Unraid ZFS Calculator
Estimate your usable storage capacity, parity efficiency, and data overhead for Unraid ZFS pools instantly.
Estimated Usable Capacity
0.00 TB
0.00 TB
0.00 TiB
Formula: Usable = ((Total Drives – Parity) × Drive Size × 0.9095) – ZFS Slop Overhead.
Capacity Allocation Breakdown
Visual representation of usable space vs parity and overhead.
| Metric | Value | Description |
|---|
What is an Unraid ZFS Calculator?
An unraid zfs calculator is a specialized tool designed to help NAS enthusiasts and server administrators estimate the actual usable storage capacity when implementing ZFS pools on the Unraid platform. Since Unraid 6.12, ZFS has become a native feature, allowing users to move beyond the traditional Unraid array and utilize the enterprise-grade data integrity features of ZFS.
Who should use an unraid zfs calculator? Anyone planning a high-performance media server, a secure data backup target, or a virtualization host. A common misconception is that ZFS capacity works exactly like the standard Unraid array; however, ZFS introduces specific concepts like VDEVs, RAIDZ levels, and binary vs. decimal conversion overhead that significantly impact your final available space.
Unraid ZFS Calculator Formula and Mathematical Explanation
The math behind our unraid zfs calculator involves several layers of subtraction and conversion. We start with the manufacturers’ decimal capacity and convert it to the binary format used by operating systems, while accounting for the mandatory parity drives required by RAIDZ configurations.
Step-by-Step Derivation:
1. Raw Capacity: Total Drives × Drive Size (TB).
2. Redundancy Subtraction: (Total Drives – Parity Drives) × Drive Size.
3. Binary Conversion: TB × 0.9095 = TiB (Tebibytes).
4. ZFS Slop Space: Subtract approximately 1/64th (1.56%) for ZFS “slop” space and metadata.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| n | Number of Disks | Count | 2 – 24 |
| s | Drive Size | TB | 1 – 24 TB |
| p | Parity Level | Drives | 1 (Z1), 2 (Z2), 3 (Z3) |
| o | Overhead | Percentage | 2% – 5% |
Practical Examples (Real-World Use Cases)
Example 1: High-Density Media Server
Using the unraid zfs calculator for a build with eight 18TB drives in RAIDZ2. The raw capacity is 144TB. However, after subtracting two drives for parity (36TB) and applying the TiB conversion and ZFS slop space, the actual usable capacity is approximately 96.5 TiB. This provides dual-drive failure protection, which is critical for large drives.
Example 2: Budget Home Office NAS
A user utilizes the unraid zfs calculator for three 4TB drives in RAIDZ1. The raw 12TB results in one drive loss for parity (4TB), leaving 8TB raw. After calculations, the user sees roughly 7.1 TiB usable. This helps the user decide if they should instead use a Mirror for better performance or add a fourth drive for RAIDZ2.
How to Use This Unraid ZFS Calculator
Following these steps ensures you get the most accurate results from the unraid zfs calculator:
| Step | Action | Why it matters |
|---|---|---|
| 1 | Enter Drive Capacity | Manufacturers label drives in decimal (1000^4), OS uses binary (1024^4). |
| 2 | Select Drive Count | Determines the total raw pool size before redundancy. |
| 3 | Choose VDEV Type | Determines how many drives are dedicated to parity and data safety. |
| 4 | Review Results | Use the “Usable TiB” value for your actual folder/share planning. |
Key Factors That Affect Unraid ZFS Calculator Results
While the unraid zfs calculator provides a highly accurate estimate, several external factors influence the final capacity in Unraid:
- Binary vs. Decimal: Hard drive companies sell 12TB drives, but computers see 10.9 TiB. Our unraid zfs calculator handles this automatically.
- ZFS Slop Space: ZFS reserves about 1.6% of pool space to ensure the file system doesn’t lock up when it becomes full.
- Metadata Overhead: Files and snapshots require metadata, which consumes a small portion of the pool.
- Recordsize: Using a 1M recordsize for media can be more efficient than the default 128K for large files.
- Swap and Quotas: If you set quotas or use ZVOLs, your perceived “free space” will change.
- VDEV Padding: In certain RAIDZ configurations, “ashift” settings and block sizes can lead to small amounts of wasted space.
Frequently Asked Questions (FAQ)
Why does the unraid zfs calculator show less space than I expected?
This is usually due to the TB to TiB conversion. A 10TB drive is only 9.09 TiB in the eyes of Unraid and ZFS.
Can I mix drive sizes in an Unraid ZFS pool?
Yes, but the unraid zfs calculator assumes all drives are the same size because ZFS will limit all drives in a VDEV to the size of the smallest drive.
Is RAIDZ1 safe for large drives?
Most experts recommend RAIDZ2 for drives larger than 8TB to prevent data loss during long rebuild times.
What is a VDEV?
A VDEV (Virtual Device) is the building block of a ZFS pool. Redundancy is handled at the VDEV level.
How does ZFS overhead differ from standard Unraid?
Standard Unraid uses a dedicated parity drive; ZFS distributes parity across all drives in a RAIDZ group, requiring more metadata.
Does this calculator include SSD cache?
This unraid zfs calculator focuses on the main storage pool. Cache pools are calculated separately.
What is ashift=12?
It refers to the sector size (4K). It is the standard setting for modern drives in Unraid ZFS pools.
Can I expand a ZFS pool later?
In Unraid, you can expand by adding more VDEVs or, in newer ZFS versions, expanding an existing RAIDZ VDEV.
Related Tools and Internal Resources
- Ultimate Unraid Setup Guide – Learn how to install Unraid and configure your first ZFS pool.
- ZFS vs Standard Unraid Array – A deep dive into which storage method is right for your data.
- Best Hard Drives for Unraid – Our top picks for reliable NAS drives in 2026.
- Unraid ZFS Performance Tuning – Optimize your ashift and recordsize for maximum speed.
- Configuring VDEVs in Unraid – A step-by-step guide to building complex pool structures.
- Unraid Cache Pool ZFS Guide – Speed up your writes using ZFS-based cache pools.