Kepler's third law states that the square of the period of revolution $\text{(T)}$ of a planet around the sun is proportional to the third power of the average distance $\text{r}$ between sun and planet i.e., ${\mathrm{T}}^2=\mathrm{K}{\mathrm{r}}^3$.
Here $\text{K}$ is constant. If the masses of sun and planet are $\text{M}$ and $\text{m}$ respectively then as per Newton's law of gravitation force of attraction between them is
$\mathrm{F}=\frac{\mathrm{G}\mathrm{M}\mathrm{m}}{{\mathrm{r}}^2}$, here $\text{G}$ is gravitational constant.
The relation between $\text{G}$ and $\text{K}$ is described as: