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To calculate weight due to gravity, you would use Newton’s law of universal gravitation. The force of gravity (F) is equal to the gravitational constant (G) times the mass of one object (m1) times the mass of a second object (m2) divided by the square of the distance between them (r).

F =  Gm1m2
            r2

On, or near a planet, m1 is the mass of the planet, and m2 is the mass of the object you are trying to weigh. This is not the same for all planets. See what an object would weigh on different planets with this interactive. Away from a planet, you would need to calculate the force due to gravity from a number of different planets stars and other objects.