6.67428 x 10^-11

According to the law of universal gravitation, the attractive force (F) between two bodies is proportional to the product of their masses (m1 and m2), and inversely proportional to the square of the distance (r) between them:

F = G x (m1 x m2)/r^2

The constant of proportionality, G, is the gravitational constant.

The gravitational constant is perhaps the most difficult physical constant to measure to high accuracy. In SI units, the 2006 CODATA-recommended value of the gravitational constant is 6.67428 x 10^-11.