ZFS RAID Capacity Calculator
Estimate raw capacity, usable capacity, and safe fill level for common ZFS layouts (Stripe, Mirror, RAIDZ1/2/3).
Note: Results are estimates. Real usable space is affected by metadata, ashift, compression, special vdevs, recordsize, snapshots, and pool fragmentation.
Why a RAID ZFS Calculator Matters
Planning a ZFS pool is one of those decisions that can save you years of headaches—or create years of regret. A simple capacity estimate from “disk count × disk size” is not enough because ZFS redundancy models change usable space dramatically. This RAID ZFS calculator gives you a practical forecast before you buy hardware or rebuild an array.
It helps answer the most common design questions:
- How much capacity do I really get from RAIDZ1, RAIDZ2, or RAIDZ3?
- How much space should I leave free to keep performance healthy?
- How many drive failures can a vdev tolerate?
How the Calculator Works
1) Raw Capacity
Raw capacity is the total disk size before parity, mirroring, and filesystem overhead:
- Raw TB = drives per vdev × vdev count × disk size (TB)
2) Usable Capacity by Layout
ZFS capacity is calculated per vdev, then multiplied by the number of vdevs:
- Stripe: all drives are usable (no protection)
- Mirror: one drive worth of capacity per mirror vdev
- RAIDZ1: lose one drive worth per vdev to parity
- RAIDZ2: lose two drives worth per vdev to parity
- RAIDZ3: lose three drives worth per vdev to parity
3) Practical Headroom
The calculator applies a small overhead estimate and then a recommended maximum fill target. Many admins keep pools at roughly 70–85% full to reduce fragmentation and performance drop-offs.
RAIDZ Levels in Plain English
RAIDZ1
Good capacity efficiency, but only one failure tolerated per vdev. Usually better for smaller drives or less critical workloads.
RAIDZ2
The most common balance of safety and capacity for home labs and prosumer NAS setups. Two drive failures per vdev can be survived.
RAIDZ3
Best for very large arrays or large-capacity disks where resilver windows are long and risk is higher. You pay with more parity overhead.
Best Practices Before You Build
- Prefer RAIDZ2 or stronger for important data.
- Keep consistent drive sizes in each vdev for predictable results.
- Do not run pools near 100% utilization.
- Remember: RAID is not backup. Always maintain independent backups.
- Plan vdev layout for future growth; expansion choices depend on topology.
Example Scenario
Suppose you build a pool with 2 vdevs, each 6 drives of 12 TB, using RAIDZ2:
- Raw = 2 × 6 × 12 = 144 TB
- Usable = 2 × (6 - 2) × 12 = 96 TB
- Practical safe fill (80% target) lands significantly lower, which is what you should plan around for performance.
Final Thought
Capacity planning is easy to underestimate. A few minutes with a RAID ZFS calculator can prevent expensive redesigns, painful migrations, and avoidable risk. Use this tool as a quick planning baseline, then validate against your exact ZFS version, workload, and recovery strategy.