fjbalaux is correct as long as you are calculating the "weight" in the gravitational field of Earth. (9.81m/s2 is the gravitational acceleration on the Earth)
But kg is a unit of the mass, not of the force.
The term "weight" can be misleading. If "weight" means force, then you calculate something like "kgf" - kilogram of force). On the Earth it does not matter - body with the mass of 1kg pushes down with the force of 9.81 Newtons (or call it 1 kgf). The mass of a body does not change if you relocate it anywhere - but the force supporting the same body will change; on the Moon it will be less, right? Then we need to use the Newton's law to calculate the mass (or force, or acceleration; depends what is known)
