Weight is technically the downward force that a mass exerts based on the force of gravity and the mass of the object. On the surface of the Earth, mass and weight are often interchanged. In some places, objects are sold by the gram (mass), and in other locations, the same objects are sold by the ounce (weight).
The Weight Calculators use the dimensions of an object and its shape to compute the objects volume. They then apply a density associated with a material to estimate the objects weight.