Why is weight measured in kg and not in Newtons?

The problem is that while mass is the same everywhere on earth, weight is not - it can vary as much as 0.7% from the North Pole (heavy) to the mountains of Peru (light). This is in part caused by the rotation of the earth, and in part by the fact that the earth's surface is not (quite) a sphere. read more

When weight is used to refer to mass, then use units of mass (kilograms), and when weight is used to refer to force, then use units of force (newtons). Do not get bent out of shape over the two meanings. read more

Since all weight is measured on earth (except in theory and for astronauts) it uses roughly the same gravitational acceleration (around 9.81 m/s^2). This means that the mass (kg) of an object is directly proportional to its weight (N). People use kg because they're more familiar with the unit than the Newton. read more