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RAID calculator

How much space will you actually get? Put in your drives and say how much protection you want, and it'll work it out. Mixed sizes are fine, up to twelve drives.

1How many drives?
4
2What size are they?

Got a mix of sizes? Change any drive on its own.

3How much protection?
Advanced: pick a layout

RAID 5, SHR, RAIDZ, Unraid and the rest. A layout picked here wins over the protection answer above.

You'll get 24 TB usable and can lose one drive safely.

24TB
usable

Usable
24 TB
Protection
8 TB
Unused
0 TB

Windows will show about 21.8 TB, because it counts in binary units.

    4 × 8 TB · SHR · 32 TB raw

    Every layout on these drives
    LayoutUsableDrives it can loseShare of raw
    24 TB 1 75%
    16 TB 2 50%
    32 TB 0 100%
    8 TB 3 25%
    24 TB 1 75%
    16 TB 2 50%
    16 TB 1–2 50%
    16 TB 1–2 50%
    24 TB 1 75%
    16 TB 2 50%
    – needs 5+
    24 TB 1 75%
    16 TB 2 50%
    24 TB 1 75%
    16 TB 2 50%
    32 TB 0 100%

    Where it says 1–2, a second failure is only safe if it lands in a different mirror pair.

    The address bar keeps your drives, so you can share the link. If you're set on one system, there's an SHR calculator for Synology and an Unraid one too.

    Mixed drive sizes

    Say you've got two 12 TB drives and two 8 TB ones, 40 TB raw. RAID 5 treats all four as 8 TB drives, which leaves you 24 TB with 8 TB doing nothing at all. SHR is cleverer about it. It builds an 8 TB layer across all four drives and a 4 TB layer across the two big ones, and you end up with 28 TB. Unraid with one parity drive lands on the same 28 TB by a different route, with a 12 TB drive on parity and the other three counting in full.

    With a drawer full of odd sizes, then, it's SHR on a Synology and Unraid on your own hardware. ZFS is the fussy one. Buy matching drives for it, or pair them up into mirrors of equal size.

    TB or TiB?

    Drive makers count a terabyte as a trillion bytes. Windows counts in tebibytes (240 bytes, nearly 10% bigger) and then labels them TB anyway, which is why a brand-new 16 TB drive turns up in Explorer as 14.55 TB and looks like it's been short-changed. macOS counts in decimal like the drive makers and shows the full 16 TB.

    On the boxIn Windows
    4 TB3.64 TB
    8 TB7.28 TB
    12 TB10.91 TB
    16 TB14.55 TB
    20 TB18.19 TB
    24 TB21.83 TB

    What each layout does

    This is the list behind "Advanced". Most people never need it, because the three questions pick the right one for a Synology box, and Unraid gets to the same numbers on matching drives.

    SHRSynology · 1+ drives
    Synology slices the drives into layers at each size and protects each layer on its own, and that's how the extra space on bigger drives gets used once two of them share it.
    SHR-2Synology · 4+ drives
    Same layering, two drives of protection. Needs four drives or more.
    RAID 0RAID · 2+ drives
    Adds every drive together and keeps nothing spare. One failure and the lot's gone.
    RAID 1RAID · 2+ drives
    Every drive holds the same data, and usable space is one drive (the smallest).
    RAID 5RAID · 3+ drives
    One drive's worth of parity spread over all of them. Usable is (drives − 1) × the smallest drive.
    RAID 6RAID · 4+ drives
    RAID 5 with two drives' worth of parity. Usable is (drives − 2) × the smallest.
    RAID 10RAID · 4+ drives
    Mirrored pairs striped together, giving you half the drives × the smallest.
    ZFS mirrorZFS · 2+ drives
    Two-way mirror vdevs in one pool, with each pair counting as its smaller drive.
    RAIDZ1ZFS · 3+ drives
    One drive of parity in a single vdev, sized to the smallest drive. ZFS's answer to RAID 5.
    RAIDZ2ZFS · 4+ drives
    Two drives of parity in one vdev, the ZFS take on RAID 6.
    RAIDZ3ZFS · 5+ drives
    Three drives of parity, for wide vdevs of big drives.
    Unraid, 1 parityUnraid · 2+ drives
    The biggest drive holds parity and every other drive adds its full size.
    Unraid, 2 parityUnraid · 3+ drives
    The two biggest drives hold parity. Everything else counts in full.
    SnapRAID, 1 parityDAS / PC · 2+ drives
    Counts like Unraid, with a parity drive at least as big as the largest data drive. Parity only updates when you run a sync, which suits a media library that rarely changes.
    SnapRAID, 2 parityDAS / PC · 3+ drives
    Two parity drives, synced on a schedule. Worth it on bigger DAS or PC arrays.
    JBOD, separate disksDAS / PC · 1+ drives
    Every drive on its own and no parity. A dead drive takes only its own files with it.

    What the calculator leaves out

    • File system overhead. Btrfs and ext4 keep a few per cent back for metadata.
    • ZFS slop space, the 1/32 of the pool (up to 128 GiB) that OpenZFS holds back.
    • Hot spares and SSD cache drives. Leave those bays out of the count.
    • The small system partition Synology and QNAP put on every drive.