TrueNAS Calculator
Expert ZFS Pool Capacity & RAID Planning Tool
50.40 TiB
Calculated using the ZFS truenas calculator logic including binary conversion.
Storage Distribution
Visual representation of Usable vs Parity vs Reserved space for this truenas calculator configuration.
| Metric | Value | Description |
|---|
What is a TrueNAS Calculator?
A truenas calculator is an essential tool for system administrators and home lab enthusiasts designed to predict the actual usable storage space within a ZFS pool. When building a Network Attached Storage (NAS) system, the advertised capacity on your hard drive labels is never what you actually see in the operating system. This discrepancy occurs due to binary-to-decimal conversion, ZFS metadata overhead, and RAID parity requirements.
Who should use it? Anyone planning a build on TrueNAS Core or TrueNAS SCALE. A common misconception is that a 10TB drive provides 10TB of usable space in ZFS. In reality, ZFS calculates space in Tebibytes (TiB), which immediately reduces the “number” by approximately 9%. The truenas calculator accounts for these nuances to ensure you don’t run out of space prematurely.
TrueNAS Calculator Formula and Mathematical Explanation
The math behind the truenas calculator involves several steps to move from physical hardware to functional file storage. The core derivation follows this sequence:
- Decimal to Binary Conversion: Physical Drive Size × (10^12 / 2^40)
- Redundancy Deduction: (Total Drives – Parity Drives) × Binary Size
- ZFS Slop Space: Subtract approximately 1/32nd (3.125%) for pool reservation.
- Swap Reservation: TrueNAS typically reserves 2GB per drive for swap partitions.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| N | Number of Disks | Integer | 1 – 255 |
| S | Manufacturer Size | TB | 1 – 24 TB |
| P | Parity Level | Disks | 0 (Stripe) to 3 (Z3) |
| O | ZFS Overhead | Percentage | 3.125% |
Practical Examples (Real-World Use Cases)
Example 1: The Home Media Server
A user buys 6 drives, each 12TB, and wants to use RAID-Z2 for safety. Using the truenas calculator, we find:
- Raw: 72 TB
- Usable: ~35.4 TiB
- Interpretation: Even though 72TB was purchased, only 35.4 TiB is available for high-performance storage.
Example 2: Enterprise Virtualization
A business deploys 12 drives of 4TB in a RAID-Z3 configuration. The truenas calculator shows:
- Raw: 48 TB
- Usable: ~28.2 TiB
- Interpretation: The high parity (3 disks) ensures extreme data durability at the cost of nearly 40% of raw capacity.
How to Use This TrueNAS Calculator
Following these steps will yield the most accurate results for your storage planning:
- Enter the total number of drives you intend to place in a single VDEV.
- Input the capacity listed on the drive’s box in Terabytes (TB).
- Select your RAID level. For most users, RAID-Z2 offers the best balance of safety and capacity.
- Review the “Safe Usage Limit.” In ZFS, performance degrades significantly once a pool exceeds 80% capacity.
- Use the Copy Results button to save these metrics for your hardware procurement list.
Key Factors That Affect TrueNAS Calculator Results
Several variables impact the final output of the truenas calculator and your real-world experience:
- Binary Conversion (TiB vs TB): Windows and ZFS use binary (1024), while marketing uses decimal (1000). This is the biggest “loss” of perceived space.
- RAID-Z Overhead: Higher parity levels (Z2, Z3) provide better protection against drive failure but reduce usable capacity.
- ZFS Slop Space: ZFS reserves a small portion of the pool to ensure administrative tasks (like deleting files) can still occur when the disk is “full.”
- Padding Overhead: Depending on your record size (ashift), small files can consume more physical space than their actual size.
- VDEV Count: If you have multiple VDEVs, you must run the truenas calculator for each and sum them up.
- Compression: Using LZ4 or ZSTD compression in TrueNAS can actually increase your usable space beyond the calculated physical limits.
Frequently Asked Questions (FAQ)
Why does the truenas calculator show less space than my hard drive label?
This is primarily due to the 1000 vs 1024 math conversion and the mandatory ZFS parity and metadata overhead.
What is the “80% rule” in TrueNAS?
ZFS is a Copy-on-Write file system. When it gets too full, it struggles to find contiguous blocks, leading to massive performance drops.
Can I mix drive sizes in the truenas calculator?
ZFS will treat all drives in a VDEV as having the capacity of the smallest drive. It is best to use identical sizes.
Does RAID-Z1 still make sense?
For drives larger than 8TB, RAID-Z1 is risky because the “rebuild” time is long, increasing the chance of a second drive failing.
Is mirror faster than RAID-Z2?
Generally, yes. Mirrors offer better IOPS (Input/Output Operations Per Second) but provide less total storage efficiency.
What is ashift and does it affect the truenas calculator?
Ashift refers to the sector size. Modern drives (4K sectors) use ashift=12. This tool assumes standard ZFS padding.
Can I expand a RAID-Z VDEV later?
Historically no, but recent OpenZFS updates are adding “RAID-Z Expansion.” However, it’s safer to plan with the truenas calculator from the start.
Does the calculator include L2ARC or SLOG?
No, those are cache devices and do not add to the persistent storage capacity of your main pool.
Related Tools and Internal Resources
- ZFS RAID Levels Explained: A deep dive into choosing between Z1, Z2, and Z3.
- Hard Drive Burn-in Testing: Why you should test your disks before trusting the truenas calculator results.
- TrueNAS Core vs SCALE: Choosing the right operating system for your hardware.
- FreeNAS History: Understanding the evolution of the software into modern TrueNAS.
- NAS Hardware Guide: Best motherboards and HBAs for a ZFS build.
- ZFS Performance Tuning: Advanced settings to optimize your truenas calculator planned pool.