‘qalc’ is the command-line version of Qalculate![0] and runs both directly from the command line as well as interactively with tab-completion of variables (Planck mass, for example) and arbitrary unit conversion:
$ qalc
> 15 kilogram * (4 m/s)^2 / 2
(15 * kilogram * (((4 * meter) / second)^2)) / 2 = 120((kg*m^2) / s^2)
> convert best
120((kg*m^2) / s^2) = 120 J
> convert gTNT
120 J = approx. 28.680688 mgTNT