# How the gravitational constant $G$ is determined?

We know the gravitational constant is $G=6.67545 \times 10^{-11} m^3 kg^{-1} s^{-2}$. But how this value is determined at first?

The gravitational constant has been experimentally measured. Wikipedia has a brief but quite nice article on the history of measurements of $G$. The first measurement was made in 1798 (yes, that long ago!) by Henry Cavendish, who measured the gravitational force between lead spheres. The force from small masses is exceedingly small, but with sensitive equipment it can be measured.
• It is also interesting to note that the first measurement of $G$ was made in 1798, well after Newton's publication of the law of universal gravitation in 1687. – NeutronStar May 23 '14 at 14:57