The formula can be obtained from the Newton's Law of Universal Gravitation.
And it is given as:
Gravity (g) = GM/R^2
Where,
->G is the gravitational constant. It's value is 6.67 × 10^(-11). And it's unit is m^3 kg^(-1) s^(-2)
->M is the mass of the planet (in kg)
->R is the radius of the planet (in m)
Considering earth's example:
Mass of Earth = 5.98 × 10^24 kg
Radius of Earth = 6370 km = 6370 x 10^3 m
Substituting these values in the formula, we get:
g = [ 6.67 × 10^(-11) x 5.98 × 10^24 ] / (6370 x 10^3)^2
g = [3.986 x 10^14] / [4.057 x 10^13]
g= 9.81 m/s^2 (approximately)