; Efficiency vs installed: 28.6%; Fault tolerance: 2 drives; Parity / mirror reserve: 4 TB; Unpaired active capacity: 0 TB.

Limitations and common mistakes

All listed drives are active. Dedicated spares, filesystem allowances and vendor hybrid pools need a separate configuration. JBOD aggregate does not assume one logical volume; check how the system exposes or pools the disks. RAID capacity arithmetic does not establish rebuild reliability or replace a backup.

Do not subtract the largest disks as parity or assume a controller can use every larger-drive remainder. Keep smallest-drive loss separate from parity and unpaired capacity.

Sources and assumptions

The smallest-drive rule is a stated planning assumption; controller-specific hybrid behavior is outside the model.

Method MIXED_DRIVE_RAID-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.