Newton's Constant, also known as "big G," is often expressed as meters cubed per kilogram per second squared, but this use of SI units is largely for the convenience of physicists who work with it. The gravitational constant can be more precisely expressed in Planck units.
Big G is properly expressed as being exactly equal to the cube of the Planck length divided by the product of Planck mass multiplied by the square of Planck time. This reliance on Planck units lends the constant a degree of precision, but it has the disadvantage of being somewhat more difficult with which to work. This is especially true for work on very large relativistic scales.