You are probably not expecting a calculator but since you are here, let me present to you:

The LIve BC - a calculator that is not just interactive, it is live!

See the result while typing!

Works with ANSI escape codes.

Press enter to finish the formula and start a new one below.

<ctrl>+a / <ctrl>+e are moving the cursor to line start and line end respectively.

Press <left/right> to move cursor.

Press <up/down> to navigate through old calculations.

Press yy or ya to put the result (yank answer) into the clipboard (currently only pbcopy is supported).
Press yq (yank query) to copy the formula into the clipboard.

I use it in tmux in a popup:

bind-key -T prefix = display-popup -T"calc" -w50 -h22 -xC -y25 -E ./libc.sh