def disk [] {
    df -h
      | str replace "Mounted on" "Mountpoint"
      | detect columns
      | rename filesystem size used avail used% mountpoint
      | into filesize size used avail
      | upsert used% {|it| 100 * (1 - $it.avail / $it.size)}
}

  Convert string to filesize in table
  > [[bytes]; ['5'] [3.2] [4] [2kb]] | into filesize bytes

Error: nu::parser::input_type_mismatch

  × Command does not support table input.
   ╭─[entry #1:5:1]
 5 │       | rename filesystem size used avail used% mountpoint
 6 │       | into filesize size used avail
   ·         ──────┬──────
   ·               ╰── command doesn't support table input
 7 │       | upsert used% {|it| 100 * (1 - $it.avail / $it.size)}
   ╰────

> disk | where mountpoint == "/" | into record
╭────────────┬───────────────────╮
│ filesystem │ /dev/sda2         │
│ size       │ 217.9 GiB         │
│ used       │ 158.3 GiB         │
│ avail      │ 48.4 GiB          │
│ used%      │ 77.77777777777779 │
│ mountpoint │ /                 │
╰────────────┴───────────────────╯