ZFS RAIDZ Calculator | Usable Pool Capacity

ZFS RAIDZ calculator for usable pool space: RAIDZ1, RAIDZ2, RAIDZ3 and mirrors, with padding, metaslab and slop overhead matched to OpenZFS. Free.

Also known as: zfs raidz calculator · zfs capacity calculator · raidz2 calculator · zfs usable space calculator · truenas zfs calculator · proxmox zfs calculator · raidz1 vs raidz2 capacity · zfs storage calculator · zfs slop space · raidz padding overhead

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Pool Layout

4 disks

vdev type

Disks per vdev (width)

RAIDZ needs parity + 1 or more

Number of vdevs

identical vdevs striped together

Disk size

as printed on the drive, or lsblk -b bytes

Unit

1 TB = 10^12 bytes

ashift (sector size)

Whole disks (zpool create on Linux)

zpool writes a GPT: data from 1 MiB, 8 MiB reserved at the end. Turn off for partitions or files used as given.

Matches OpenZFS 2.4.4 byte for byte: RAIDZ allocation and padding, metaslab sizing, the embedded log metaslab and slop space, checked against libzpool on 64 pool layouts.

Results are for an empty pool with no special vdevs, dRAID or RAIDZ expansion.

Usable Capacity

Usable space as zfs list will report it, where every lost terabyte goes, how each block size is stored and how the same disks compare in other layouts will appear here.

About ZFS RAIDZ Calculator

A ZFS RAIDZ calculator tells you how much space a pool will really give you before you buy the drives. Pick RAIDZ1, RAIDZ2, RAIDZ3, mirrors or a stripe, enter the disk count and size, and it works out what zfs list will report, including the parity, RAIDZ padding, metaslab rounding, embedded log metaslab and slop space that a simple disks-minus-parity estimate leaves out.

Key Features

Usable space as zfs list reports it, in TiB and TB, plus the SIZE that zpool list shows
Step-by-step breakdown from raw disk to usable: parity, partitions, labels, metaslabs, padding and slop
RAIDZ1, RAIDZ2, RAIDZ3, 2-way and 3-way mirrors and stripes, with several vdevs per pool
Block size table that shows how 4K to 1M records are allocated and why small zvol blocks waste space
Side-by-side comparison of the same disks in every other layout
Ported from OpenZFS 2.4.4 and checked against libzpool on 64 layouts, byte for byte

Use Cases

Size a TrueNAS or Proxmox VE pool before ordering drives for a home lab
Decide between one 8-wide RAIDZ2 vdev and two 4-wide RAIDZ1 vdevs for a Plex server
Explain why a new pool of six 20 TB drives shows 72.5T instead of 80T in zfs list
Pick a volblocksize for VM zvols on a wide RAIDZ pool without doubling their footprint
Budget usable backup capacity for a small business NAS before a hardware refresh
Check how much space adding a second vdev of the same width will add

Supported Formats

RAIDZ1, RAIDZ2, RAIDZ3

Mirrors and stripes

TB, TiB, GB, GiB or exact bytes

ashift 9 to 14

Related Keywords

zfs raidz calculator

zfs capacity calculator

raidz2 calculator

zfs usable space calculator

truenas zfs calculator

proxmox zfs calculator

raidz1 vs raidz2 capacity

zfs storage calculator

zfs slop space

raidz padding overhead

Why does my ZFS pool show less space than expected?

Four things eat into a RAIDZ pool beyond the parity disks. RAIDZ rounds every block up to a multiple of parity + 1 sectors, so a 128 KiB record on a 6-wide RAIDZ3 takes 68 sectors instead of 64, and zfs list bakes that into the space it reports. ZFS divides each vdev into metaslabs of up to 16 GiB and ignores the remainder. Since OpenZFS 2.1, a vdev with more than 64 metaslabs holds one back for the ZFS intent log. Finally, slop space reserves 1/32 of the pool, up to 128 GiB, so you can still delete files when the pool is full. Drive makers also sell decimal terabytes while zfs list shows binary TiB, which alone makes a 20 TB drive look like 18.2T.

Privacy & Security

All processing happens locally in your browser. Your data never leaves your computer, ensuring complete privacy and security.

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Frequently asked questions

What is ZFS RAIDZ Calculator used for?

ZFS RAIDZ calculator for usable pool space: RAIDZ1, RAIDZ2, RAIDZ3 and mirrors, with padding, metaslab and slop overhead matched to OpenZFS. Free.

Is ZFS RAIDZ Calculator free to use?

Yes. ZFS RAIDZ Calculator is completely free with no usage limits, and there is no paid tier required to use it.

Do I need an account to use ZFS RAIDZ Calculator?

No. ZFS RAIDZ Calculator works immediately in your browser with no signup, login, or email address required.

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