Weight is a unit of force.
Newtons measure force.
5 kg is a mass and is in kg. Kg and mass are independent of location. 5 kg is 5 kg on the moon, here on earth, on Jupiter and on Mars or anywhere else you care to name.
F = m*a
F = 5 * 9.81
F = 49.05 Newtons. This is how things should be weighed on planet earth.
