TrueNAS Hardware Calculator

How the RAM number is built

Every line in the breakdown comes from a published TrueNAS sizing rule rather than a rule of thumb. The base is 8GB, which the hardware guide gives as the minimum and treats as sufficient for pools of up to eight drives. Past eight drives it adds 1GB per additional drive. Directory services add 2GB for the Winbind cache. iSCSI is treated as a floor of 16GB rather than an addition, because that is how the guide states it, and the guide points at 32GB or more for optimal iSCSI performance.

Deduplication is the line that dominates every build that turns it on. The hardware guide suggests 5GB of RAM per TB of storage. The deduplication reference notes that pools reaching a ratio of 3x or better might need only 1-3GB per TB, so setting an expected ratio of 3 or higher moves the calculator to that lower band. L2ARC is not free either: roughly 1GB of RAM goes to headers for every 50GB of cache device, which is why adding a large L2ARC to a memory-starved box makes it slower rather than faster.

The old advice of 1GB of RAM per TB of storage is not in the current guide. It comes from the FreeNAS era and it is why so many builds are specified wrong in both directions. Use the published rules instead.

Why the capacity number looks smaller than the drives you bought

Two separate things shrink it. Parity or mirroring takes whole drives out of the usable total, which is the difference between a RAIDZ2 and a mirror at the same drive count. Then the units change: drives are sold in decimal TB and TrueNAS reports binary TiB, so a 32TB pool shows up as roughly 29TiB before you have written anything to it. The calculator shows both so the number in the web interface is not a surprise.

The third figure is the 80% fill guidance. ZFS performance degrades as a pool fills, because free space gets fragmented and allocation gets harder, and TrueNAS flags pool usage above 80% in its own interface. Plan capacity against that line, not against the raw total.

ECC, boot devices, and the parts people skip

ECC memory is not required. The hardware guide is explicit that many TrueNAS systems run every day without it, while also noting that most users strongly recommend it as another data-integrity defense. For a homelab that holds backups of things that exist elsewhere, non-ECC is a defensible choice. For the only copy of anything, buy the ECC.

The boot device minimum is 20GB, and a larger one buys room for more boot environments, which is what lets you roll back a bad update. Deduplication also wants CPU: the deduplication reference recommends a high-end processor with at least 4-6 cores.

Picking the hardware

Once you have a RAM target, the constraint on most small machines is not price but the memory ceiling, and whether that memory is socketed at all. See ourbest mini PC for TrueNAS picks, theTrueNAS hardware requirements guide, and the wider NAS mini PC roundup. To see what a build costs to leave running, use thepower cost calculator.

Sources

• TrueNAS Hardware Guide — minimum RAM, per-drive allowance, directory services, iSCSI, deduplication, L2ARC, boot device
• TrueNAS ZFS Deduplication reference — the 1-3GB per TB band at high dedup ratios, and the CPU recommendation
• The ZFS 80% Rule — why pool performance degrades as capacity fills

This is a planning calculator built from published guidance. It is not a measurement of your workload, and a pool with unusual record sizes or snapshot churn can want more than any table predicts.