The value of G is the gravitational force acting between two unit masses kept at a unit distance away from each other. Thus, in SI units, the value of G is equal to the gravitational force between two masses of 1 kg kept 1 m apart. Meanwhile, in SI units, the unit of G is Newton m2 kg-2. The value of G was first experimentally measured by Henry Cavendish. As per SI units, its value is 6.673 x 10-11 N m2 kg-2.