Found out, i was looking at the wrong formula
You still haven't answered the basic question, "what do you mean by "heavy", here?"
One way of defining "heavy" is the object's weight: "the force of gravity on an object" which is always "mg", the mass times the acceleration due to gravity. Since you give the mass in kg, use g= 9.81 m/s^2.
But more common is the force with which an object is pressing down on a floor, scale, your hand, etc. Here, since the object is not pressing down on anything there is no force- its "heaviness" is 0.