truenas zfs calculator

What this TrueNAS ZFS calculator helps you estimate

Planning a NAS build can be confusing because raw disk capacity is never the same as usable capacity. This TrueNAS ZFS calculator gives you a quick estimate of how much storage your pool can provide after redundancy overhead, vdev grouping, and optional reserved free space.

It is useful for home labs, backup servers, media archives, and small business storage where you need to answer: “If I buy these drives, how much space will I really get?”

How to use the calculator correctly

1) Enter your total drives and drive size

Use the manufacturer size shown on the disk (for example, 18 TB). The tool will display capacity in both TB and TiB.

2) Choose a layout and vdev width

• Stripe: maximum capacity and speed, zero protection.
• Mirror: strong redundancy, lower capacity efficiency.
• RAIDZ1: 1 disk parity per vdev.
• RAIDZ2: 2 disk parity per vdev (common recommendation for larger drives).
• RAIDZ3: 3 disk parity per vdev for very high resiliency needs.

3) Add spares and reserve percentage

Hot spares are available for replacement but are not used for active data capacity. Reserve percentage helps estimate a practical operating limit so your pool stays healthy and responsive over time.

Quick ZFS planning guidance

Understand vdev math first

In ZFS, pool redundancy is based on vdevs. If one vdev fails beyond its fault tolerance, the entire pool is lost. That is why vdev design matters more than just counting total disks.

Prefer balanced vdevs

Mixed widths and mixed performance drives can make growth harder. Many TrueNAS admins keep each vdev identical for predictable expansion and performance behavior.

Don’t run near 100% full

ZFS copy-on-write behavior can degrade write performance and increase fragmentation when pools are very full. Keeping 15% to 25% free is a practical rule for many workloads.

Example scenarios

8 x 12TB in RAIDZ2 (single vdev)

Two drives worth of parity are used, leaving roughly 6 drives of usable equivalent capacity. This is a popular compromise between protection and capacity.

12 x 8TB as 6 mirrored pairs

Mirroring usually gives excellent random I/O and rebuild behavior, but usable capacity is around 50% of raw. It is often chosen when performance consistency and resiliency are more important than raw efficiency.

Important limitations of any calculator

• Real usable space varies with block size, metadata, snapshots, and recordsize behavior.
• Different workloads (VMs, media, databases) can see different practical efficiency.
• This tool is for planning estimates, not an exact byte-for-byte guarantee.

FAQ

Is RAIDZ1 safe with large drives?

RAIDZ1 can be acceptable in smaller environments, but many users prefer RAIDZ2 for larger modern drives due to rebuild risk.

Should I use one large RAIDZ vdev or multiple vdevs?

It depends on your goals. Multiple vdevs can improve performance and growth flexibility, but design should match your fault tolerance and expansion strategy.

Why does TrueNAS show less than advertised disk size?

Drive vendors use decimal TB, while many storage tools also show binary TiB. The calculator provides both to reduce confusion.

If you are building a new system, use this calculator as a first-pass capacity planner, then validate your final layout in TrueNAS before purchasing hardware.