ZFS Capacity Calculator
What is a ZFS Calculator?
ZFS is a copy‑on‑write file‑system that stores data inside vdevs (virtual devices) made of one or more physical disks. Usable space depends on your vdev layout (mirror, RAIDZ‑1/2/3), the number of groups, spare‑disk policy, and the mandatory ZFS “slop” reserve the system keeps to avoid fragmentation slow‑downs. The calculator crunches all that for you and instantly returns:
- Raw vs usable capacity in terabytes for the whole pool.
- Efficiency % – space you keep after parity, spares, and slop.
- Fault‑tolerance summary – disks each vdev can lose without data loss.
- (Advanced mode) spare reservation, record‑size, ashift, AFR, and resilver‑time inputs.How to use it
How to Use the ZFS Capacity Calculator
The ZFS Capacity Calculator gives you two ways to plan your storage. You can use a lightweight mode for a quick estimate or switch to the advanced mode when you want deeper planning. The lightweight mode works well when you already know your general layout and want instant results without many inputs.
Lightweight Mode
Lightweight mode gives a fast estimate with only a few inputs.
Step 1 – Enter vdev Groups
Enter the number of identical vdevs in the pool. A vdev group is one repeated structure in your pool. If your design has four identical mirror vdevs, type 4. The calculator uses this number to multiply capacity across your entire pool.
Field | Description | Example Input | Unit |
vdev groups | Number of identical vdev stripes | 4 | groups |
Step 2 – Select vdev Type
Choose the layout for each vdev. Your choice affects usable capacity and redundancy. Mirror gives high redundancy with fast rebuilds, while RAIDZ1, RAIDZ2, and RAIDZ3 reduce risk with one, two, or three parity disks. Pick the type that matches your design plan.
Field | Description | Example Input | Unit |
vdev type | Layout used for each vdev | Mirror | type |
Step 3 – Enter Disks per Group
Now enter how many disks are inside each vdev. This number is important because ZFS calculates parity and usable space based on vdev width. For example, type 3 if you are building a three-disk mirror or a three-wide RAIDZ group.
Field | Description | Example Input | Unit |
Disks per group | Width of each vdev | 3 | disks |
Step 4 – Enter Disk Size
Add the size of each disk in your array. All disks in the same vdev must have the same usable size because ZFS will match the smallest disk in the group. Enter the size exactly as your drives are labeled.
Field | Description | Example Input | Unit |
Disk size | Size of each disk | 4 | TB |
Step 5 – Include or Exclude ZFS Slop
ZFS keeps a reserved area called slop space. It helps the pool stay healthy and prevents fragmentation issues. If you want your output to reflect real-world usable capacity, turn this option on. If you want to see raw capacity without the reserved space, turn it off.
Step 6 – Click Calculate
After entering the values, press Calculate. The calculator shows raw capacity, usable capacity, efficiency, fault tolerance, and related outputs.
Advanced Mode
Advanced mode gives a deeper breakdown with more settings.
Step 1 – Enter Base Disk Configuration
Add the main disk details such as disk size, number of global spares, slop percentage, recordsize, ashift, annual failure rate, and target resilver time.
Step 2 – Add vdev Layouts
Select the vdev type and width, then add it to the pool. You can add several different vdev layouts if needed.
Step 3 – Click Calculate for Full Breakdown
After adding all vdevs, press Calculate. The calculator presents raw and usable capacity, efficiency, fault tolerance for each vdev, spare overhead, and any advanced factors included in the setup.
The Main Highlights
Lightweight mode – 30‑second estimate
- vdev groups – how many identical stripes (e.g. four mirrors).
- vdev type – Mirror, RAIDZ‑1/2/3.
- Disks per group – width of a single vdev.
- Disk size – choose TB or GB; calculator converts.
- Tick/untick “Include ZFS slop” (default 6.25 %).
- Hit Calculate – results appear underneath.
Advanced mode – design‑grade planning
- Enter base disk size, number of global spares, slop %, record‑size, ashift (sector exponent), disk AFR, and target resilver hours.
- Add one or more vdev layouts with the quick “type + width + Add” line (e.g. Mirror × 2 drives, RAIDZ‑2 × 6 drives).
- Click Calculate for a full breakdown: raw/usable TB, efficiency, fault tolerance per vdev, spare overhead, plus your advanced parameters for the record.
Key Features
Raw vs Usable Capacity
This ZFS capacity calculator shows raw capacity from all drives combined and usable capacity after mirrors, parity, and reserved space. A four-disk mirror setup gives four times the disk size in raw space but only two times the disk size in usable space because each piece of data is written twice. Both terabytes and tebibytes appear, so you can see the real amount of storage available.
Efficiency Percentage
Efficiency shows how much of the raw space remains usable. A stripe gives full efficiency, a two-disk mirror gives half, and RAIDZ layouts fall in between. This percentage helps you compare layouts and understand how much space you keep after redundancy. You can also evaluate how efficiently storage resources are used in other systems with our capacity utilization calculator.
Fault Tolerance Summary
This calculator also shows how many drives each layout can lose. A two-way mirror can survive one failed drive in each pair. RAIDZ1 can survive one failed drive per vdev, RAIDZ2 can survive two, and RAIDZ3 can survive three. This summary helps you judge the level of protection you get.
Advanced Configuration Options
Our calculator also includes extra settings such as global spares, slop reserve, record size, and ashift. These options make the estimate more accurate and help you plan storage based on your hardware and workload.
ZFS Capacity Calculator Benefits
True ZFS Math
Our calculator follows real ZFS rules. It includes important overheads such as slop reserve, parity space, and padding. This gives you a usable capacity estimate that stays close to what ZFS reports in practice. You avoid the optimistic numbers that simple RAID tools often show.
Dual-Mode Interface
This calculator offers two ways to work. A quick mode gives fast estimates when you want a simple answer. The advanced mode lets you explore detailed scenarios with more control. You decide whether you want speed or precision without dealing with a cluttered screen.
Mixed vdev Support
This calculator supports pools that use more than one vdev layout. You can mix mirror vdevs with RAIDZ vdevs in the same design. Each vdev is calculated separately and then combined. This matches real storage plans where new vdevs might be added over time or built with different layouts.
Spare-Aware Capacity
If your pool includes hot spares, the calculator includes them in the estimate. Spares reduce usable space even before they take part in a rebuild. The calculator subtracts this space so you know exactly how much capacity remains available for data.
Sector-Size and Record-Size Inputs
This calculator allows you to set ashift and recordsize values. These settings can change how ZFS stores blocks and may affect usable space. You can match these values to your hardware or workload to see if they change the final estimate.
Browser-Based with No Uploads
This ZFS capacity calculator runs inside your browser. Nothing is uploaded and nothing is stored on a server. All calculations stay on your device, which keeps your configuration private and makes the tool easy to use anywhere without installation.
The Main Highlights
- True ZFS math – includes the 6.25 % slop reserve often forgotten in generic RAID calculators.
- Dual‑mode UI – one screen for quick ball‑parks, one for “what‑if” design‑studio sessions.
- Mixed vdev support – build pools with any combination of mirrors and RAIDZ levels; most online tools allow only one shape.
- Spare‑aware capacity – instantly see how many TB you sacrifice when you earmark hot spares (no manual subtraction).
- Sector‑size & record‑size inputs – plan for 4 Kn disks or DB‑friendly record sizes without hidden assumptions.
- No sign‑up, no telemetry – runs entirely in the browser; nothing is uploaded.
Supported ZFS Layouts
Mirrors and Multi-way Mirrors
Our calculator supports both two-disk mirrors and larger multi-way mirrors. In an N-way mirror, every disk stores the same data. A three-disk mirror can survive two failed disks. Usable capacity in an N-way mirror is always one part usable for every N parts raw. The calculator lets you set any mirror width and then shows the usable space and level of protection.
RAIDZ1, RAIDZ2, RAIDZ3
This calculator supports all RAIDZ levels. RAIDZ1 uses one disk of parity in each stripe, RAIDZ2 uses two, and RAIDZ3 uses three. When you choose the RAIDZ level and the number of disks in the vdev, the calculator applies the correct parity cost. A five-disk RAIDZ1 setup gives the usable space of four disks, while a five-disk RAIDZ2 setup gives the usable space of three disks. This helps you compare capacity and redundancy across different layouts.
Stripe-Only Configurations
You can also build a simple stripe with no redundancy. In this layout, raw capacity equals usable capacity, and efficiency is one hundred percent. A striped vdev offers maximum space and speed but no protection. If one disk fails, data is lost. The calculator shows this clearly by reporting full efficiency and zero fault tolerance.
Hybrid and Mixed-pool Designs
This calculator supports mixed pools that use different vdev types together. You can combine mirror vdevs with RAIDZ vdevs in the same plan. Each vdev is calculated on its own and then added to the totals. This helps when planning expansions or building pools with a mix of layouts.
Special vdev, L2ARC, and slog Awareness
Special vdevs, L2ARC devices, and SLOG devices do not increase usable data space. The calculator treats them correctly and leaves them out of the usable capacity total. If you include them in the design, they appear as support devices rather than as capacity contributors.
Best Practices for ZFS Capacity Planning
Avoiding Too-wide RAIDZ Groups
RAIDZ vdevs with too many disks can look attractive because they offer more usable space, but they often reduce performance and increase risk. Moderate widths work better for most setups. A three-disk RAIDZ1, a six-disk RAIDZ2, or a nine-disk RAIDZ3 keeps parity at roughly one-third and stays predictable during random I O. Very wide vdevs, such as groups with fifteen or more disks, take a long time to rebuild after a failure and may feel slow. When unsure, create several smaller vdevs instead of one large one.
Planning for Replacement and Rebuild Time
Large disks take time to resilver. A rebuild can last many hours or even days, depending on workload and hardware speed. If your storage system depends on backup power during rebuilds, you can estimate runtime using our battery runtime calculator. During this period, your pool has less protection. Always consider how long a rebuild will take when planning capacity.
For example, a pool with one hundred terabytes of data and drives that write at two hundred megabytes per second can take more than sixteen hours to rebuild under perfect conditions. The calculator can include a target resilver time so you can see how changes in layout, disk count, or spare usage may improve recovery.
Keeping Spare Disks Ready
Spare drives make recovery faster. A hot spare lets ZFS start a rebuild as soon as a disk fails. Without spares, you need to obtain a replacement before the rebuild begins, which increases the risk period. Many setups work well with at least one spare for each RAIDZ vdev or one global spare for the entire pool. The calculator shows how spares reduce usable capacity, so you can plan for the trade-off. In most cases, a small drop in efficiency is worth the faster recovery.
Choosing Proper Disk Size and Type
Use drives with the same size and type in each vdev. If you mix capacities, ZFS uses only the smallest size in that group, which wastes space. It is better to match new drives to existing ones or replace them in sets. Also consider drive reliability.
NAS-rated or enterprise HDDs are safer than SMR drives, and SSDs rebuild much faster than spinning disks. Larger disks offer more capacity but take longer to rebuild. Smaller or faster disks may improve performance even if they reduce total capacity.
Planning for Future Expansion
Think about how you want the pool to grow. You can add new vdevs to expand a ZFS pool, but shrinking old vdevs is difficult. It is often easier to start with multiple smaller vdevs and add new ones later.
Older ZFS versions required replacing every disk to expand a RAIDZ vdev, and newer versions allow adding disks one at a time, but the process is still slow. Many users prefer adding complete vdevs because it is simpler and faster.
Disclaimer: This ZFS calculator provides estimated storage capacity values to help with general planning and comparison of different pool layouts. Actual usable space, performance, and fault tolerance may vary depending on your hardware, ZFS version, system configuration, and workload. For production environments or critical deployments, it is recommended to review your design with a systems administrator or storage professional.
FAQs
ZFS keeps a small portion of every pool reserved to maintain stability. This reserved part is known as slop. It prevents the pool from becoming full, which helps avoid fragmentation and performance issues.
A vdev group is a set of disks that work together as one building block in a ZFS pool. Each vdev contributes capacity and redundancy to the pool, and the overall layout depends on how many vdevs you create and how they are arranged.
Yes. The calculator can handle mixed layouts. You can combine mirror vdevs and RAIDZ vdevs in the same plan, and the calculator adds the results from each vdev to form the total capacity.
Raw capacity is the total space from all disks. Usable capacity is the space left after ZFS reserves room for mirrors, parity, and system overhead. This difference ensures data protection and long-term stability.
Yes. The calculator works for small pools, large pools, and complex designs. It can help plan capacity for home servers, business systems, and enterprise storage.
Yes. The calculator reads your layout and reports how many disk failures your design can survive. It gives a clear summary based on the mirror width or the RAIDZ level.