The mass of Jupiter is estimated using a mathematical formula of Kepler's Third Law, which states gravity pulls the same on all planetary objects. A formula for calculating mass takes the distance of an orbit cubed divided by the period of the object's orbit squared, and then that figure is multiplied by the number of four times pi squared divided by Kepler's constant. Astronomers observe Jupiter's moons to calculate mass.
The formula reads "M=(a^3/P^2)*(4pi^2/G)," where M is the mass, "a" denotes the distance of an orbit, P is the period of an orbit, pi is 3.1459 and G is Kepler's constant. The period of an orbit is the time it takes a moon or planet to return to the same spot in the orbit every revolution. Pi is the mathematical constant that describes the relationship between the diameter of a circle and its circumference.
The mass of Jupiter is estimated to be 1.9 x 10^27 kilograms, or approximately 318 times the mass of Earth. The mass of Jupiter is 2.5 times greater than all of the other planets the solar system combined. Jupiter's mass can only be estimated, since the moons of Jupiter do not orbit the planet in a perfectly circular orbit as described by Kepler's Third Law.