Calculate “1/(40rods/hogshead) to L/100km” from your Zsh prompt

Vincent Bernat March 22, 2026

I often need a quick calculation or a unit conversion. Rather than reaching for a separate tool, a few lines of Zsh configuration turn = into a calculator. Typing = 660km / (2/3)c * 2 -> ms gives me 6.60457 ms¹ without leaving my terminal, thanks to the Zsh line editor.

The equal alias
The quoting problem
- Automatic quoting with ZLE
- Storing unquoted history

The equal alias#

The main idea looks simple: define = as an alias to a calculator command. I prefer Numbat, a scientific calculator that supports unit conversions. Qalculate is a close second.² If neither is available, we fall back to Zsh’s built-in zcalc module.

Qalculate is less understanding with units. For example, it parses “Mbps” as megabarn per picosecond: ☢️

$ numbat -e '5 MB/s -> Mbps'
40 Mbps
$ qalc 5 MB/s to Mbps
5 megabytes/second = 0.000005 B/ps

❦

As the alias built-in uses = as a separator for name and value, we need to alter the aliases associative array:

if (( $+commands[numbat] )); then
  aliases[=]='numbat -e'
elif (( $+commands[qalc] )); then
  aliases[=]='qalc'
else
  autoload -Uz zcalc
  aliases[=]='zcalc -f -e'
fi

With this in place, = 847/11 becomes numbat -e 847/11.

The quoting problem#

The first problem surfaces quickly. Typing = 5 * 3 fails: Zsh expands the * character as a glob pattern before passing it to the calculator. The same issue applies to other characters that Zsh treats specially, such as > or |. You must quote the expression:

$ = '5 * 3'
15

We fix this by hooking into the Zsh line editor to quote the expression before executing it.

Automatic quoting with ZLE#

Zsh calls the line-finish widget before submitting a command. We hook a function that detects the = prefix and quotes the expression:

_vbe_calc_quote() {
  case $BUFFER in
    "="*)
      typeset -g _vbe_calc_expr=$BUFFER # not used yet
      BUFFER="= ${(q-)${${BUFFER#=}# }}"
      ;;
  esac
}
add-zle-hook-widget line-finish _vbe_calc_quote

When you type = 5 * 3 and press ↲, _vbe_calc_quote strips the = prefix, quotes the remainder with the (q-) parameter expansion flag, and rewrites the buffer to = '5 * 3' before Zsh submits the command. As a bonus, you can save a few keystrokes with =5*3! 🚀

You can now compute math expressions and convert units directly from your shell. Zsh automatically quotes your expressions:

$ = '1 + 2'
3
$ = 'pi/3 + pi |> cos'
-0.5
$ = '17 USD -> EUR'
14.7122 €
$ = '180*500mg -> g'
90 g
$ = '5 gigabytes / (2 minutes + 17 seconds) -> megabits/s'
291.971 Mbit/s
$ = 'now() -> tz("Asia/Tokyo")'
2026-03-22 22:00:03 JST (UTC +09), Asia/Tokyo
$ = '1 / (40 rods / hogshead) -> L / 100km'
118548 × 0.01 l/km

“That's the way I like it!” says Grampa
Simpson — The metric system is the tool of the devil! My car gets forty rods to the hogshead, and that’s the way I like it! ― *Grampa Simpson*, A Star Is Burns

Storing unquoted history#

As is, Zsh records the quoted expression in history. You must unquote it before submitting it again. Otherwise, the ZLE widget quotes it a second time. Bart Schaefer provided a solution to store the original version:

_vbe_calc_history() {
  return ${+_vbe_calc_expr}
}
add-zsh-hook zshaddhistory _vbe_calc_history

_vbe_calc_preexec() {
  (( ${+_vbe_calc_expr} )) && print -s $_vbe_calc_expr
  unset _vbe_calc_expr
  return 0
}
add-zsh-hook preexec _vbe_calc_preexec

The zshaddhistory hook returns 1 if we are evaluating an expression, telling Zsh not to record the command. The preexec hook then adds the original, unquoted command with print -s.

The complete code is available in my zshrc. A common alternative is the noglob precommand modifier. If you stick with to instead of -> for unit conversion, it covers 90% of use cases. For a related Zsh line editor trick, see how I use auto-expanding aliases to fix common typos.