First we should know the difference between weigh and mass. Weigh depends upon gravity whilst mass doesn't. There is no gravitational force on moon so we can measure mass.
The mass of the moon is about (7.35 x 1022 kg), or about 1/81 of the Earth's mass.
The mass of the moon is about (7.35 x 1022 kg), or about 1/81 of the Earth's mass.