Planning a ZFS storage pool requires more than simply adding up the capacities of all the hard drives or SSDs you intend to use. When drives are configured with RAID Z, part of the total capacity is dedicated to parity so that the storage pool can continue operating when one or more drives fail. The amount of storage available for data depends on the RAID Z level and the number and capacity of the drives.
RAID Z Calculator
The RAID Z Calculator is a convenient tool for estimating the storage characteristics of a RAIDZ1, RAIDZ2, or RAIDZ3 configuration. By entering the number of drives, the capacity of each drive, and your desired RAID Z level, you can quickly estimate the total raw capacity, usable capacity, parity capacity, storage efficiency, drive failure tolerance, and number of data drives.
This can be particularly useful when designing a home server, NAS, media server, backup system, virtualization environment, or business storage pool. Instead of manually performing several calculations, the RAID Z Calculator provides the essential figures in one place.
RAID Z is part of the ZFS storage architecture and is designed to combine redundancy with pooled storage. The three commonly used RAID Z levels covered by this calculator are RAIDZ1, RAIDZ2, and RAIDZ3. Each provides a different balance between usable capacity and protection against drive failure.
What Is RAID Z?
RAID Z is a ZFS storage configuration that uses parity to provide redundancy across multiple drives. It is conceptually similar to traditional RAID levels, but it is integrated into the ZFS storage model and has characteristics that make it particularly useful for data integrity and storage management.
The basic idea is straightforward: multiple physical drives are combined into a storage pool, while some capacity is reserved for parity information. If a drive fails, the parity information can be used to reconstruct the missing data.
The three RAID Z levels supported by this calculator are:
- RAIDZ1 – one drive's worth of capacity is used for parity and one drive can fail without losing the pool's redundancy.
- RAIDZ2 – two drives' worth of capacity are used for parity and the configuration can tolerate two drive failures.
- RAIDZ3 – three drives' worth of capacity are used for parity and the configuration can tolerate three drive failures.
Choosing between these levels involves a trade-off. Higher redundancy generally means sacrificing more raw capacity for parity.
What Does the RAID Z Calculator Calculate?
The calculator provides six important results.
1. Total Raw Capacity
Total raw capacity represents the combined nominal capacity of all drives before accounting for parity.
For example, if you have eight 10 TB drives:
8 × 10 TB = 80 TB raw capacity
This is the total physical capacity represented by the drives, not the amount of storage available for normal data.
2. Usable Capacity
Usable capacity is the estimated capacity remaining after allocating the equivalent of the selected number of drives for parity.
For the calculator's simplified model:
Usable Capacity = Data Drives × Capacity Per Drive
The number of data drives is calculated by subtracting the RAID Z level from the total number of drives.
3. Parity Capacity
Parity capacity represents the amount of drive capacity allocated to parity.
The calculator uses:
Parity Capacity = RAID Z Level × Capacity Per Drive
Therefore:
- RAIDZ1 = 1 drive's capacity for parity
- RAIDZ2 = 2 drives' capacity for parity
- RAIDZ3 = 3 drives' capacity for parity
4. Storage Efficiency
Storage efficiency shows what percentage of the total raw capacity is available as usable capacity.
The calculator uses:
Storage Efficiency = (Usable Capacity ÷ Raw Capacity) × 100
This makes it easier to compare RAIDZ1, RAIDZ2, and RAIDZ3 configurations.
5. Drive Failure Tolerance
The calculator reports the number of drive failures that the selected RAID Z level is designed to tolerate:
| RAID Z Level | Failure Tolerance |
|---|---|
| RAIDZ1 | 1 drive |
| RAIDZ2 | 2 drives |
| RAIDZ3 | 3 drives |
This is one of the most important considerations when choosing a configuration.
6. Data Drives
Data drives represent the simplified number of drives available for storing data after subtracting the parity drives.
The calculator uses:
Data Drives = Total Drives − RAID Z Level
For example, a 10-drive RAIDZ2 configuration has:
10 − 2 = 8 data drives
How to Use the RAID Z Calculator
Using the RAID Z Calculator requires only three inputs.
Step 1: Enter the Number of Drives
Enter the total number of drives that will be included in the configuration.
The calculator accepts a minimum of three drives. The required minimum depends on the RAID Z level:
- RAIDZ1 requires at least 3 drives.
- RAIDZ2 requires at least 4 drives.
- RAIDZ3 requires at least 5 drives.
Use a whole number, such as 4, 6, 8, 10, or 12.
Step 2: Enter Capacity Per Drive
Enter the capacity of each drive in terabytes (TB).
For example:
- 4 TB
- 8 TB
- 10 TB
- 12 TB
- 16 TB
- 20 TB
The calculator assumes all drives have the same capacity.
Step 3: Select the RAID Z Level
Choose one of the available RAID Z configurations:
RAIDZ1, RAIDZ2, or RAIDZ3
Your choice determines how much capacity is allocated to parity and how many drive failures the configuration is designed to tolerate.
Step 4: Click Calculate
After entering all three values, select Calculate.
The calculator displays:
- Total Raw Capacity
- Usable Capacity
- Parity Capacity
- Storage Efficiency
- Drive Failure Tolerance
- Data Drives
Step 5: Compare Different Configurations
You can change the number of drives, drive capacity, or RAID Z level and calculate again. Comparing multiple configurations is useful when deciding whether additional drives are worth the cost of increased redundancy.
RAID Z Formulas Explained
Understanding the formulas behind the calculator helps you interpret the results correctly.
Raw Capacity Formula
The total raw capacity is:
Raw Capacity = Number of Drives × Capacity Per Drive
Example
Suppose you have 8 drives, each with 12 TB:
8 × 12 = 96 TB
The raw capacity is therefore 96 TB.
Parity Capacity Formula
The calculator estimates parity capacity as:
Parity Capacity = RAID Z Level × Capacity Per Drive
For an 8-drive RAIDZ2 configuration using 12 TB drives:
2 × 12 = 24 TB
The estimated parity capacity is 24 TB.
Data Drive Formula
The calculator determines the number of data drives using:
Data Drives = Number of Drives − RAID Z Level
For 8 drives in RAIDZ2:
8 − 2 = 6 data drives
Usable Capacity Formula
The estimated usable capacity is:
Usable Capacity = Data Drives × Capacity Per Drive
For six data drives at 12 TB each:
6 × 12 = 72 TB
Therefore, the calculator estimates 72 TB of usable capacity.
Storage Efficiency Formula
Storage efficiency is:
Efficiency = (Usable Capacity ÷ Raw Capacity) × 100
Using the previous example:
(72 ÷ 96) × 100 = 75%
The calculated storage efficiency is 75%.
RAIDZ1 vs RAIDZ2 vs RAIDZ3
Choosing a RAID Z level is essentially a balance between storage efficiency and redundancy.
| Feature | RAIDZ1 | RAIDZ2 | RAIDZ3 |
|---|---|---|---|
| Minimum drives in calculator | 3 | 4 | 5 |
| Parity drives' worth | 1 | 2 | 3 |
| Drive failure tolerance | 1 | 2 | 3 |
| Data drives | Total − 1 | Total − 2 | Total − 3 |
| Relative usable capacity | Highest | Medium | Lowest |
| Redundancy | Lower | Higher | Highest |
RAIDZ1 provides the greatest usable capacity of the three options for a given number of drives, but it provides the least redundancy.
RAIDZ2 sacrifices an additional drive's worth of capacity in exchange for protection against two drive failures.
RAIDZ3 sacrifices three drives' worth of capacity but provides the highest failure tolerance supported by the calculator.
Worked Example: 8 × 12 TB Drives
Let's use the calculator with the following values:
- Number of drives: 8
- Capacity per drive: 12 TB
- RAID Z level: RAIDZ2
Step 1: Calculate Raw Capacity
8 × 12 TB = 96 TB
Raw capacity = 96 TB
Step 2: Calculate Parity Capacity
RAIDZ2 uses two drives' worth of capacity for parity:
2 × 12 TB = 24 TB
Parity capacity = 24 TB
Step 3: Calculate Data Drives
8 − 2 = 6
Data drives = 6
Step 4: Calculate Usable Capacity
6 × 12 TB = 72 TB
Usable capacity = 72 TB
Step 5: Calculate Efficiency
(72 ÷ 96) × 100 = 75%
Storage efficiency = 75%
Step 6: Determine Failure Tolerance
Because this is RAIDZ2, the calculator reports:
2 drives
So, under the calculator's model, the configuration can tolerate the failure of two drives.
Result Summary
| Result | Value |
|---|---|
| Number of drives | 8 |
| Drive capacity | 12 TB |
| RAID Z level | RAIDZ2 |
| Raw capacity | 96 TB |
| Parity capacity | 24 TB |
| Data drives | 6 |
| Usable capacity | 72 TB |
| Storage efficiency | 75% |
| Failure tolerance | 2 drives |
This example demonstrates the fundamental RAID Z capacity trade-off. You start with 96 TB of raw drive capacity, but the calculator estimates 72 TB for data after allocating the equivalent of two drives to parity.
Example Comparison With the Same 12 TB Drives
Suppose you have eight 12 TB drives and want to compare the three RAID Z levels.
RAIDZ1
Data drives:
8 − 1 = 7
Usable capacity:
7 × 12 = 84 TB
Efficiency:
84 ÷ 96 × 100 = 87.5%
RAIDZ2
Data drives:
8 − 2 = 6
Usable capacity:
6 × 12 = 72 TB
Efficiency:
72 ÷ 96 × 100 = 75%
RAIDZ3
Data drives:
8 − 3 = 5
Usable capacity:
5 × 12 = 60 TB
Efficiency:
60 ÷ 96 × 100 = 62.5%
| Configuration | Raw Capacity | Usable Capacity | Parity Capacity | Efficiency | Failure Tolerance |
|---|---|---|---|---|---|
| RAIDZ1 | 96 TB | 84 TB | 12 TB | 87.50% | 1 |
| RAIDZ2 | 96 TB | 72 TB | 24 TB | 75.00% | 2 |
| RAIDZ3 | 96 TB | 60 TB | 36 TB | 62.50% | 3 |
This illustrates why RAID Z selection should be based on more than usable storage alone.
Why Drive Failure Tolerance Matters
Hard drives and SSDs can fail unexpectedly. A storage pool that contains important data therefore needs a strategy for handling hardware failures.
RAIDZ1 can tolerate one failed drive according to the calculator's model. RAIDZ2 increases that tolerance to two drives, while RAIDZ3 increases it to three.
However, redundancy should not be confused with backup.
A redundant storage pool can help keep data available after a drive failure, but it is not a replacement for a backup. Accidental deletion, malware, corruption, theft, fire, equipment damage, or other events can affect data even when multiple drives are functioning correctly.
For important information, a separate backup strategy remains essential.
Understanding Raw Capacity vs Usable Capacity
One of the most common storage-planning mistakes is assuming that the combined capacity printed on the drives will all be available for files.
For example, eight 12 TB drives have:
96 TB raw capacity
But an eight-drive RAIDZ2 configuration in this calculator produces an estimated:
72 TB usable capacity
The difference exists because redundancy requires capacity for parity.
This distinction is especially important when purchasing drives. If you need a particular amount of usable storage, calculate the required configuration before purchasing hardware.
Choosing a RAID Z Level
When RAIDZ1 May Be Appropriate
RAIDZ1 can be attractive when usable capacity is a high priority and the storage environment can tolerate the risk associated with single-drive failure protection.
It may be considered for less critical workloads or situations where the capacity cost of additional redundancy is undesirable.
However, storage requirements and drive sizes should be carefully considered before selecting RAIDZ1.
When RAIDZ2 May Be Appropriate
RAIDZ2 offers a stronger redundancy level while retaining more usable capacity than RAIDZ3.
For many storage planning scenarios, RAIDZ2 provides a useful middle ground between capacity and fault tolerance.
It can be especially appealing when the data is important and the storage pool contains several drives.
When RAIDZ3 May Be Appropriate
RAIDZ3 provides the highest level of redundancy available in this calculator.
It uses three drives' worth of capacity for parity but can tolerate three drive failures under the calculator's simplified model.
The capacity sacrifice can be worthwhile when protecting important data is more important than maximizing usable capacity.
Important Considerations When Planning a RAID Z Pool
Drive Capacity
The calculator assumes that every drive has the same capacity. Real-world ZFS configurations can behave differently when drive sizes are mixed.
For straightforward planning, using drives with matching capacities makes calculations easier and generally provides more predictable results.
Number of Drives
Adding more drives increases raw capacity, but the usable result also depends on the RAID Z level.
Before purchasing additional drives, use the calculator to compare different drive counts.
Usable Storage Is Not the Same as Raw Storage
Always distinguish between:
Raw capacity → total physical drive capacity
and
Usable capacity → estimated capacity after parity allocation
This distinction prevents overestimating how much space will be available for files.
Keep Free Space Available
A storage pool should not necessarily be filled to its theoretical maximum. Maintaining free space can be important for normal storage operations and future growth.
When planning a system, consider your expected data growth rather than designing only around today's requirements.
RAID Is Not Backup
RAID Z provides redundancy, not a complete backup solution.
A sensible storage strategy can include:
- Local redundancy
- Separate backups
- Off-site or cloud backups
- Periodic backup verification
- Monitoring of drive health
- A plan for drive replacement
The appropriate strategy depends on the importance of the stored data.
RAID Z Capacity Planning Table
The following table shows the simplified usable capacity for eight drives of different sizes.
| Drive Count | Drive Size | RAIDZ1 Usable | RAIDZ2 Usable | RAIDZ3 Usable |
|---|---|---|---|---|
| 8 | 4 TB | 28 TB | 24 TB | 20 TB |
| 8 | 6 TB | 42 TB | 36 TB | 30 TB |
| 8 | 8 TB | 56 TB | 48 TB | 40 TB |
| 8 | 10 TB | 70 TB | 60 TB | 50 TB |
| 8 | 12 TB | 84 TB | 72 TB | 60 TB |
| 8 | 16 TB | 112 TB | 96 TB | 80 TB |
| 8 | 20 TB | 140 TB | 120 TB | 100 TB |
These figures follow the simplified formulas used by the calculator and are useful for initial planning.
Benefits of Using a RAID Z Calculator
A RAID Z calculator can simplify storage planning in several ways.
Faster Capacity Estimates
Instead of calculating raw, parity, usable capacity, and efficiency manually, you can enter your configuration and receive the results immediately.
Easier RAID Level Comparison
You can compare RAIDZ1, RAIDZ2, and RAIDZ3 to see how increasing redundancy affects available storage.
Better Hardware Planning
Before purchasing drives, you can estimate the capacity you will have after redundancy is taken into account.
Clearer Understanding of Efficiency
The efficiency percentage helps explain how much of your total raw capacity is available for data.
Useful for NAS and Server Planning
The calculator can help with preliminary planning for home labs, NAS systems, file servers, media libraries, backup systems, and other ZFS-based storage environments.
Limitations of the RAID Z Calculator
The calculator is designed as a simple capacity-planning tool. Its formulas provide a straightforward estimate based on the number of drives, identical drive capacities, and selected RAID Z level.
Actual ZFS capacity can involve additional considerations, including filesystem overhead, pool configuration, sector sizes, allocation behavior, reserved space, and other factors. Therefore, the calculator's usable-capacity figure should be treated as an estimate rather than a guaranteed amount of user-visible free space.
It also assumes identical drive capacities. If your planned pool uses drives of different sizes, actual capacity behavior can differ from this simplified calculation.
For production storage deployments, verify the final configuration using the documentation and tools appropriate for your ZFS implementation.
Frequently Asked Questions
1. What is a RAID Z Calculator?
A RAID Z Calculator is a storage-planning tool that estimates raw capacity, usable capacity, parity capacity, efficiency, data drives, and drive failure tolerance for RAIDZ1, RAIDZ2, and RAIDZ3 configurations.
2. What is the minimum number of drives for RAIDZ1?
The calculator requires at least 3 drives for RAIDZ1.
3. How many drives are required for RAIDZ2?
The calculator requires at least 4 drives for RAIDZ2.
4. What is the minimum number of drives for RAIDZ3?
The calculator requires at least 5 drives for RAIDZ3.
5. How much capacity does RAIDZ1 use for parity?
The calculator treats RAIDZ1 as using one drive's worth of capacity for parity.
6. How much capacity does RAIDZ2 use for parity?
RAIDZ2 uses two drives' worth of capacity for parity in the calculator's model.
7. How much capacity does RAIDZ3 use for parity?
RAIDZ3 uses three drives' worth of capacity for parity according to the calculator.
8. Can RAIDZ2 survive two failed drives?
The calculator reports a two-drive failure tolerance for RAIDZ2. However, redundancy and real-world recovery depend on the complete storage environment, and maintaining backups is still important.
9. Is RAID Z a backup?
No. RAID Z provides redundancy and can help maintain storage availability when drives fail, but it does not replace an independent backup.
10. Does the RAID Z Calculator support different drive capacities?
The calculator asks for one capacity per drive and therefore assumes that all drives have the same capacity. Mixed-drive configurations may require more detailed planning.
Final Thoughts
The RAID Z Calculator makes it easier to understand the relationship between drive count, drive capacity, parity, usable storage, efficiency, and redundancy. By entering just three values—number of drives, capacity per drive, and RAID Z level—you can quickly estimate the characteristics of a potential storage configuration.
The key concept is that raw capacity and usable capacity are not the same. RAIDZ1 reserves one drive's worth of capacity for parity, RAIDZ2 reserves two, and RAIDZ3 reserves three in the calculator's simplified model. As redundancy increases, usable capacity decreases, but drive failure tolerance increases.
For example, eight 12 TB drives provide 96 TB of raw capacity. Under the calculator's model, that becomes approximately 84 TB with RAIDZ1, 72 TB with RAIDZ2, or 60 TB with RAIDZ3. This simple comparison shows the fundamental trade-off between storage efficiency and redundancy.
Before building a storage pool, use the calculator to test different combinations of drive count, drive size, and RAID Z level. Then consider factors such as expected data growth, workload importance, drive availability, recovery requirements, and—most importantly—your independent backup strategy.
Whether you're planning a personal NAS, home lab, media server, backup system, or larger storage environment, calculating capacity before purchasing hardware can help you make a more informed decision and avoid unpleasant surprises after the pool is created.