raidz1 calculator

What this RAIDZ1 calculator does

This tool gives a quick planning estimate for a RAIDZ1 pool: how much raw capacity you install, how much is used by parity, and how much space you should realistically plan to use. It is designed for homelab and SMB storage planning where simplicity matters.

RAIDZ1 protects against a single drive failure in a vdev. You gain better usable capacity than a mirror, but with less redundancy than RAIDZ2 or RAIDZ3. Capacity planning is straightforward, but performance and risk should be part of the decision too.

RAIDZ1 capacity formula

Core equation

For equal-sized drives in one RAIDZ1 vdev:

• Raw capacity = number of drives × drive size
• Parity overhead = 1 × drive size
• Theoretical usable = (number of drives − 1) × drive size
• Efficiency = ((drives − 1) / drives) × 100%

This calculator also applies optional overhead and a planned fill limit so you can estimate practical working space, not just the maximum mathematical value.

Example

If you build a pool with 6 × 12 TB disks in RAIDZ1, your raw is 72 TB, parity cost is 12 TB, and theoretical usable is 60 TB before filesystem overhead. If you reserve 3% overhead and only fill to 80%, your practical working target is lower and usually much safer for day-to-day operation.

TB vs TiB: why your numbers look smaller in the OS

Drive labels use decimal units (TB), while many operating systems report binary units (TiB). That mismatch makes available space look smaller than the drive packaging suggests. This calculator shows both styles so expectations match what you'll see in real systems.

Important RAIDZ1 caveats

1) One failed drive is your entire safety margin

During rebuild (resilver), the pool is degraded. A second failure in the same vdev can cause data loss. With modern large disks, rebuild windows can be long, which is why many administrators prefer RAIDZ2 for larger arrays.

2) Mixed drive sizes waste space

In a vdev, effective disk size is limited by the smallest member. If sizes differ, extra capacity on larger drives is not fully usable in that vdev.

3) Capacity is not backup

RAIDZ1 protects availability, not against accidental deletion, corruption propagated to replicas, malware, or site disaster. Keep independent backups.

Best practices before you commit

• Use identical or closely matched drives where possible.
• Run regular SMART checks and scheduled scrubs.
• Keep spare drive inventory for faster recovery.
• Avoid running near 100% full; performance and management get harder.
• If uptime/data value is high, compare RAIDZ2 and mirrored designs.

Quick FAQ

Can RAIDZ1 survive two failed drives?

No. RAIDZ1 protects against one failed drive per vdev.

Is RAIDZ1 the same as RAID5?

It is conceptually similar (single parity), but ZFS implementation details and data integrity features differ from classic hardware RAID5.

Should I fill my pool completely?

Usually no. Many operators target around 70–85% utilization to preserve performance and avoid operational pain.

Bottom line

RAIDZ1 is capacity-efficient and works well for some workloads, but it is a tradeoff. Use this calculator to plan realistic space, then choose a redundancy level that matches the importance of your data and your tolerance for risk.