HexOS Calculator ZFS Capacity Planner

HexOS / TrueNAS Pool Planner

Estimate usable ZFS capacity before you build or expand.

Plan RAIDZ, mirror, stripe, and advanced dRAID layouts with a cleaner HexOS-style interface, realistic headroom targets, and mixed-drive behavior that reflects how ZFS actually groups disks inside each VDEV.

Start Planning Advanced dRAID
  • RAIDZ1 / 2 / 3
  • Mirror
  • Stripe
  • dRAID

The estimate is geometry-based, not byte-perfect. Real ZFS pool capacity will land slightly lower once filesystem overhead and metadata are applied.

Top Recommended NAS Drive Recommended now

HexOS Calculator recommends

WD Red Pro 14TB

Excellent middle ground for 4-to-8-bay HexOS builds.

WD Red Pro 14 TB
Current street price View current Amazon pricing
View on Amazon
Updated from Amazon data when available. Pricing and rankings can change throughout the day.

Pool Builder

Choose a layout, set how many VDEVs are in the pool, and size the disks inside each VDEV.

Advanced pool options Mixed VDEV widths, exact ZFS accounting, and extra planning controls when you want more than the default geometry estimate.
Exact ZFS accounting Optional. Model RAIDZ padding, sector size, swap reservation, and OpenZFS slop more conservatively.
Affects RAIDZ padding efficiency. 128 KiB is the common default assumption.
Most modern NAS HDDs are effectively planned around ashift 12.
Reserved on every drive before pool math. Leave at 0 GiB unless you explicitly plan for swap.
Used for the Recommended Max Fill card after the allocatable estimate is calculated.
Turn this on if you want a closer match to exact ZFS calculators. The default planner still uses simpler geometry math.

Need dRAID for a large or specialized array? Open the advanced estimator below.

Recommended NAS Drives

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HexOS Storage Terms

The old parity/data-disk language does not map well to ZFS, so the calculator now uses HexOS and TrueNAS terms instead.

Pool vs. VDEV

A pool is the top-level storage space. A VDEV is the building block inside that pool. Total usable pool space is the sum of the usable capacity from every VDEV.

RAIDZ Math

RAIDZ1, RAIDZ2, and RAIDZ3 reserve one, two, or three drives worth of capacity per VDEV for redundancy. Mixed-size VDEVs behave like every disk is the size of the smallest disk in that group.

Plan for Headroom

ZFS performs best with free space available. The calculator shows a recommended 80% max fill so you can plan snapshots, resilvers, and normal growth without painting yourself into a corner.

Advanced calculator: dRAID estimator Not the default. Find it here when you need TrueNAS/OpenZFS dRAID math.

Based on the TrueNAS dRAID primer formula: Capacity = (C - S) × (D / (D + P)) × DS. TrueNAS documents dRAID for very specific use cases, especially very large arrays, and warns that it can be less space-efficient than RAIDZ for general workloads.

Estimated dRAID Pool Capacity 0 TiB
Recommended Max Fill 0 TiB
Raw Disk Capacity 0 TiB
Parity + Spare Reservation 0 TiB
Distributed Spares Only 0 TiB
Total Drives 0
  • dRAID Fault Model Waiting for dRAID inputs
  • Layout Formula Waiting for dRAID inputs
  • Expansion Note Waiting for dRAID inputs
Open the panel and enter your layout to estimate dRAID capacity using the documented TrueNAS formula.