After trying a lot of different calculators, I just settled on this little shell script to launch/raise a nodejs repl with FN+Backpspace:

  if [ $(xdotool search --name "calc repl") ]; then
   xdotool search --name "calc repl" windowraise
  else
   xfce4-terminal \
    --title="calc repl" \
    --color-bg=#123 \
    --color-text=#fec \
    --hide-scrollbar \
    --font="Deja Vu Sans Mono 14px" \
    --geometry 40x14 \
    --execute node -i -e "$JSREPL"
  fi

($JSREPL just contains some extra functions that I want in a calculator)

I can easily go back to previous calculations and results, define vars, run loops whatevs. Having it on a key combo and launching the terminal with a distinct text/bg color really makes a difference. It also launches faster than most other calculators I tried. For my purposes (non-scientific, mostly programming stuff), it's been great.

You need to be very careful, as you may get wrong results due to rounding and floating point errors.

  $ node -i
  > 2**54+2
  18014398509481984
  > 3-3.01
  -0.009999999999999787

Other calculators such as calc [1] don't have these issues

1. https://github.com/lcn2/calc